Anatomy of Animals

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 Formation of Coelom [view large image]

Visceral mass: the soft-bodied portion that contains internal organs.
Foot: the strong, muscular portion used for locomotion.
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.

In addition to these three parts, many mollusks show cephalization and have a head region with eyes and other sense organs.

The division of the body into distinct areas seems to have allowed diversification to occur because there are many different types of mollusks adapted to various ways of life. The molluskan groups can be distinguished by the modification of their foot. In bivalves, such as clams, the foot is laterally compressed and is sometimes called pelecypods ("hatchet-footed"). In the gastropods ("belly-footed"), such as snail, the foot is ventrally flattened, and the animal moves by muscle contractions that pass along the foot. In cephalopods ("head-footed"), such as octopuses, the foot has evolved into tentacles about the head. The tentacles are used to seize prey.

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

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
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blood and arranged like the teeth of a comb.

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 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.

Figure 12 Digestive System
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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. Research report in 2016 suggests that cephalopods may be able to perceive colors due to the un-usual construction of their eyes (see "Weird pupils let octopuses see their colorful gardens").

		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.