Anatomy of Animals

Contents

Animals

All animals digest their food, carry on gas exchange, excrete waste, circulate nutrient and waste products to and from the cells, coordinate their movements, protect themselves, and reproduce and disperse the species. The more complex animals have organ systems to carry out these functions; in simple animals, these functions sometimes are carried out by specialized tissues. All these functions can be found in the Human Organ Systems as shown in Figure 01a or in Table 10-01, Topic 10. Although endocrine system does not exist in invertebrates, neurosecretory systems are widespread. Such systems probably exist in all phyla. Work to date has demonstrated the important role of neurosecretion in growth and reproduction of many model systems.

Figure 01a Human Organs [view large image]

All phyla of animals had evolved by the beginning of the Paleozoic Era some 540 million years ago. The evolutionary tree of animals in Figure 01b indicates that animals are the descendants of protozoans - perhaps in a colonial form whose cells differentiated into various types of cells. The evolutionary tree of animals resembles a tree with 2 main branches. The animal phyla located on the main trunk of the tree are referred to as the primitive invertebrates, and the animals of the main 2 branches include the advanced invertebrates and the vertebrates. Invertebrates lack a dorsal backbone, while vertebrates have a backbone made up of vertebras. A study of the evolution of animals reveals that the most complex animals have the most advanced features as listed in Table 01 below. Classification of animals therefore is based on type of body plan, symmetry, number of

Figure 01b Evolutionary Tree of Animals [view large image]

germ layers, level of organization, type of body cavity, and presence or absence of segmentation.

Table 01 Animal Features

Sponges

		necessary for multicellularity already. Their evolutionary steps are clearly demonstrated from single-celled aquatic protists to colonies and then appear as the collar cells in sponges. A sponge does not have anything in their bodies that can be called tissues or organs. Instead each type of sponge cell is responsible for a different activity to keep the sponge alive. Some sponges grow on rocks and are brightly colored. Sponges often are shaped like vases with a central cavity (Figure 02). Figure 03 shows the anatomy of the sponge. The internal structures are described in the followings.
Figure 02 Sponge [view large image]	Figure 03 Sponge Anatomy [view large image]	Although the terminologies (and hence the functions) for the various systems are in close parallel to those for the human body, the composition and distribution are vastly different.

• Circulatory - The amoeboid cells within the wall act as a circulatory device to transport nutrients from cell to cell. For example, they would pick up some food, move out to the sheets of cells that cover the surface, and feed them.
• Digestive - Sponges are sessile filter feeders. This means that they remain in one place as an adult, and the food they acquire filters through the pores. Microscopic food particles brought by the water are engulfed by the collar cells and are digested by them in food vacuoles or they are passed to the amoeboid cells for digestion.
• Exocrine - There are no endocrine glands in sponges. But they have exocrine glands that release a secretion to the surface. Studies over the last two decades have revealed that sponges produce a surprisingly broad spectrum of biotoxins, some of which are quite potent. A few, such as those of Tedania and Neofibularia can cause painful skin rashes in humans. Not only are those chemicals used to deter predators they are also helpful to prevent infection by microbes and to compete for space with other sessile invertebrates such as ascidians, corals and even other sponges.
• Excretory - The main opening of a sponge is used as an exit to expel waste.
• Immune - Compounds with anti-inflammatory, and antibiotic activity have been identified from many marine sponges.
• Musculo-skeletal - The wall of a sponge contains 3 types of cells. The outer cells are flattened epidermal cells. The inner cells are collar cells with flagella, whose constant movement produces water currents that flow through the pores into the central cavity and out through the upper opening of the body, called the osculum. The mesohyl is a gelatinous layer between the outer body of the sponge and the spongocoel (the inner cavity). The "skeleton" of the sponge is composed of tiny needle-like splinters called spicules (composed of glass or calcium carbonate), a mesh of protein called spongin, or a combination of both.
• Nervous and Sensory - None. But there is some evidence of electrical signals moving along the overall cell membrane.
• Reproductive - Sponges can reproduce sexually or asexually. Sexual reproduction can be either hermaphroditic (organism having both sexes) or gonochoristic (separate sexes). The eggs and the sperms are released into the water, where fertilization occurs. Fertilization results in a zygote, which develops into a ciliated larva that may swim to a new location. Sponges also reproduce by budding, and this process produces whole colonies that can become quite large. Like many less-specialized organisms, sponges are capable of regeneration, or growth of a whole from a small part.

  Germ cells provide the continuity of life between generations. In higher animals (up to roundworms) there is a clear and early separation of germ cells from somatic cell types. For the other species somatic cells can readily become germ cells even in adult organisms such as the buds, and polyps of many invertebrate phyla. For those organisms where there is an established germ line that separates early in development, the germ cells come from the primordial germ cells (PGCs) in the yolk sac and then migrate into the developing gonads (the genital ridges, originally not in the lower part

  Figure 04a Primordial Germ Cells [view large image]

  of the body) as shown in Figure 04a. Once in the gonad, primordial germ cells continue to divide mitotically, producing millions of potential gametes. The PGCs then undergo meiosis to produce the male and female germ cells, i.e., the sperms and eggs.

  Fertilization is the process whereby two sex cells (gametes) fuse together to create a new individual with genetic potentials derived from both parents. Although the actual details of fertilization vary enormously from species to species, the events of conception generally consist of four major activities: 1. Contact and recognition between sperm and egg. In most instances, this insures that the sperm and egg are of the same species.

  Figure 04b Sperm and Egg [view large image]

  2. Regulation of sperm entry into the egg. Only one sperm can ultimately fertilize the egg. 3. Fusion of the genetic material of sperm and egg. 4. Activation of egg metabolism to start development.

            The shape of the sperm is highly conserved in the evolution of the animal kingdom. Each sperm is known to consist of a haploid nucleus, a propulsion system to move the nucleus, and a sac of enzymes that enable to nucleus to enter the egg. most of the cytoplasm of the sperm has been eliminated during the sperm's maturation as shown in Figure 04b. The structure can be partitioned into the following components (see Figure 04b):
  • Plasma Membrane - In some species such as the parasitic roundworm, the sperm travel by the amoeboid motion of extending the cell membrane.
  • Acrosomal Vesicle - It is derived from the Golgi apparatus and contains enzymes (that digest proteins and complex sugars) to lyse the outer covering of the egg. Recognition between sperm and egg may involve the acrosomal process for some species.
  • Nucleus - The nucleus is very streamlined and its DNA becomes tightly compressed in the head of the sperm.
  • Axoneme - It is the major motor portion of the flagellum emanating from the centriole at the base of the nucleus.
  • Mitochondria - The energy to whip the flagellum and there propel the sperm comes from rings of mitochondria located in the neck region of the sperm.
  • Centriole - The centrosome (Figure 04c) is an area in the cell where microtubles are produced. Within an animal cell centrosome there is a pair of small organelles, the centrioles, each made up of a ring of nine groups of

    microtubules. There are three fused microtubules in each group. The two centrioles are arranged such that one is perpendicular to the other. During animal cell division, the centrosome divides and the centrioles replicate. The result is two centrosomes, each with its own pair of centrioles. The two centrosomes move to opposite ends of the nucleus, and from each centrosome, microtubules grow into a "spindle" which is responsible for separating replicated chromosomes into the two daughter cells. It is found that the nucleus of the sperm cell is not necessary for the early divisions at the very beginning of development. It is the centrosome (and the centriole within) that is absolutely essential for the initial divisions. There are cases in which a centrosome can actually be created within the egg. This process is called parthenogeneseis, or virgin birth (see more below)

    Figure 04c Centrosome and Centriole [view large image]

            All the material necessary for the beginning of growth and development must be stored in the mature egg (the ovum). Therefore, whereas the sperm has eliminated most of its cytoplasm, the developing egg (the oocyte before it is haploid) not only conserves its material but is actively involved in accumulating more (see Figure 04b for the relative size of sperm and egg). The cytoplasmic trove includes proteins, ribosomes, tRNA, mRNA, and morphogenetic factors (the molecules that direct the differentiation of cells into certain cell types at different locations). The basic components are listed in the followings:
  • Nucleus - In some species such as the sea urchins, the nucleus is already haploid at the time of fertilization. In other species (including many worms and most mammals) the egg nucleus is still diploid and the sperm enters before the meiotic divisions are completed.
  • Plasma Membrane - It must regulate the flow of certain ions during fertilization and must be capable of fusing with the sperm plasma membrane.
  • Vitelline envelope - It forms a fibrous mat above the egg and is essential for the species-specific binding of sperm. It is called zona pelucida in mammals.
  • Cortical Granules - These membrane-bound structures are homologous to the acrosomal vesicle of the sperm. They contain proteins that control the entrance of only one sperm, and support the process of cleavage.
  • Yolk Granule - Yolk is sequestered inside this membrane-bounded sac.
  • Jelly Layer - This glycoprotein meshwork can have numerous functions. Most often, though, it is used to either attract or activate sperm entrance.

		Almost all animal species reproduce sexually, which is believed to have evolved as a mean of strengthening the "fitness" of the species by mixing the genes of two different individuals from meiosis. About 1% of animal species reproduce by partenogenesis (Greek for "virgin birth"), while an even smaller fraction switch between sexual and asexual reproduction (known as cyclical parthenogenesis).
Figure 04d Parthenogenesis Examples [view large image]	Figure 04e Partheno- genesis [large image]	Both kinds of parthenogenesis occur most often in low-level species including some fish, amphibians and reptiles (see some examples in Figure 04d). Some experiments try to

induce partenogenesis in chickens and turkeys with limited success. Figure 04e shows the different between normal fertilisation and parthenogenesis. There are actually two ways in parthenogenesis - one method involves sex cell division and recombination, while the other just produces an egg with a full complement of DNA. Many species reproduce parthenogenetically in the absence of a wide pool of potential mates (as alleged in the movies "Jurassic Park"), while some species switch to sexual reproduction only in adverse conditions for increasing the chance of survival.
• Respiratory - Various cells absorb oxygen from the passing water stream.

Cnidarians

		shape with the mouth region directed downward, are called jellyfishes, or medusae. The polyp is adapted to a sessile life, while the medusa is adapted to a floating or free-swimming existence. At one time, both body forms may have been a part of the life cycle of all cnidarians, because today we see an alternation of generations life cycle of these two forms in certain cnidarians (see Figure 05d). In such life cycle, the polyp stage produces medusae, and the medusae, which produce eggs and
Figure 05a Cnidaria [view large image]	Figure 05b Hydra Anatomy [view large image]	sperm, disperse the species. Cnidarians are quite diversified including the Portuguese man-of-war, sea anemones, hydra and many other species.

• Circulatory - Nutrient molecules are passed by diffusion to the rest of the body from the cells of the gastrodermis.
• Digestive - The cells of the gastrodermis secrete digestive juices that pour into a central cavity. The enzymes begin the digestive process, which is completed within food vacuoles when small pieces of the prey are engulfed by the cells of the gastrodermis (see Figures 05a and 05b).
• Exocrine - There are no endocrine glands in cnidarians. But they have specialized stinging cells called cnidocytes, from a Greek word meaning sea nettles. Within these cells are the nematocysts, which are long spirally coiled hollow threads. When the trigger of a cnidocyte is touched, the discharged thread, which sometimes contains poison, stuns either prey or enemy (see Figure 05b).
• Excretory - The main opening of the cnidarians serves as a mouth and anus.
• Immune - Cnidarians have primitive immune system. They use phagocytosis to eliminate attacking cells (in grafts between strains), apoptosis (cell death) plays an important part as well. In addition, there is evidence that IgG (immuno- globulins type G - a kind of antibody) related genes is involved in the response of cnidarian cells to microbial infection.
• Musculo-skeletal - The body of a cnidarian is a hollow, 2-layered sac, which accounts for the former name of these organisms - coelenterate means hollow sac. The outer layer of the sac, the ectoderm, is separated from the inner layer, the endoderm, by a jellylike material called mesoglea (see Figure 05a). Each coral polyp is an individual but has a chalky skeleton that is joined to its neighbor's to form the coral reef. Hydras usually remain in one place, they may glide along on their base, or foot, or even move rapidly by means of somersaulting. Like other animals capable of locomotion, they possess both muscle and nerve cells.

		Nervous and Sensory - The nerve cells form a connecting network throughout the mesoglea known as the nerve net. The nerve net makes contact with the outer layer of cells, called the epidermis, and the inner layer of cells, called the gastrodermis. These cells contain contractile fibers. The first light-sensitive cells appeared in Cnidarians animals such as the hydras. Figure 05c illustrates the sequence of vision evolution with an insert to indicate the kind of light perception in each step.
Figure 05c Evolution of Vision [view large image]	Figure 05d Compound Eye [view large image]

1. The box jellyfish is an amazing animal with more than 20 eye pits and many eyes very similar to ours. They seem to double for ears as well.
2. Hydras are among those animals that are limited to light-detecting cells spread around their bodies to pick up change in brightness. The eyespots consist of a single photoreceptor cell and a surrounding pigment cell.
3. In animals like flatworms, the eyespots grew inward to form cup-shaped structures to receive light from different angles.
4. Over time the cup became a deep chamber. As the opening to the chamber became narrower, the pinhole-eye developed. This type of eye can provide coarse images, allowing an animal to discern the shape of objects. Optic nerve to convert light energy to electric impulse also appeared in this type of eye, now found only.
5. Eventually the chamber opening closed over with transparent cells, which helped to prevent eye infections, creating a lens that focuses light onto the retina (a layer of photoreceptor cells). Marine snails have this type of eye, which provides the ability to see more clearly through water.
6. The lens evolved into a refractive lens that could change shape to focus many rays of light onto one spot on the retina. Cephalopods such as cuttlefish, squid and octopuses have eyes that are similar to those of vertebrates like humans (with small variation in the orientation of the retinal cells). Insects have compound eye with multiple facets, each with a fixed lens and crystalline cone that focuses light on the photoreceptor cells (Figure 05d).
• Reproductive - In hydra, the gastrodermis contains interstitial or embryonic cells capable of becoming other types

  of cells (Figure 05b). For example, they can produce the ovary and the testes and probably account for the animal's great regenerative powers. Like the sponge, a whole cnidarian can grow from a small piece. When conditions are favorable, small outgrowths, or buds, appear, pinch off, and begin to live independently. Figure 05e shows the cnidaria life cycle involving both sexual and asexual reproductions. The larva in the life cycle represents a juvenile form of the animal undergoing metamorphosis (form change).

  Figure 05e Cnidaria Life Cycle [view large image]

  Respiratory - The presence of the large inner cavity makes it possible for all cells to exchange gases directly with the surrounding medium. Because this function is carried out by a vascular system in more complex animals, the cavity is known as a gastrovascular cavity.

Flatworms

Flatworms are nonsegmented, lack a coelom, and have the sac body plan with only one opening. Therefore, if we analyze them according to Table 01, they have a combination of primitive and advanced features. There are three classes of flatworms: one is free living and two are parasitic. The free-living specimen, the planarian, best exemplifies the characteristics of the phylum. Tapeworms and flukes are parasitic with structure reflecting the modifications that occur in parasitic animals. Concomitant with the loss of predation, there is an absence of cephalization; the anterior end notably carries hooks and/or suckers for attachment to the host. The parasite acquires nutrient molecules from the host, and the digestive system is reduced. It is

Figure 06 Tapeworm Life Cycle [view large image]

covered by a specialized body wall resistant to host digestive juices. The extensive development of the reproductive system, with the production of millions of eggs, may be associated with difficulties in dispersing the species. Figure 06 shows the life cycle of the tapeworm.

• Circulatory - Since the body is flattened, nutrient molecules are easily passed by diffusion from cell to cell.
• Digestive - The digestive organ is tripartite (having 3 branches) and ramifies thorughout the body. It begins with a pharynx (throat), which ejects from the mouth to suck food particles into the digestive organ (see Figure 07(a)).
• Endocrine - Flatworms lack endocrine glands.
• Excretory - Because planarians live in fresh water, which tends to enter the body by osmosis. They have an excretory organ that largely rids the body of excess water. The organ consists of a series of interconnecting canals, which run the length of the body on each side. The beating of cilia in the flame cells (so named because the beating of the cilia reminded some early investigator of the flickering of a flame) keeps the water moving toward the excretory pores (see Figure 07(d)).

Immune - Immune function requires inter-cellular communication. A number of requisite messenger molecules, such as hormones and neuro-peptides, have been discovered in recent years. Once thought to be restricted to complex multi-celled creatures, similar messengers have been found to influence primitive nervous system and simple non-specific immune cells in low order creatures such as flatworms. Musculo-skeletal - Flatworms have three germ layers. The presence of mesoderm not only gives bulk to the animal, it also allows for greater complexity of internal structure. They have well-developed muscles, and their ciliated epidermis allows them to glide along a film of mucus. Nervous and Sensory - The planarian has a ladder-type nervous organ (Figure 07(b)). There is a small brain and two lateral nerve cords joined by cross branches. Planarians show good cephalization. Aside from the brain, there are light-sensitive organs (the eyes, which do not form image) and chemosense organs. For primitive invertebrates the eyesight is blurry as shown in diagram 3 of Figure 05c. It improves gradually until the image forming eyesight with the camera-type eye in human. Reproductive - Planarians are hermaphroditic animals, which means that they possess both male and female sex organs (Figure 07(c)). The worms practice cross-fertilization; the penis of one is inserted into the genital pore of the other. The fertilized

Figure 07 Planarian Anatomy [view large image]

eggs hatch in 2-3 weeks as tiny worms. Planarians are capable of regeneration, if a worm is cut crosswise, it usually grows a new head or a new tail as is appropriate.

• Respiratory - The flatworms lack respiratory organ, but since the body is flattened, diffusion alone is adequate for the passage of oxygen from cell to cell.

Roundworms

with the rotifers. Roundworms possess two anatomical features not seen in more primitive animals: a tube-within-a-tube body plan and a body cavity. The body cavity is a pseudocoelom, or a cavity incompletely lined with mesoderm. This fluid-filled pseudocoelom provides space for the development of organs, and serves as a type of skeleton. When roundworms are analyzed according to Table 01, they are seen to have features associated with advanced animals except that they are nonsegmented.

Figure 08b Roundworm Anatomy [view large image]

• Circulatory - Food digested in the gut is not distributed by any specialized vascular system. Nutrients and waste are distributed in the body cavity, whose contents are regulated by one-celled glands (in simpler species) or excretory canal along each side of the body (see Figure 08b).
• Digestive - The head of a nematode has a few tiny sense organs, and a mouth opening into a muscular pharynx where food (mostly bacteria and detritus) is pulled in and crushed. This leads into a long simple gut cavity, and then to an anus near the tip of the body (see Figure 08b).
• Endocrine (pseudo) - Among the many remarkable discoveries from the roundworm, Caenorhabditis elegans, is the fact that disrupting an hormonal insulin-like pathway can dramatically extend lifespan (see Genes of Ageing).

Excretory - Sponges and Cnidarians have radial symmetry. These animals possess a single digestive opening served as both mouth and anus. Even though evolution doesn't care about aesthetic, such arrangement is highly inefficiency to say the least. It is commonly believed that during the transition from radial to bilateral symmetry, both openings evolved simultaneously by the lateral closure of a slit-like blastopore with the ends giving rise to a mouth and anus (Figure 08c). In deuterostomes (secondary mouth) the site of gastrulation (the site at which the endomesoderm forms) becomes the anus and the mouth is formed at a different end. The relationship is not that clear in roundworm (protostomis), and the flatworm (nemertodermatida) still retains only one opening although it has bilateral symmetry. Research in 2008 suggests another model, which hypothesizes the single opening shifts ventrally during the transition to bilateral symmetry with the anus fully formed only after protonephridia (which establishes a system of canals to expel watery waste, like the kidney in mammals) as indicated in Figure 08c.

Figure 08c Evolution of Mouth and Anus [view large image]

• Immune - Roundworms have innate immunity that fights pathogens using defenses that are quickly mobilized and triggered by receptors that recognize a broad spectrum of pathogens. Lower animals such as the roundworms lack adaptive immune system, which may take days or weeks after an initial infection to have an effect.
• Musculo-skeletal - Roundworms in general do not have an internal or external skeleton, but they do have a hydrostatic skeleton, a fluid-filled interior that supports muscle contraction and enhances flexibility. In addition, the epidermis of a nematode is highly unusual; it is not composed of cells like other animals, but instead is a mass of cellular material and nuclei without separate membranes. This epidermis secretes a think outer cuticle (Figure 08b) which is both tough and flexible. The cuticle is a feature shared with arthropods and other ecdysozoans. As in those other groups, the cuticle is periodically shed during the life of a nematode as it grows, usually four times before reaching the adult stage. The cuticle is the closest thing a roundwrom has to a skeleton, and in fact the worm uses its cuticle as a support and leverage point for movement. Long muscles underneath the epidermis move the body from side to side (no crawling or lifting).
• Nervous and Sensory - The muscles are activated by two nerves that run the length of the nematode on both the dorsal and ventral side. Unlike other animals, where the nerves branch out to the muscle cells, a nematode's muscle cells branch toward to nerves. The ventral nerve has a series of nerve centers along its length, and both nerves connect to a nerve ring and additional nerve centers located near the head (see Figure 08b).
• Reproductive - The reproductive system is complex, and many parasitic species have a very high reproductive potential. Some nematodes bear live young, the eggs matured in the female reproductive tract; but most release eggs, which develop into larvae that molt one or more times before reaching maturity. C. elegans exists either as a hermaphrodite or a male. The predominant sexual form of C. elegans is the hermaphrodite, which produces both sperm and eggs. Thus, it can self-fertilize. When it does, each animal produces about 300 progenies.
• Respiratory - There is no respiratory system for the uptake or distribution of oxygen. The body cavity allows easy passage of molecules including oxygen.

Mollusks

All the advanced invertebrate phyla have a true coelom. Nevertheless, they can be divided into two groups on the basis of embryological evidence (Figure 09). In mollusks, annelids, and arthropods, the coelom forms by splitting of the mesoderm. Therefore, they are called the schizocoelomates. In echinoderms and chordates, the coelom forms by outpocketing of the primitive gut. It is thus called enterocoelomates. Note the interchange of mouth and anus in these two different types of development.

Figure 09 Coelom Formation [view large image]

Mollusks are a very large and diversified group containing many thousands of living and extinct forms. However, all forms of mollusks have a body composed of three distinct parts:


1. Visceral mass: the soft-bodied portion that contains internal organs.
2. Foot: the strong, muscular portion used for locomotion.
3. Mantle: the membranous or sometimes muscular covering that envelops but does not completely enclose the visceral mass. The mantle cavity is the space between the two folds of the mantle. The mantle may secrete a shell.

Clam (bivalve) Anatomy:
• Circulatory - The heart of a clam lies just below the hump of the shell within the pericardial cavity (Figure 10), the only remains of the coelom. Therefore, the coelom of the clam is said to reduce. The heart pumps blue blood, containing the pigment hemocyanin instead of red hemoglobin, into vessels that lead to the various organs of the body. Within the organs, however, blood flows through spaces, or sinuses, rather than through vessels. Such a circulatory system is called an open circulatory system because the blood is not contained within blood vessels all the time. This type of circulatory system can be associated only with an inactive animal because it is an inefficient means of transporting oxygen and nutrients throughout the body.
• Digestive - The clam is a filter feeder. Food particles and water enter the mantle cavity by way of the incurrent siphon (Figure 10), a posterior opening between the two valves. Mucous secretions cause smaller particles to adhere to the gills, and cilia action sweeps them toward the mouth. The digestive system includes a mouth, a stomach, and an intestine, which coils about in the visceral mass and then goes right through the heart before ending in an anus (see Figure 10). The anus empties at an excurrent siphon, which lies just above the incurrent siphon.
• Exocrine - There is an accessory organ of digestion called a digestive gland. The digestive gland surrounds the stomach and is composed of numerous tubules, which are formed at the distal ends of branching ciliated ducts. Nutrients from the stomach enter the tubules through these ciliated ducts and are absorbed by the glandular cells lining the tubules and digested intracellularly. The crystalline style (stalk) extends from the style sac into the stomach. The style contains amylase, which helps digest the starches present in the food. Thus, the digestive gland is both secretory and absorptive. In addition, the style usually (in other bivalves) harbours many spirochete bacteria that are thought to secrete additional digestive enzymes. Wastes from the digestive gland tubules are returned to the stomach and are eventually swept into the intestine
• Excretory - There are two excretory kidneys in the clam (Figure 10), which lie just below the heart and remove waste from the pericardial cavity for excretion into the mantle cavity. The clam excretes ammonia, a poisonous substance that requires the concomitant excretion of water, Land-dwelling animals tend to excrete a less toxic substance in a more concentrated form.
• Immune - Internal defence in bivalve molluscs, like all invertebrate species, is based on an innate, non-lymphoid immune system which consists of a variety of cell types and effector molecules interacting to maintain efficient elimination of foreign bodies. Phagocytic blood cells (cell that removes unwanted substances) or haemocytes, are the principal effector cell for immune defence. The phagocytic response of the haemocytes is complemented by an array of killing mechanisms which may include, release of degradative enzymes and the generation of reactive oxygen metabolites. Antioxidant enzymes may also be present to minimise the potential damage for adjacent tissues and cells from these reactive oxygen metabolites. Haemocytes may also release other soluble compounds as part of their defence strategies. Recent research has identified the composition of a number of antibacterial peptides in bivalve molluscs.
• Musculo-skeletal - In a clam, the shell is secreted by the mantle and is composed of calcium carbonate with an inner layer of mother-of-pearl. If a foreign body is placed between the mantle and the shell, pearls form as concentric layers of shell are deposited about the particle. The adductor muscles are used to open and close the shells (Figure 10).
• Nervous and Sensory - The clam nervous system is composed of three pairs of ganglia (anterior, foot, and posterior, see Figure 10), which all are connected by nerves. Clam lack cephalization. The foot projects anteriorly from the shell, and by expanding the tip of the foot and pulling the body after it, the clam moves forward.

Reproductive - The male or female gonad of a clam can be found about the coils of the intestine (Figure 10). Reproduction takes place when male and female clams release sperm and eggs into the water. Fertilized eggs develop into free-swimming larvae before settling to the bottom. While all clams have some type of larval stage, only marine clams have a trochohore larva. The presence of the trochophore larva among some mollusks indicates a relationship to the annelids, some members of which also have this type of larval stage. Respiratory - Within the mantle cavity, the gills (Figure 10) hang down on either

Figure 10 Clam Anatomy [view large image]

side of the visceral mass, which lies above the foot. Gills are composed of vascularized, highly convoluted, thin-walled tissue specialized for gas exchange.

Snail (gastropod) Anatomy:
• Circulatory - The heart of the snail is found on the left side and consists of one auricle and one ventricle (Figure 11). The ventricle pumps blue blood through an aortic trunk to all parts of the body through a series of arteries and capillaries. From the capillaries, the blood passes into sinuses, or spaces in the tissues called the hemocoel. From the hemocoel blood passes into the veins and back to the auricle.
• Digestive - Snail have sets of jaws inside their mouths used to cut off bits of food. Just behind the jaws the digestive tract is swollen to form a large buccal mass with muscles attached. This area is covered by the radula, the snail version of the human tongue. The radula moves back and forth very rapidly to grind up pieces of food. It wears away with use, but is continuously replaced since it is formed in a radular sac at the end of the buccal mass and grows constantly, like the human fingernail. The teeth are fastened to the radula in rows. Snails may have up to thousands of individual teeth, with tiny cutting points called cusps. The esophagus leaves the buccal mass and passes from the foot into the visceral mass within the shell to form a crop. Behind the crop is a dilated stomach, which is followed by the long intestine, whose posterior end is dilated to form the rectum. The anus opens into the mantle cavity near the edge of the mantle and the shell (see Figure 11).
• Endocrine (neuro) - Hormone production is not well documented in mollusks other than gastropods and cephalopods. Antagonistic neurohormonal control of reproductive activity and metabolic processes is performed in the gastropods through cerebral dorsal bodies and lateral lobes or juxtaposed organs. There is a pair of salivary glands line the crop or esophagus, a large digestive gland called the liver empties into the stomach, and a gland secreting albumin.
• Excretory - There are two kidneys (Figure 11), or nephridia, in only the primitive gastropods, such as the archaeo- gastropods, while, in the advanced forms, one kidney is small or lost. The kidney plays different roles, depending upon the environment in which the snail lives. Usually, the posterior chamber excretes uric acid and purines, while the anterior chamber has osmoregulatory function. Snails living on the land are uricotelic, this means that to preserve water they excrete almost solid uric acid. Snails living in the water excrete ammonia, and are not uricotelic. The renal aperture (urine opening) is situated in the upper region of the right mantle cavity. The urine produced by the kidneys is expelled here. Terrestrial gastropods reduce water loss by sealing the mantle cavity with an extended mantle collar.
• Immune - Similar to the bivalve's.
• Musculo-skeletal - Most gastropods have a coiled shell in which the visceral mass spirals. Snails move using several important muscles. The columellar muscle is attached to the shell internally and is used to withdraw the snail's body into its shell. The foot, which is mostly muscle tissue, is the main source of propulsion for the snail. The pedal gland at front end of the foot secretes a thin, flat ribbon of mucus for the snail to move along. When the snail is caught in vegetation or out of the water, it may move by "hunching", or muscular contractions of the foot along with a jerky pulling forward of the shell.
• Nervous and Sensory - The greater portion of the gastropod nervous system, called the brain, consists of nine large ganglia, eight of which are paired. They are connected to each other by commissures and are found around the esophagus just behind the buccal mass. Large branching nerves originating in these ganglia innervate all parts of the body, while several small ganglia are associated with sense organs. Snails have well-developed eyes at the base of their tentacles. Their sense of hearing is centered in two tiny sacs called statocysts, which contain fluid in which bodies called statoliths are suspended. The snail uses its hearing more as an ability to detect vibrations and maintain a sense of balance. Taste is centered in the mouth region.
• Reproductive - While land snails have separate sexes (there are males and females), aquatic snails are hermaphrodites, they have male as well as female genital organs in a genital apparatus. A snail's fertilization by itself is mostly prevented by the development of sperm and egg cells, which, passing hermaphroditic areas of the genital apparatus are unripe and ripen only when those areas used by male as well as female genital cells are passed (see Figure 11). The dart sac serves to transfer a hormone cocktail to the recipient, which is supposed to achieve maximal effectivity of the donator's sperm cells after the copulation. Among the more highly developed snails fertilization takes place internally in the female or at least in the snail acting as female. For that to be possible, fertilization has to be preceded by a copulation, during which sperm cells are transferred to the sexual partner. After about two weeks the eggs are laid, although in some species the seminal fluid can be stored for up to year. The eggs look like little white pearls and are laid

separatly or in groups under stones, logs or fallen leafs. After a few weeks tiny snails are born. They have a transparent shell. Some snailspecies are ovo-viviparious, this means that the eggs hatch inside of the snail. Most species reach maturity in a year, but the larger ones can take two to four years to reach maturity. Respiratory - Snails are adapted to life on land. Their mantle is richly supplied with blood vessels and functions as a lung when air is moved in and out through respiratory pores (Figure 11). Respiration in aquatic snails occurs through an internal gill, or ctenidium, which consists of a series of narrow, flat leaflets well supplied with

Figure 11 Snail Anatomy [view large image]

blood and arranged like the teeth of a comb.

Octopus (cephalopod) Anatomy:
• Circulatory - Octopuses have blue blood and three hearts. There is a heart at the end of each of their gills; these hearts pump blood through the gills. The third heart pumps blood through the rest of the body.
• Digestive - Octopuses catch prey with their arms, then kill it by biting with their horny, beaklike jaws and the radula or tooth ribbon, paralyzing the prey with a nerve poison, and softening the flesh. They then suck out the flesh. Figure 12 shows the digestive system of the octopus.

Endocrine (neuro) - Hormone production is not well documented in mollusks other than gastropods and cephalopods. Antagonistic neurohormonal control of reproductive activity and metabolic processes is performed in the cephalopods through optic glands. The anterior salivary gland secrets the clear liquid (of water, mucin, protein, and enzymes) into the mouth. It moistens food and starts the breakdown of starches. The posterior salivary gland (poison gland) produces venom to paralyze prey. The oviducal gland surrounding the end of the primary oviduct is responsible for secreting some of the external coatings over spawned eggs. In octopuses it also acts as a spermatheca. Excretory - Excretory functions are carried out by a pair of nephridia (kidneys), tubular structures that collect fluids from the coelom and exchange salts and other substances with body tissues as the fluid passes along the tubules for excretion. The nephridia empty into the mantle cavity. Immune - Similar to the bivalve's. Musculo-skeletal - An octopus has a soft body, eight arms, and pouch-shaped. Each arm has two rows of suction cups. If it loses an arm, it will eventually regrow another one. Octopuses move by pulling themselves along with their arms or by forcibly expelling water through the funnel or siphon. They eject dark ink from the

Figure 12 Digestive System [view large image]

ink sac to escape danger. They can also change color according to mood and environ- ment, sometimes exhibiting rapid waves of color changes that sweep over the body. Cuttlefish has internal skeleton; it seems the skin is dotted with light sensors that help to control camouflage.

• Nervous and Sensory - An octopus has an eye on each side of its head and has very good eyesight. It cannot hear. Octopuses can get around quite well due to the extreme sensitivity of the suckers in their tentacles. In laboratory tests, blindfolded octopuses could tell different objects apart as well as visually-unimpeded octopuses. Octopuses also have the most complex brains of any invertebrates; they quickly learn new tasks by trial-and-error, and they seem to form the same short-term and long-term memories as vertebrate animals. Figure 13a shows a jar-opening octopus.

		Reproductive - Instead of a penis, male octopuses have a specialized tentacle (usually the third tentacle on their right side) that deposits spermatophores in the mantle cavity of the female. The eggs are encased in capsules and attached to rock. A mother octopus doesn't eat during the entire 1 to 2 months she is caring for her eggs. The young hatch directly, without a larval stage.
Figure 13a Jar-opening Octopus [view large image]	Figure 13b Octopus Anatomy [view large image]	Respiratory - Octopuses use the ctenidium (molluscan gill) to carry out gas exchange. It is located just next to the hearts. The octopus anatomy is shown in Figure 13b.

Annelids

The primary characteristic of annelids is segmentation; obvious rings encircle the body, and even the well-developed coelom is partitioned by membranous septa. Both segmentation and an ample coelom are important advances, facilitat- ing the development of specialization of parts. Figure 14 shows the earthworm anatomy as an example of the annelids.
• Circulatory - The earthworm has an extensive closed circulatory system. Hemoglobin-containing blood moves anteriorly in a dorsal blood vessel and then is pumped by five pairs of hearts into a ventral blood vessel. As the ventral blood vessel takes blood toward the posterior regions of the worm's body, it gives off branches in every segment.
• Digestive - The earthworm feeds on leaves or any other organic matter, living or dead, that can be taken conveniently into its mouth along with dirt. Food drawn into the mouth by the action of the muscular pharynx is stored in a crop and is ground up in a thick, muscular gizzard. Digestion and absorption occur in a long intestine, whose dorsal surface is expanded by a typhlosole that allows additional surface area for absorption. Notice that the tube-within-a-tube plan has allowed specialization of the digestive system to occur.
• Endocrine (neuro) - It is known that annelid reproduction is controlled by some kind of endocrine system.
• Excretory - The excretory system consists of paired nephridia, or coiled tubules, in each segment. Nephridia have two openings: one is a ciliated funnel that collects coelomic fluid, and the other is an exit in the body wall. Between the two openings is a convoluted region where waste material is removed from the blood vessels about the nephridium.
• Immune - Earthworms have a highly effective immune system since cancer cannot be induced in them nor does it seem to occur in natural populations. They possess a highly evolved, unique and efficient immune system that has facilitated long-term survival.
• Musculo-skeletal - Locomotion in the earthworm is suitable to its way of life, and each segment of the body has four pairs of setae, or slender bristles. The setas are inserted into the dirt, and then the body is pulled forward. Both a circular layer and a longitudinal layer of muscle in the body wall make it possible for the worm to move and to change its shape. Muscle contraction is aided by the fluid-filled coelomic compartments, which act as a hydrostatic skeleton.
• Nervous and Sensory - The nervous system consists of an anterior, dorsal, ganglionic mass, or a brain, and a long ventral solid nerve cord with ganglionic swellings and lateral nerves in each segment. When invertebrates are compared to vertebrates, it often is said that the former have a ventral solid nerve cord, while the latter have a dorsal hollow nerve cord. The earthworm lacks obvious cephalization.

Reproductive - The earthworms are hermaphroditic, with a complete set of organs for both sexes. The male organs of an earthworm are the testes, the seminal vesicles, and the sperm ducts; the female organs are the ovaries, the oviducts, and the seminal receptacles. Copulation occurs when two worms lie ventral surface to ventral surface with the heads pointing in opposite directions. The clitellum, a smooth girdle about each worm secretes mucus, after which the sperm leave the sperm ducts and travel to the seminal receptacles of the partner. The clitellum later produces a slime tube, which is move along over the head of the worm by muscular contractions. Into this tube are deposited eggs from the oviducts and sperm from the seminal receptacles. The slime tube forms a cocoon within which the miniature worms develop. There is no larval stage. Respiratory - A earthworm has no respiratory organ. It takes in oxygen directly through its skin and gives off carbon dioxide. Its skin is always moist for allowing

Figure 14 Earthworm Anatomy [view large image]

efficient absorption of oxygen dissolved efficient in the water.

Arthropods

		Figure 15B and D) are often fused into regions such as the head, thorax, and abdomen. The head and thorax may be fused to form a cephalothorax (class Crustacea, Figure 15A). Since insects comprise one of the largest animal groups - both in number of species (about 1 million) and in number of individuals, the grasshopper will be used as the anatomical example (Figure 16e), also see a crayfish anatomy in Figure 16b. Figure 16a captures an image of
Figure 15 Arthropods [view large image]	Figure 16a Flight of the Bumblebee [view large image]	the flight of bumblebee.

• Circulatory - They have an open circulatory system. Colorless blood is pumped dorsally from the heart and then enters sinuses where it comes in direct contact with the tissues. These sinuses are referred to as a hemocoel.
• Digestive - The mouth contains grinding mouthparts. Food in the mouth is mixed with saliva, which contains enzymes that begin the process of digestion. The crop functions in storage. Upon leaving the crop, food enters the gizzard where it is ground into smaller particles. Most chemical digestion occurs in the stomach. Absorption of nutrients occurs in the stomach and pouches that are attached to the stomach called gastric caeca. Absorbed nutrients move into the hemocoel. The intestine functions mostly to absorb water.
• Endocrine - Metamorphosis is controlled by hormones.

Excretory - Terrestrial forms have an excretory system that conserves water. The malpighian tubules collect wastes from the blood and reabsorb water so that only dry material is excreted. Marine Crustaceans such as grayfish have excretory system consists of a pair of green glands lying in the head region anterior to the esophagus. Each organ possesses a glandular region for waster removal - a bladder and a duct that opens ventrally at the base of the antennae (Figure 16b).

Figure 16b Crayfish Anatomy [view large image]

• Immune - Insects and other arthropods have several kinds of blood cells (hemocytes) two of which (the Granulocyte and/or Plasmatocyte) provide cellular immunity (e.g., phagocytosis and encapsulation) - the first line of defense. These immunocytes also synthesize and secrete into the blood (hemolymph) several humoral, antiviral, and other anti-bacterial factors; the hemolymph also contains neuropeptides and opioids, all of which often provide the second line of defense or assist in cellular immunity.
• Musculo-skeletal - The external skeleton (exoskeleton) is composed of chitin, which is a strong, flexible polysaccharide. It serves to protect the animal as well as provide an attachment site for muscles, and prevention of desiccation on land. Because chitin is hard and nonexpandable; it requires periodic molting (shedding) for growth. The new skeleton must expand and harden when the old one is shed. The appendages are also covered by the skeletal material, but they are jointed. The Jointed appendages act like a system of levers and are ideal for any activity that requires movement. Arthropods have evolved a variety of different kinds of specialized appendages for activities such as walking, swimming, reproduction, or feeding.

Nervous and Sensory - Well-developed sense organs are needed for active animals. Arthropods have both compound and simple eyes. Insects and crustaceans have compound eyes (see Figure 16c, magnified 370X), each with many separate visual units called ommatidia. Each ommatidium contains a lens and a light-sensitive cell. Eyes of this design allow for the detection of small movements, but are weak in distinguishing detail. The tympanum is used for the reception of sound waves. The nervous system includes a dorsal brain and a double, ventral, solid nerve cord. Such well-developed nervous system is needed to integrate the sensory information and

Figure 16c Compound Eyes of Mosquito [view large image]

allow the animal to be active.

Reproductive - Males deposit sperm directly into females where fertilization occurs. This type of breeding, called internal fertilization, allows animals to breed on dry land. Another advantage is that fewer gametes are needed than if both sexes deposit their gametes in water. The change in body form as an insect grows to adult is called metamorphosis (Figure 16d). In some species such as the grasshopper, the larvae look like the adult; the animal simply gets larger as it matures. This type of change is incomplete metamorphosis. Species that undergo a complete reorganization of the body tissues exhibit complete metamorphosis, in which case they have 3 stages of development: larval stage, pupal stage, and adult stage, for example, the three stages of caterpillar, cocoon, and

Figure 16d Metamorphosis

butterfly as shown in Figure 16d. Worm-like larvae form a cocoon called a pupa and then become transformed into the adult form. Metamorphosis occurs during the pupal stage, when the animal is enclosed within a hard covering. Complete metamorphosis allows the animal to take advantage

of two different ecological settings (niches). For example, caterpillars are specialized for feeding on plant leaves. The butterfly of many species is specialized for finding mates and reproduction. Respiratory - Insects have a network of tubules called tracheae that bring oxygen directly to the tissues and allow carbon dioxide to escape. The openings to the outside, called spiracles, are located on the side of the abdomen. Air sacs attached to the tubules pump air through the system.

Figure 16e Grasshop -per [view large image]

Echinoderms

radially symmetrical, in contrast to vertebrates; they have no internal skeleton, no trace of any of the three major chordate characters of notochord, nerve cord, or gill slits, and they have many peculiar and complicated organs of their own. But the embryology sheds an unexpected gleam of light. The early embryo of the echinoderm is a tiny creature, which floats freely in the sea water. Unlike the adult, the larva is bilaterally symmetrical, suggesting that the radial symmetry of the starfish is a secondary affair, assumed when the ancestors of these forms look up a sedentary existence. Then, too, the type of development of certain of the body cavities is identical with that found in the embryos of some primitive vertebrates. It is believed that the bilateral larva developed types which retained the original symmetry,

Figure 17 Echinoderms [view large image]

and gradually evolved into the chordates and, finally, the true vertebrates.

• Circulatory - Coelomic fluid, circulated by ciliary action, performs many of the normal functions of a circulatory system.
• Digestive - Starfishs feed on mollusks. When a starfish attacks a clam, it arches its body over the shell, and by the concerted action of the tube feet, forces the clam to open. Then it everts a portion of its stomach to digest the contents of the clam. The mouth of a starfish opens into a narrow esophagus, which in turn leads to an expanded stomach. The stomach has two portions: the saclike cardiac, which can be everted as described, and the narrower pyloric, which is connected to a short intestine. The anus opens on the aboral or upper side of the animal.
• Exocrine - Each of the five arms contains a well-developed coelom, a pair of large digestive glands that secrete powerful enzymes into the pyloric portion of the stomach, and gonads.
• Excretory - Starfish has no excretory organs.
• Immune - Sea urchins are long lived, normally healthy animals that display remarkable abilities to heal wounds and combat major infections. From an external point of view, their immune systems obviously work very well. Thus, their cellular defense systems are extremely sensitive, and they respond rapidly to minor perturbations, all without any specific adaptive capabilities. These systems probably function through the transduction of signals conveying information on injury and infection, just as do the equivalent systems that underlie and back up the human immune systems, and that provide the initial series of defenses against pathogenic invasions.
• Musculo-skeletal - The water vascular system is purely for locomotion. Water enters this system through a structure on the aboral side called the sieve plate, or madreporite. From there, it passes through a short canal called the stone canal, to a ring canal, which surrounds the mouth. From the ring canal, five radial canals extend into the arms. From the radial canals, many lateral canals extend into the tube feet. One lateral canal goes to each tube foot, where it ends in the ampulla. When the ampulla contracts, the water is forced into the tube foot, expanding it and giving it suction. By alternating the expansion and the contraction of the tube feet, the starfish moves along slowly. Typically, echinoderms have an endoskeleton (internal skeleton) consisting of hard calcite ossicles embedded in the body wall and often bearing protruding spines or tubercles.
• Nervous and Sensory - The nervous system consists of a central nerve ring that supplies radial nerves to each arm. A light-sensitive eyespot is at the tip of each arm.
• Reproductive - The starfish usually becomes sexually mature at about 12-14 months. Mating mainly takes place between May and June (water temp. about 8 °C) when the whole population are at the same depth and like an epidemic. The lifecycle of most starfishes starts by shedding their eggs and sperm freely into the water, so fertilization is externally. The very small chance of fertilization is compensated by the enormous amounts of eggs and sperm cells. A female starfish sheds in two hours several millions of eggs into the water, with a mean diameter of 0.16-0.19 mm . After fertilization, a hollow ball develops, called the blastula. The cells of the blastula possess cilia on the outside for swimming. After one day a deep groove develops, leading to the gastrula. The gastrula's of all types of echinoderms are very similar. But then differentiation starts. The common starfish develops a so-called bipinnaria larva, with ciliated bands running about the periphery. After several weeks the bipinnaria larva takes on a more elaborate form, with longer

  projecting arms and after some more weeks, a brachiolaria larva is formed. The larvae have their own gut, with inside cilia to inhale and transport food particles. They feed themselves with diatoms and other organisms in the plankton. The stomach is large and round and situated at the backside. After this phase a large part of the larva degenerates and at the rear side a rudimentary formed juvenile starfish develops. The organs of the young starfish are formed anew. Respiratory - There are skin gills, which project from the coelomic cavity, serve the

  Figure 18 Starfish Anatomy [view large image]

  function of respiratory exchange.

Chordates

Among the chordates are those animals with which we are most familiar, including human beings. Figure 19a shows all the animal classes with indicators about added features. All members of this phylum are observed to have the following three basic characteristics at some time in their life history.


1. A dorsal supporting rod called a notochord, which is replaced by the vertebral column in the adult vertebrates.
2. A dorsal hollow nerve cord, in contrast to invertebrates, which have a ventral solid nerve cord. By hollow, it is meant that the cord contains a canal that is filled with fluid.
3. Pharangeal pouches or gill clefts (slits), which are seen only during embryological development in most vertebrate groups, although they persist in adult fishes. Water passing into the mouth and the pharynx goes through the gill slits, which are supported by gill bars and used for gas exchange.

		the only chordate feature retained by the adult. The larva of the tunicate, however, has a tadpole shape and possesses the three chordate characteristics. It has been suggested that such a larva may have become sexually mature without developing the other adult tunicate characteristics, and may have evolved into a fishlike vertebrate similar to the lancelet, which is a chordate that shows the three chordate characteristics as an adult.
Figure 19a Vertebrates [view large image]	Figure 19b Protochordates [view large image]

Vertebrates have all the most advanced characteristics listed in Table 01.


• Circulatory - All vertebrates have a closed circulatory system in which red blood is contained entirely within blood vessels. They have ventral heart with 2-4 chambers.
• Digestive - The digestive system is very complex in vertebrates. It consists of the gastrointestinal tract (gut), an extensive tube extending from the mouth to the anus, through which the swallowing, digestion, and assimilation of food and the elimination of waste products are accomplished. The system includes large digestive glands, liver, and pancreas.
• Endocrine - Endocrine is related to the presence of ductless glands of the type typically found in vertebrates. The endocrine system involves hormones, the glands, which secrete them, the molecular hormone receptors of target cells, and interactions between hormones and the nervous system. The endocrine and nervous systems are similar in that both are chemical messenger systems (although the nervous system is more like electrochemical in nature) that send messages from a source to a target. Physiology and metabolism in multicellular animal are controlled by messengers from these two control systems, which complement each other and tend to be used for different purposes under different circumstances. Both systems use chemical signals to bring about a desired response in some target, perhaps a muscle (in the case of the nervous system) or the liver (in the case of the endocrine system). Further, the two systems interact closely and control each other so that endocrine organs may be the target of the CNS and vice versa.
• Excretory - The kidneys are important excretory and water-regulating organs that conserve or rid the body of water as appropriate.
• Immune - It has only recently become apparent that all vertebrates with jaws share an astonishing adaptation: an extraordinarily complex immune system. Living jawless vertebrates do not have it. So a little before, or a little after, the evolution of jaws, our ancestors acquired a new ability to detect and react to foreign substances, typically of biological origin, by developing antibodies, T cells, and so on. This huge evolutionary jump corresponds to the gap between living agnathans (jawless fishes) and jawed vertebrates, which are separated by about 100 million years in the fossil record.

Musculo-skeletal - Vertebrates are segmented chordates in which the notochord is replaced in the adult by a vertebral column composed of individual vertebrae. The skeleton is internal, and in all the vertebrates, there is not only a backbone but also a skull, or cranium, to enclose and to protect the brain. In higher vertebrates, other parts of the skeleton serve as attachment for muscles and for protection of internal organs of the thoracic cavity and the abdomen. Movements are provided by muscles attached to the endoskeleton. All but the fishes are tetrapods, meaning that they have four limbs. General body plan consists of head, trunk, two pairs of appendages, and postanal tail (but these structures are highly modified in many vertebrates and

Figure 19c Limbs [view large image]

sometimes absent). There is a fundamental design in the skeleton of all vertebrates. Humans, bats, lizards, and whales are all just variations on a theme (see for example the common pattern of limbs for some vertebrates in Figure 19c).

Bone is the major component of the skeletons in adult vertebrates. It is composed of both living tissues, such as bone cells, fat cells, and blood vessels, and nonliving materials (such as collagen) secreted by the bone cells called osteoblasts into the inter-cellular space. The collagen fibers are coated with a calcium phosphate salt, making it strong without being brittle. As shown in Figure 19d, a bone can be divided into four parts:

Figure 19d Bone [view large image]

1. Periosteum - This fibrous membrane is the outer layer of the bone. It is rich with blood vessels and nerve endings and it ends at the edge of the joint area or where the ligaments and the tendons insert themselves.

2. Compact bone - This part makes up most of the bone of arms and legs. The structural units of compact bone are osteons, elongated cylinders that act as weight-bearing pillars, able to withstand any mechanical stress placed on the bone. The center of each osteon contains a hollow canal that acts as a central passageway (Haversian canal) for blood vessels and nerves.
3. Spongy bone - In some bones, internal to the compact bone is spongy bone composed of a honeycomb network of bones called trabeculae that act as supporting beams. Spongy bone is designed to bear stress from several directions, such as that exerted on the pelvis in bending or stretching. The spaces between the trabeculae are filled with red bone marrow containing the blood vessels that nourish spongy bone. In additon to produce cells, which make up the bone, the cells of red bone marrow also produce the majority of the cellular elements of the blood and of the lymph. Spongy bone is found in bones of the pelvis, ribs, breastbone, vertebrae, skull, and at the ends of the arm and leg bones.
4. Bone marrow - It is the soft tissue found in the hollow interior of bones. In adults, marrow in large bones produces new blood cells. There are two types of bone marrow: red marrow (consisting mainly of myeloid tissue) and yellow marrow (consisting mainly of fat cells). Red blood cells, platelets and most white blood cells arise in red marrow; some white blood cells develop in yellow marrow. Both types of bone marrow contain numerous blood vessels and capillaries. At birth, all bone marrow is red. With age, more and more of it is converted to the yellow type. About half of the bone marrow is red. Red marrow is found mainly in the flat bones, such as the hip bone, breast bone, skull, ribs, vertebrae and shoulder blades, and in the spongy material at the proximal ends of the long bones femur and humerus. Yellow marrow is found in the hollow interior of the middle portion of long bones. In cases of severe blood loss, the body can convert yellow marrow back to red marrow in order to increase blood cell production.

An important contribution to the shape of animals with backbones is the number of vertebrae (bones in spinal column) that make up the structure. While human has only 33, snakes have more than 300, with some species having more than 500. Vertebrae develop from segments of tissue called somites, which form, one after another, in a head-to-tail sequence in the embryo (diagram a, Figure 19e). They bud off from the "head" end of the presomitic mesoderm (PSM), an immature tissue fated to generate the somites. This budding is regulated by a "clock-and-wavefront"

Figure 19e Somitogenesis [view large image]

model. In snakes, the clock genes seems to express 4 times faster than in shorter-bodies animal (such as mice), leading to many more, though smaller, somites (see diagram b in Figure 19e).

• Nervous and Sensory - Vertebrates show good cephalization with sense organs; the eyes develop as outgrowths of the brain, and the ears serve as equilibrium devices in aquatic vertebrates plus sound wave receivers in land vertebrates.
• Reproductive - Multicellular organisms do not spring forth fully formed. Rather, they arise by a relatively slow process of progressive change that is called development. Vertebrates develop a little bit differently with a notochord appears in the "organogeneses" stage (see Figure 19f). This cord is filled with a jelly-like substance and provides support for the body. The notochord in vertebrates breaks up in later stage and ultimately becomes part of the disks that lie between the vertebrae. Animal development starts with the fusion of genetic material from the two gametes - the sperm and the egg. This fusion, called fertilization, stimulates the egg to begin development. The subsequent sequence of stages is collectively called embryogeneses. Throughout the animal kingdom an incredible variety of embryonic types exists, but most patterns of embryogenesis comprise variations on four themes as shown by the development of a representative organism, the frog, in Figure 19f:

1. Cleavage - a stage of extremely rapid mitotic divisions wherein the zygote cytoplasm is divided into numerous smaller cells. By the end of cleavage these cells generally form a fluid-filled sphere known as blastula. 2. Gastrulation - The cells in the blastula undergo dramatic movements wherein they change their positions relative to one another. As a result, the typical embryo contains three germ layers - the etoderm in the outside, the mesoderm in the middle and the endoderm is the innermost layer. The invagination of cells is something like poking a curved finger into a balloon. 3. Organogenesis - The cells interact with one another and rearrange themselves to produce the bodily organs. In vertebrates, the mid-dorsal ectodermal cells fold to form the neural tube with the notochord laying under. Also during this stage certain

Figure 19f Embryonic De- velopment [view large image]

cells such as the blood, lymph, pigment, and the gamete migrate from their place of origin to the final location. 4. Gametogenesis - As shown in Figure 19f, a portion of egg cytoplasm gives rise

to cells that are the precursors of the gametes (in red colour). These cells are called germ cells, and they are set aside for their reproductive function. All the other cells of the body are called somatic cells. The germ cells eventually migrate to the gonads at maturity. Then the gametes are released and undergo fertilization to begin a new life. The adult organism with all its somatic cells eventually undergoes senescence and dies. Thus, it is often said that sex is deadly, and despite all the human aspiration to higher level of existence, we are basically the carrier of germ cells.
• Respiratory - Vertebrates have muscular, perforated pharynx, which functions as a filter-feeding apparatus in protochordates. Pharyngeal pouches or slits are not unique to chordates; hemichordates also have pharyngeal slits. Fishes added a capillary network with gas-permeable walls; this network evolved into gills. It is much reduced in adult land-dwelling forms (although it is extremely important in embryonic development of all vertebrates). Land animals use

		lungs for gas exchange instead of gills. It is believed that the gill arches have been modified to become the jaw as shown in Figure 20a. Further modification of these gill arches can be traced from an embryo to an adult (in humans as shown in
Figure 20a Evolution of Jaws [view large image]	Figure 20b Evolution of Gill Arches [view large image]	Figure 20b). The origins of jaws, ears, larynx, throat, bones, muscles, nerves, and arteries can all be found in these gill arches.

Fishes

There are three classes of fishes (Figure 21a):


1. Jawless fishes - They are cylindrical, up to a meter long, with smooth, scaleless skin and no jaws or paired fins. There are two families of jawless fishes, e.g., the hagfishes are scavengers, feeding mainly on dead fishes, while some lampreys are parasitic.
2. Cartilaginous fishes - They are the sharks, the rays, and the skates, which have skeletons of cartilage instead of bone.
3. Bony fishes - They are by far the most numerous and varied of the fishes. Most of these fishes, such as the grouper in Figure 21a, are a type of bony fish called ray-finned fishes. They have a swim bladder that aids them in changing their depth in the water. The swim bladder seems to have evolved from a primitive lung, some fishes such as bowfins, gars and bichirs, still use it for breathing. Perhaps it forked in evolution and went two ways - one carried its old breathing function out onto the land, and the other had been modified to form the swim bladder. Species of fish that do not possess a swim bladder sink to the bottom if they stop swimming. "Ray-finned" refers to the fact that the fins are thin and are supported by bony rays. Another type of bony fish, called the lob-finned fishes, evolved into the amphibians. These fishes not only have fleshy appendages that could be adapted to land locomotion, they also have a lung that is

		used for respiration. The coelacanth and shark in Figure 21a are the "living fossil" among the fishes. They are species that have defied the evolutionary odds to survive virtually unchanged for tens or hundreds of millions of years. Other examples with such distinction are shown in Figure 21b.
Figure 21a Fishes [view large image]	Figure 21b Living Fossils [view large image]	Figure 21a also includes a fish squirting a shot at the cricket. Figure 22 shows the internal anatomy of a common fish.

• Circulatory - The heart of a fish is a simple pump, and the blood flows through the chambers, including a nondivided atrium and ventricle, to the gills only. Oxygenated blood leaves the gills and goes to the body proper.
• Digestive - The mouths shape is a good clue to what fish eat. The larger it is the bigger the prey it can consume. Fish have a sense of taste and may sample items before swallowing if they are not obvious prey items. The stomach and intestines break down (digest) food and absorb nutrients. Fish such as bass that are piscivorous (eat other fish) have fairly short intestines because such food is easy to chemically break down and digest. Fish such as tilapia that are herbivorous (eat plants) require longer intestines because plant matter is usually tough and fibrous and more difficult to break down into usable components. The function of the pyloric caeca is not entirely understood, but it is known to secrete enzymes that aid in digestion, may function to absorb digested food, or do both. The liver has a number of functions. It assists in digestion by secreting enzymes that break down fats, and also serves as a storage area for fats and carbohydrates. The liver is also important in the destruction of old blood cells and in maintaining proper blood chemistry, as well as playing a role in nitrogen (waste) excretion.
• Endocrine - All vertebrate animals (fish, amphibians, reptiles, birds and mammals, including humans) have the same endocrine glands and release similar hormones to control development, growth, reproduction and other responses. However, the pineal gland of fish and amphibians is located near the skin and functions to detect light. This is often referred to as the third eye.
• Excretory - The kidney filters liquid waste materials from the blood; these wastes are then passed out of the body. The kidney is also extremely important in regulating water and salt concentrations within the fish’s body, allowing certain fish species to exist in freshwater or saltwater, and in some cases both. The vent is the external opening to digestive urinary and reproductive tracts. In most fish it is immediately in front of the anal fin. Ammonia is formed immediately after the amino group is removed from protein. This process requires very little energy. Ammonia is highly soluble in water but very toxic. Aquatic animals such as bony fishes, aquatic invertebrates, and amphibians excrete ammonia because it is easily eliminated in the water.
• Immune - The fish immune system comprises of the non-specific and specific immune defences, having both humoral and cellular mechanisms to resist against infectious diseases. Studies in various species of fish have shown that the spleen and head kidney are major locations of immunological activity. The relative importance of these two organs varies among different species. Previous studies have demonstrated that the head kidney is a major source of lymphocytes (including B cells) in Bluegill. More research are being run on other species to determine the extent of the variation. Germs and bugs constantly probe and try to breach the fish’s immune system to gain a strong foothold. In general, for them to be successful there has to be an underlying predisposing factor such as poor environmental conditions, poor nutrition, overcrowding or poor water quality. In addition to causing stress, which will depress the fish’s immune system, such conditions will often encourage increased numbers of opportunistic pathogens.
• Musculo-skeletal - Fishes is covered by scales, which protect the body but do not prevent water loss. The spine is the primary structural framework upon which the fish’s body is built. It connects to the skull at the front of the fish and to the tail at the rear. The spine is made up of numerous vertebrae, which are hollow and house and protect the delicate spinal cord.
• Nervous and Sensory - Fish see through their eyes and can detect color. Paired nostrils, or nares, in fish are used to detect odors in water and can be quite sensitive. The lateral line is a sensory organ consisting of fluid filled sacs with hair-like sensory apparatus that are open to the water through a series of pores (creating a line along the side of the fish). The lateral line primarily senses water currents and pressure, and movement in the water.

Reproductive - General speaking, reproduction in the fishes requires external water; sperm and eggs usually are shed into the water, where fertilization occurs. The zygote develops into a swimming larva that can fend for itself until it develops into the adult form. Respiratory - Fishes breathe by means of gills, respiratory organs that are kept continuously moist by the passage of water through the mouth and out the gill slits.

Figure 22 Fish Anatomy [view large image]

As the water passes over the gills, oxygen is absorbed by blood and carbon dioxide is given off.

Amphibians

Circulatory - With the development of lungs, there is a change in the circulatory system. The amphibian heart has a divided atrium but a single ventricle. The right atrium receives impure blood with little oxygen from the body proper, and the left atrium receives purified blood from the lungs that has just been oxygenated, but these two types of blood are mixed partially in the single ventricle. Mixed blood is then sent, in part, to the skin, where further oxygenation can occur. The frog heart is the only organ contained within the coelom, which has its own protective covering. This is the pericardium.

Figure 23 Amphibians [view large image]

• Digestive - The frog's mouth is where digestion begins. It is equipped with feeble, practically useless teeth. These are present only in the upper jaw. The frog's tongue is highly specialized. Normally, the tip of its tongue is folded backward toward the throat. From this position the frog can flick it out rapidly to grasp any passing prey. To better hold this prey, the tongue is sticky. Food passes from the frog's mouth into the stomach by way of the esophagus. From the stomach, the food moves into the small intestine, where most of the digestion occurs. Large digestive glands, the liver and the pancreas, are attached to the digestive system by ducts. A gall bladder is also present.
• Endocrine - Similar for all vertebrates.
• Excretory - Liquid wastes from the kidneys travel by way of the ureters to the urinary bladder. Solid wastes from the large intestine pass into the cloaca. Both liquid and solid waste material leave the body by way of the cloaca and the cloacal vent. Terrestrial amphibians and mammals excrete nitrogenous wastes in the form of urea because it is less toxic than ammonia and can be moderately concentrated to conserve water. Urea is produced in the liver by a process that requires more energy to produce than ammonia does.
• Immune - Studies of the ontogeny of immunity in a limited number of representative amphibians have shown that while the immune systems of the larval forms are competent to defend against potential pathogens in the temporary ponds they inhabit, they are not equivalent to the mature immune systems that develop after metamorphosis. Metamorphosis is a critical time of transition when increased concentrations of metamorphic hormones orchestrate the loss or reorganization of many tissues and organ systems, including the immune system. Immune system reorganization may serve to eliminate unnecessary lymphocytes that could be destructive if they recognized newly emerging adult-specific antigens on the adult tissues. Frog deformities were reported intermittenly over the years. A working hypothesis suggests that common herbicide called atrazine somehow suppresses the amphibian immune system and leaves the animals more susceptible to parasite infection. The variation in occurrence is related to the year-to-year changes of water level that take place in the natural environment.
• Musculo-skeletal - These animals have distinct walking legs, each with five or fewer toes. This represents an adaptation of land locomotion. The skull is flat, except for an expanded area that encases the small brain. Only nine vertebrae make up the frog's backbone, or vertebral column. The human backbone has 24 vertebrae. The frog has no ribs. The frog does not have a tail. Only a spikelike bone, the urostyle, remains as evidence that primitive frogs probably had tails. The urostyle, or "tail pillar," is a downward extension of the vertebral column. The shoulders and front legs of the frog are somewhat similar to man's shoulders and arms. The frog has one "forearm" bone, the radio-ulna. Man has two forearm bones, the radius and the ulna. Both frog and man have one "upper arm" bone, the humerus. The hind legs of the frog are highly specialized for leaping. The single "shinbone" is the tibiofibula. Man has two lower leg bones, the tibia and the fibula. In man and in the frog, the femur is the single upper leg (thigh) bone. A third division of the frog's leg consists of two elongated anklebones, or tarsals. These are the astragalus and the calcaneus. The astragalus corresponds to the human talus (anklebone). The calcaneus in the human skeleton is the heel bone. See also transformation of limbs for land dwelling.
• Nervous and Sensory - The important parts of the frog brain correspond to comparable parts in the human brain. The medulla regulates automatic functions such as digestion and respiration. Body posture and muscular co-ordination are controlled by the cerebellum. The cerebrum is very small in the frog. By comparison the human cerebrum is very large. In man the cerebrum is involved in many important life processes. Only 10 cranial nerves originate in the frog's brain.

Man has 12. Similarly, the frog has only 10 pairs of spinal nerves. Man has 30 pairs. Two simple holes make up the nostrils for the frog. There are complex valves but no long nasal passages as there are in man. The frog's sense of smell is registered by olfactory lobes. These make up the forward portion of the brain. The eye is crude. Its fixed lens cannot change its focus. Poorly developed eyelids do not move. To close its eye, the frog draws the organ into its socket. A third eyelid, or nictitating membrane, may be drawn over the pulled-in eyeball. There is no external ear. Both eardrums, or tympanic membranes, are exposed. There is only one bone in the frog's middle ear. The human middle ear contains three bones (malleus, incus, and stapes in the ossicles). As in man, semicircular canals help to maintain body balance.

Figure 24a Frog Anatomy [view large image]

Fishes have evolved a unique diversity of mechanisms for acoustical communication. This diversity is found both in

sound-generating mechanisms and organs for acoustic perception. Fishes are able to produce different types of sounds and to perceive acoustic signals of different frequencies, temporal patterns and intensities. They possess inner ear and a few have rudimentary middle ear (the ossicles). Human diving experience indicates that sound wave in water can only be perceived through bone conductivity via vibration of the bones of the skull. The efficiency of this method of sound wave detection is 40% weaker than air conductivity in land animals and it lacks sufficient orientation to identify the direction of the audial source. It is not accidental that the full apparatus of ear developed only after animals making a living on land. Figure 24b traces the development of the stapes from the gill arches in the upper jaw of fish to the stapes in amphibian. The same figure also shows that the malleus and incus come from the lower jawbones of the reptile. The transformation sequence is supported by fossils bearing a continuum of forms as shown. The construction of the mammalian middle

Figure 24b Ear Bones Evolution [view large image]

ear allows detection of higher frequency sounds. The distance between the two ears enables its owner to discern direction of the sound source (because the sound speed in air is lower than that in water). See also evolution of the inner ear.

Reproductive - Nearly all the members of this class lead an amphibious life, i.e., the larval stage lives in the water and the adult stage lives on the land. The adults must return to the water, however, for reproduction. Just as with the fishes, the sperm and the eggs are discharged into the water and fertilization results in a zygote that develops into the tadpole. The tadpole undergoes metamorphosis into the adult before taking up life on the land. Respiratory - Respiration is accomplished by the use of small, relatively inefficient lungs, supplemented by gaseous exchange through the skin. Therefore, the skin is smooth, moist, and glandular. This is call cutaneous gas exchange, which occurs among many vertebrates in various degree. It assumes a major share (~ 90%) in amphibian respiration as shown in Figure 24c. This is a distinct disadvantage in a dry environment; therefore, frogs spend most of their time in or near freshwater. All amphibians possess two nostrils that are connected directly with the mouth cavity. Air enters the mouth by way of the nostrils, and when the floor of the mouth is raised, air is forced into the lungs. Lungless frogs have been found on the Borneo island in 2007. They have skin flaps coming off their arms and legs. The species is the first

Figure 24c Cutaneous Gas Exchange [view large image]

frog known to science without lungs and joins a short list of amphibians with this unusual trait, including a few species of salamanders and a wormlike creature known as a caecilian.

Reptiles

Circulatory - The atrium of the heart is always separated into right and left chambers, but division of the ventricle varies. There is always at least one interventricular septum, but it is incomplete in all but the crocodiles; therefore, exchange of oxygenated and deoxygenated blood between the ventricles occurs in all but the crocodile. Reptiles do not regulate their body temperature. Animals that cannot maintain a constant temperature, e.g., fishes, amphibians, and reptiles, are called cold blooded. They take on the temperature of the external environment. Thus, reptiles try to regulate body temperatures

Figure 24c Reptiles [view large image]

by exposing themselves to the sun if they need warmth or by hiding in the shadows if they want to cool off.

• Digestive - Except for most snakes, reptiles have a cecum. The stomach of crocodilians has two compartments. The first is very muscular and frequently contains stones. The second is similar to the glandular stomach of mammals. All reptiles have a gall bladder. The liver of many reptiles contains melanin and can have black spots or streaks. Reptiles generally have little subcutaneous fat and store fat in discrete masses (fat bodies) in the caudal abdomen.
• Endocrine - Similar for all vertebrates.
• Excretory - The metanephric kidneys of reptiles are lobulated. One or more renal arteries can be present to receive blood from the renal portal system. The nitrogenous wastes of reptiles are in the form of ammonia, urea, uric acid or a combination of these. Crocodilians, snakes and some lizards do not have a urinary bladder. In chelonians and those

lizards with a bladder, it is connected to the cloaca by a short urethra. Urine passes into the cloaca and then into the urinary bladder, if present, or into the distal colon where water resorption occurs. The cloaca typically consists of 3 chambers. The coprodeum is the most cranial and receives fecal material and urinary wastes. The urodeum is the middle section and receives genital secretions and urinary wastes

Figure 24d Lizard Anatomy [view large image]

from the urogenital ducts. The caudal proctodeum acts as a reservoir for fecal and urinary wastes before they are excreted. This also is the location of the openings of the musk glands.

• Immune - The reptile immune response is performed by a well-developed immune system whose leukocytes have been characterized as lymphocytes, monocytes and granulocytes. A main characteristic of the reptile immune system is the effect of the seasonal cycle on its histology and function. A reptile's immune system is more efficient when the animals is warmer, however, since bacteria probably grow more slowly in lower temperatures, reptiles sometimes lower their body temperatures when they have an infection.
• Musculo-skeletal - They have thick strateum corneum (top dead layer) with compacted layers of flat horny dead cells, filled with keratin. It is replaced by swelling/pressure from diffusion of lymph between new and old layers. The surface scales can be in varied form. In some lizards the scales are small and grainlike, while in others they are large and plate- like. The scales may be smooth or they may have little ridges called keels or they may even have developed into spines.
• Nervous and Sensory - Most lizards see and hear well. They have external ear openings and their eyes have movable eyelids (unlike snakes). Some lizards have a "third eye," a tiny, light-sensitive, transparent structure on top of the head that helps them regulate how long they stay in the sun.
• Reproductive - Reptiles have a mean of reproduction suitable to a land existence. There is usually no need for external water to accomplish fertilization because the penis of the male passes sperm directly to the female. After internal

  fertilization has occurred, the egg is covered by a protective, leathery shell and is laid in an appropriate location. The shelled egg made development on land possible and eliminated the need for a swimming-larva stage during development. It provides the developing embryo with oxygen, food, and water; it removes nitrogenous wastes; and it protects the embryo from drying out and from mechanical injury. This is accomplished by the presence of extraembryonic membranes (Figure 24e).

  Figure 24e Membranes [view large image]

  Respiratory - Reptiles have well-developed lungs enclosed in a protective rib cage. The lungs expend together with the rib cage to let in air.

Birds

Birds are characterized by the presence of feathers, which are actually modified reptilian scales. There are many orders of birds, including birds that are flightless (ostrich), web footed (penguin), divers (loons), fish eaters (pelicans), waders (flamingos), broad billed (ducks), birds of prey (hawks), vegetarians (fowl), shorebirds (sandpipers), nocturnal (owl), small (hummingbirds), and songbirds, the most familiar of the birds. Some of them are showed in Figure 25a.

Figure 25a Birds [view large image]

Nearly every anatomical feature of a bird can be related to its ability to fly. Figure 26a shows the anatomy of a common bird.

	Circulatory - Birds have a four-chambered heart that completely separates oxygenated blood from deoxygenated blood. Birds are warm blooded; like mammals, they are able to maintain a constant internal (core) temperature of 40 oC. Placental mammals maintain a temperature of 37 oC, for most marsupials it is 35 oC. Figure 25b
Figure 25b Warm Blooded- ness [view large image]	shows the temperature difference between the warm-blooded human and cold blooded lizard, it also shows the temperature differences from different parts of the body. The 37 oC is measured in the mouth, it is 37.4 oC in the rectum, and 36.7 oC in the armpit.

• Digestive - Birds digest food quickly, they can't afford the extra weight. They have no teeth, the breakdown of food occurs in the gizzard - sometimes birds swallow rocks to assist the process. The crop stores food; mother birds regurgitate food stored in the crop to their babies. Waste exits through the cloaca - and so do eggs.
• Endocrine - Similar for all vertebrates.
• Excretory - Insects, reptiles, birds, and some dogs (Dalmatians) excrete uric acid. Reptiles and birds eliminate uric acid with their feces. The white material seen in bird droppings is uric acid. It is not very toxic and is not very soluble in water. Excretion of wastes in the form of uric acid conserves water because it can be produced in a concentrated form due to its low toxicity. Because it is relatively insoluble and nontoxic, it can accumulate in eggs without damaging the embryos. The synthesis of uric acid requires more energy than urea synthesis. There is no urinary bladder in birds.
• Immune - The bird's immune system mainly consists of lymphatic vessels and lymphoid tissue. Primary tissues are the thymus, located in the neck along the jugular vein, and the bursa of Fabricius, located adjacent to the cloaca. Secondary lymphatic organs and tissues would be the spleen, bone marrow, mural lymph nodules and lymph nodes. There is also a lymphatic circulatory system of vessels and capillaries that transport lymph fluid through the bird's body and communicate with the blood supply. There are three major types of response in a bird's immune system; the specific, consisting of the humoral and cell mediated responses, and the nonspecific. The humoral and cell mediated responses need a processed antigen to stimulate response, and their response is to create a specific antibody for each particular antigen. The nonspecific immune system responds to all antigens. The B (bursal produced) lymphocytes are associated with humoral response and the T (thymus produced) lymphocytes are associated with the cell-mediated response. Macrophages, heterophils and thrombocytes are the main cells associated with the nonspecific immune system.
• Musculo-skeletal - The anterior pair of appendages (wings) is adapted for flight; the posterior is variously modified, depending on the type of bird. Some are adapted to swimming, some to running, and some to perching on limbs. The breastbone is enormous and has a ridge to which the flight muscles are attached. See also the evolution of flight.
• Nervous and Sensory - Birds have well-developed brains, but the enlarged portion seems to be the area responsible for instinctive behavior. Therefore, birds, follow very definite patterns of migration and nesting.
• Reproductive - Birds often engage in elaborate courtship behavior for mating purposes. These include: building nests, dancing and posturing, bringing gifts, bright colorful displays, and singing. Some birds mate for life, and often both parents raise young. Two types of reproduction occur in birds: some birds incubate eggs for a long time - chicks are born with

  			feathers and can walk/swim (ducks); while others incubate eggs a short time - chicks are born featherless and helpless (robins). The process of making egg is illustrated in Figure 26b; while the structure of a chicken egg is
  Figure 26a Bird Anatomy [view large image]	Figure 26b Making Egg	Figure 26c Chicken Egg [view large image]	displayed in Figure 26c together with a description of its various components in Table 02.

  
  

  Component	Composition	Function(s)
  Yolk	Protein and fat	Provision of nourishment
  Albumen	Protein and water	Provide additional nutrition and protection
  Amniotic fluid	Water and other materials	Protection and aiding the growth of the fetus
  Allantois	Nitrogenous waste	Collection of waste
  Embryonic disk	Cells of the embryo	The developing embryo
  Chalaza	Part of egg white, 2 spiral bands of tissue	Stabilizing the yolk
  Chorion	Two layers of cells	Membranes for gas exchange
  Air sac	Air	Provide oxygen for baby chick just before hatching
  Shell	Protein matrix lined with calcium carbonate	Protective outer cover

  Table 02 Chicken Egg

• Respiratory - Respiration is efficient since the lobular lungs are connected to air-sacs (see Figure 26a), which fill with air during inhalation. The air then is released from the air sacs when the bird exhales - this means that the bird receives oxygen during inhalation and exhalation. Another benefit of air sacs is that the air-filled, hollow bones lighten the body and aid flying.

Mammals

The chief characteristics of mammals are hair and mammary glands that produce milk to nourish the young. Human mammary glands are called breasts. Mammals are classified according to their means of reproduction: there are egg-laying mammals called monotremes such as the duck-billed platypus; mammals with pouches for immature embryos are the marsupials such as the kangaroos; while the placental mammals are the majority of living mammals. Figure 27a shows just a few of these animals. Table 03 lists the 18 orders of living placental mammals. They are classified largely according to the mode of locomotion and how they get their food. Figure 28 illustrates a cat's anatomy,

Figure 27a Mammals [view large image]

which is very similar to the human's. Information on "mammal characteristics and comparison with other vertebrates" can be found in the appendix on "Age of Animals".

Table 03 The 18 Orders of Placental Mammals

• Circulatory - Mammal circulatory systems are divided into two circuits: pulmonary and systemic. The pulmonary circuit carries deoxygenated blood from the heart to the respiratory surface in the lungs, where it is reoxygenated, and then back to the heart. The systemic circuit carries oxygenated blood to all the body's cells via arteries, and deoxygenated blood back to the heart via veins. The mammalian double circulatory system is efficient because it uses a separate pump (the two ventricles) to power each circuit.
• Digestive - The digestive tract is a tube, with coils and branches, which begins at the mouth and ends either at a cloaca or anus. It processes food, which moves by peristalsis (waves of involuntary muscle contractions) through the process of digestion, absorption and elimination. The general pattern is to have an oral cavity, pharynx, esophagus, stomach and intestine. Accessory organs are the pancreas, liver and gallbladder, which arise as evaginations from the embryonic digestive tract.
• Endocrine - Similar for all vertebrates.
• Excretory - In mammals, the two major excretory processes are the formation of urine in the kidneys and the formation of feces in the intestines. The waste products are eliminated by urination and defecation respectively. While urine and feces are both waste material of body processes, they are in completely different categories. Urine is a waste product of the urinary system process while feces are waste products of the digestive system. Feces may contain harmful materials such as bacteria, viruses, and parasitic worms. Urine, on the other hand, contains excess water, salt, and protein waste in the form of urea, and seldom carries any pathogens.
• Immune - In order to identify a foreign organism, the mammal immune system exploits the protein binding activity of antibodies. Depending on the invader, highly specialized antibodies are secreted by suitable cells, which are selected out of a large cell pool and induced to expand their population. When the parasite Trypanosomes, which have been responsible for sleeping sickness in humans and nagana among cattle, reaches the blood of the mammal, it escapes the notice of the immune system of its host due to the fact that it constantly disguises itself: since almost all of its surface is covered with a protein called VSG - against which the infected organism is able to produce antibodies - it thwarts the defence system by constantly transforming this protein so that the antibodies do not recognize it.
• Musculo-skeletal - Mammal has an inner skeleton. It has developed muscles and generally have four limbs attached. The cat anatomy is very similar to the one for human beings. The skeleton of a cat contains 250 bones versus 206 for humans, the difference is with the extra bones in its tail. All cats have a flexible spine. The cat's body has great elasticity. Because the vertebrae of the spinal column are held together by muscles rather than by ligaments, as in humans. The cat can elongate or contract its back, curve it forward, or oscillating along the vertebral line. The construction of the shoulder joints permits the cat to turn its foreleg in almost any direction. The hip joint enables a cat to leap easily. Other special joints allow a cat to turn its head to reach most parts of its body. A cat uses its tail for balancing while jumping or falling. All cats have an excellent sense of balance, which enables them to leap and bounce with ease.

Nervous and Sensory - Mammal has a highly developed brain, nerves and sensory organs such as eyes, nose, mouth, ears and touch. The cat anatomy includes a brain structure much similar to humans, contrary to the brain of a dog. The brain of a cat is about 99% similar to a human brain. Both cats and human beings have similar areas in their brain, controlling their behaviour. The cat has a very advanced cerebral hemisphere, typical of intelligent creatures. Most cats have good vision and are able to see well in very dim light; their color vision is weak. Their sense of hearing is excellent and, at least in the small cats, can detect frequencies of up to 40,000 Hz or higher. The sense of smell is not as highly developed as in the dog; its keenness may vary from one species to another. Figure 27b is a schematic diagram comparing the brain of a fish, an

Figure 27b Brains of the Vertebrates

amphibian, a reptile, a bird, and a mammal. See also layers of the reptilian, mammalian, and human brains.

• Reproductive -
  • Monotremes - In the same manner as birds, the female monotreme incubates the eggs, but after hatching, the young are dependent upon the milk that seeps from glands on the abdomen of the female. Therefore, monotremes retained the reptilian mode of reproduction while evolving hair and mammary glands. The young are blind, helpless, and completely dependent on the parent for some months.
  • Marsupials - In marsupials, the young are born in a very immature state and finish their development in the mother's abdominal pouch, called the marsupium. Using clawed forelimbs, the newborn crawls toward the mother's fur-lined pouch. Once there, it attaches itself to a nipple. After 4 or 5 weeks in the pouch, some marsupials such as opossum spends an additional 8 or 9 weeks clinging to the mother's back.
  • Placental Mammals - In these mammals, the extraembryonic membranes (Figure 24e) have been modified for internal development within the uterus of the female. The chorion contributes to the fetal portion of the placenta, while a portion of the uterine wall contributes to the maternal portion. Here nutrients, oxygen, and waste are exchanged between fetal and maternal blood. It also ensures that the mother's immune system does not attack the embryo by separating white blood cells and other immune system components (including blood) at the boundary. These mammals not only have a long embryonic period, they also are dependent on their parents until the nervous system is developed fully and they have learned to take care of themselves. Figure 27c shows the early

  		appearance of the extraembryonic membranes and the formation of the umbilical cord in the human embryo. The embryonic disk had initially pressed to the wall of the uterus at implantation. It had moved around during the development for attaching the umbilical cord in the tummy
  Figure 27c Early Embryo [large image]	Figure 27d Fertilization - Implantation [view large image]	button to the placenta. Figure 27d depicts the progression from fertilization to implan-tation in the earliest stage of development.

  IVF (In Vitro Fertilization) replaces the natural steps leading to implantation, but it should not be interpreted as parthenogenesis (virgin birth).
• Respiratory - The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung. The environment of the lung is very moist, which makes them a hospitable environment for bacteria. Many respiratory illnesses are the result of bacterial or viral infection of the lungs. Breathing is largely driven by the diaphragm below; a muscle that by contracting expands the cavity in which the lung is enclosed. The rib cage itself is also able to expand and

  contract to some degree. As a result, air is sucked into and pushed out of the lungs through the trachea and the bronchial tubes or bronchi; these branch out and end in alveoli which are tiny sacs surrounded by capillaries filled with blood. Here oxygen from the air diffuses into the blood, where it is carried by hemoglobin. The deoxygenated blood from the heart reaches the lungs via the pulmonary artery and, after having been oxygenated, returns via the pulmonary veins.

  Figure 28 Cat Anatomy [view large image]

Primates

Humans are mammals in the order Primates. The first primates may have resembled today's tree shrews, rat-size animals with a snout, claws, and sharp front teeth. By 50 million years ago, however, primates had evolved characteristics suitable to move freely through the trees. The first primates were prosimians (meaning "premonkeys"). They are represented today by several types of animals, including the lemurs. Monkeys, along with apes and humans, are anthropoids. Monkeys evolved from the prosimians about 38 million years ago, when the weather was warm and vegetation was like that of a tropical rain forest. There are two types of monkeys: the New World (South America) monkeys such as the spider monkeys, which have long grasping tails and flat noses, and the Old World (Africa) monkeys such as the baboons, which are now ground dwellers and lack such tails. Ape (gibbon, gorilla, and chimpanzee) evolved later. The human lineage split from that of the apes occurred about 5 - 10 million years ago in Africa. Figure 29 shows some of the primates. Figure 30 illustrates the chimpanzee anatomy, which is virtually identical to the human's.

Figure 29 Primates [view large image]

• Circulatory - Like the other mammals, primates have a four-chambered heart and a double-circuit circulatory system and are able to maintain a constant body temperature. The insulating covering is provided by hair, although in the humans nearly all the hair is lost, and insulation is now provided by clothing.
• Digestive - Most primates are nearly or exclusively herbivores, but their digestive tract does not show the high degree of morphological specialization seen in many other herbivores. Even some of the smallest primates, which until recently were believed to be carnivorous, subsist on plant food. In humans the large intestine is relatively less voluminous than in apes (which are predominently plant eaters), but nevertheless, humans are surprisingly effective at digesting cellulose.
• Endocrine - Similar for all vertebrates.
• Excretory - The kidney is a major excretory organ of primates and other vertebrates. The principal responsibility of the organ is to separate urea, toxins, and other types of waste from the blood, while water, salt, and electrolytes are maintained at an appropriate level. Due to this important role, the kidney is also involved in blood pressure and acid-base regulation in the body. Nephrons are the basic filtering units of the kidney, more than a million of them being present in a normal adult human kidney. Working together, the nephrons are able to filter blood at an impressive rate, processing the entire five-quart water content of the human circulatory system about every 45 minutes. Only a minute portion of the material passing through the kidneys is actually excreted, however, the vast majority being reabsorbed by the nephrons.
• Immune - A new study indicates that evolution of the immune system may be directly linked to the sexual activity of a species. A comparative analysis of 41 primate species demonstrates that the most promiscuous species have naturally higher white blood cell (WBC) counts -- the first line of defense against infectious disease -- than more monogamous species. The findings strongly suggest that the most sexually-active species of primates may have evolved elevated immune systems as a defense mechanism against disease.
• Musculo-skeletal - The limbs of the primates became adapted to swinging and leaping from branch to branch. Their hands were especially dexterous and mobile because their thumbs were opposable; that is, they closed to meet the fingertips. Therefore, these animals easily could reach out and bring food to the mouth. Claws were replaced by nails, which allowed a tree limb to be grasped and released freely. The skeleton of most mammals including primates is simplified compared to that of most reptiles, in that it has fewer bones. For example, the lower jaw consists of a single bone, rather than several.
• Nervous and Sensory - A snout is common in animals in which a sense of smell is of primary importance. In primates, the sense of sight is more important, and the snout has shortened considerably, allowing the eyes to move to the front of the head. This resulted in three-dimensional vision, permitting primates to make accurate judgments about the distance and the position of adjoining tree limbs. Primate sense of touch became also highly developed as a result of arboreal

living. It is useful as an effective feeling and grasping mechanism to grab their insect prey, and to prevent them from falling and tumbling while moving through the trees. By far the most outstanding characteristic of primate evolution has been the enlargement of the brain among members of the order. Primate brains tend to be large, heavy in proportion to body weight, and very complex. Reproductive - One birth at a time became the norm with primates; it would have been difficult to care for several offspring as large as primates in trees. The period of postnatal maturation was prolonged, giving immature young an adequate length of time to learn complex behavior patterns. Respiratory - Similar to all mammals, primates have a constant body temperature, an efficient respiratory system featuring a separation between the nasal and mouth cavities, an efficient four-chambered heart that prevents mixing of oxygenated and deoxygenated blood, among other characteristics.

Figure 30 Chimp Anatomy [view large image]