Anatomy of Animals

Chordates :

Fishes, Amphibians, Reptiles, Birds, Mammals, Primates

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.

  Two types of eyes had developed since the Cambrian explosion. The arthropods adopt compound eyes, which is suitable for small animals but becomes too bulky for larger ones in the vertebrate kingdom. The camera-style did not come "ready-made" as claimed by creationists and intelligent design proponents. Studies in the 2000's reveal that about 500 million years ago camera-style eyes first appeared as a simple light sensor for regulating circadian rhythms, as well as seasonal activities such as feeding and breeding. The primitive retina with two layers can detect light but cannot form image. Figure 19f shows the evolutionary path by comparing different species (from primitive hagfish to human in horizontal row) and by embryonic development (as shown in vertical columns). It also shows that evolution does not always produce perfect structures as the photoreceptors are

  Figure 19f Evolution of Eye [view large image]

  blocked by two layers of cells. However, when octopuses and squids took another evolutionary path independently, their camera-style eyes do not suffer such deficiencies.

• 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 19g). 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 19g:

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 19g 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 19g, 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 also includes a fish squirting a shot at the cricket (see a video).

			Meanwhile in 2015, a frilled shark believed to be around in the Cretaceous Period about 80 million years ago is caught off South West Australia. This 6-foot-long fish has 300 teeth in 25 rows (Figure 21c).
Figure 21a Fishes [view large image]	Figure 21b Living Fossils [view large image]	Figure 21c Frilled Shark [view large image]

An article in the 2014 November-December issue of American Scientist cautions the use (or misuse) of the term "living fossils". Creationists have misappropriated the term to disprove the theory of evolution. Although it is not possible to compare the DNA of the

fossils with living fossils, detailed studies of the external structures, the relict distribution, the taxonomic diversity, and the DNA sequence of the living fossils indicate that the living fossils are not exactly the same of the corresponding fossils. As shown in Figure 21d, the horseshoe crab and ginkgo tree are now spread all over the world, the horseshoe crab has also diversified to four different species. These living fossils and the other examples in Figure 21d demonstrate that changes did occur beyond the apparent forms. Thus, the superficial similarity in appearence is deceptive. Actually all current organisms retain some features of the ancestor, be it in DNA sequence or form. For example, the homeobox genes are ubiquitous in multicellular organisms; the advanced invertebrates and all vertebrates posses similar animal features including body plan, body symmetry etc. These characteristics had been developed hundreds of million years ago. It is from the high degree of resemblance in external morphology that prompted Darwin to coin the term "living fossil" arbitrarily. It seems that besides blind luck - like surviving a cataclysmic extinction, or being first to move into a new niche - it is often a key innovation which renders living fossils successful in persisting for long stretches of time.

Figure 21d Truth about Living Fossils



• 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. Figure 22a shows the internal anatomy of a common fish.

  Figure 22a Fish Anatomy [view large image]

• 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. A 2014 research report indicates that the long-extinct fish called placoderms (the armour-plated creatures - our ancient ancestors) in the Silurian Period had internal sex by a bony 'claspers'. Somehow the practice had been reverted back to external fertilization. The act of copulation was re-invented later by the land animals (see Figure 22c) - a reversal previously thought to be evolutionarily improbable.
  Figure 22b Ancient Sex [view large image]	Figure 22c Evolution of Intromittent Organ [view large image]

• 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. 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. A 2016 research shows that scales, feathers and hairs develop from the same group of placode cells (the dark blue spots in Figure 24e) suggesting a common origin of these diverse features.

  Figure 24e Scales, Feathers, Hairs, Common Origin [view large image]

• 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 24f).

  Figure 24f Membranes [view large image]

  It is discovered in 2011 that an obscure African lizard called T. ivensii does possess a true placenta used to be monopolized by mammals. Cells on the outside of the embryo send out extensions that burrow between the cells of the oviduct wall and

  implants itself beneath the outer layer. These cells press up against the mother's blood vessels to obtain nutrients. Such feat carries the risks of being attacked by its mother's immune system and male embryos could also be feminised by her sex hormones. The problems seem to have been resolved at this point of evolution (in T. ivensii).
• 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. Nearly every anatomical feature of a bird can be related to its ability to fly. Figure 26a shows the anatomy of a common bird.

Figure 25a Birds [view large image]

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 shows the temperature difference between the warm-blooded human and cold blooded

Figure 25b Warm Bloodedness

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 displayed in Figure 26c together with a description of its various components in Table 02.
  Figure 26a Bird Anatomy [view large image]	Figure 26b Making Egg	Figure 26c Chicken Egg [view large image]

  
  

  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, which is very similar to the human's. See "Age of Animals" for "mammal characteristics and comparison with other vertebrates".

Figure 27a Mammals [view large image]

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

Figure 27b Brains of the Vertebrates

• 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 spending 4 or 5 weeks there. Some marsupials such as opossum spends an additional 8 or 9 weeks clinging to the mother's back.

    Marsupial reproductive organs differ from placental mammals. For them, the reproductive tract is doubled. The females have two uteri and two vaginas. A birth canal forms between them before birth called pseudo-vagina or central vaginal canal; while placental mammals such as human have only one passage for both birth and copulation (Figure 27c). The male marsupial has a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas.

    Figure 27c Reproductive Organs [view large image]

    Study in 2017 indicates that the wallaby does possess a placenta of only two-cell thick providing oxygen, nutrients at the end of a short pregnancy (of 26.5 days) before the underdeveloped baby crawls from the uterus into the mother's pouch. The molecular signals to drive development of the fetus and to protect it from the mother's immune system are similar to those of the placental mammals' at the beginning of pregnancy. The mammary glands of the wallaby is found to expressed the same genes as mammalian placentas do in late fetal development. The finding suggests that the more primitive marsupials are on the way to evolve a fully placental pregnancy; while it still retains some of the external features for the reptiles and birds. The mammals tend to raise the young inside the body as long as possible. The isolated environment of Australia did not provide enough challenges for the marsupials to evolve further. Such unique circumstance allows a peek into the early stage of mammalian evolution. See "Wallaby Milk Acts as a Placenta for Babies".
  • Placental Mammals - In these mammals, the extraembryonic membranes (Figure 24f) 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 27d 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 button to the placenta. Figure 27e depicts the progression from fertilization to implan-tation in the earliest stage of development.
  Figure 27d Early Embryo	Figure 27e Fertilization - Implantation [view large image]	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]

Go to Top of Page to Select
 or to Main Menu