Mollusca

Mollusca

After reviewing this section, the student should be able to

determine the identifying physiological characteristics of Mollusks, identify body supply systems, and discuss the major classes of Mollusks: class Bivalvia, class Gastropoda, and class Cephalopoda.

The mollusks constitute one of the largest phyla of animals, both in numbers of living species (at least 47,000, and perhaps many more) and in numbers of individuals.

A significant characteristic of mollusks is their possession of a coelom, a fluid-filled cavity that develops within the mesoderm. The coelom not only functions as a hydrostatic skeleton but also provides space within which the internal organs can be suspended by the mesenteries.

All mollusks have a soft body (their name is derived from the Latin word mollus, meaning "soft"), which is generally protected by a hard, calcium-

containing shell. In some forms however, the shell has been lost in the course of evolution, as in slugs and octopuses, or greatly reduced in size and internalized, as in squids.

Structurally, mollusks are quite distinct from all other animals. However, all modern mollusks have the same fundamental body plan. There are three distinct body zones: a head-foot, which contains both the sensory and motor organs; a visceral mass, which contains the well-developed organs of digestion, excretion, and reproduction; and a mantle, a specialized tissue formed from folds of the dorsal body wall, that hangs over and enfolds the visceral mass and that secretes the shell. Themantle cavity, a space between the mantle and the visceral mass, houses the gills; the digestive, excretory, and reproductive systems discharge into it.

Mollusks are also characterized by a toothed tongue, the radula, composed primarily of chitin. The radula serves both to scrape off algae and other food materials and also to convey them backward to the digestive tract. In some species, it is also used in combat.

Digestion and Excretion

The digestive tract is complete and ciliated, with a mouth, anus and complex stomach. The pattern of the stomach varies according to the mollusks diet. Food is taken up by cells lining the digestive glands arising from the stomach, and then is passed into the blood. Undigested materials are compressed and packaged, then discharged through the anus into the mantle cavity and are carried away from the animals in the water currents. This packaging of wastes in solid form prevents fouling of the water passing over the gills.

Excretory functions are carried out by a pair of nephridia, 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.

Nervous System and Sensory Capability

Mollusks have a relatively complex nervous system, which varies from species to species reaching the height of complexity at the octopus. The octopus is thought to be among the most intelligent of all invertebrates, with a mental capacity likened to that of a domestic cat. Sensory ability in some mollusks (notably the cephalopods) is considerable, with a variety of organ systems, as well as large, complex eyes. The eyes of the giant squid are the largest in the animal kingdom, approaching the size of dinner plates. It has recently been demonstrated that squid can successfully locate and capture transparent prey in the water by means of a specialized polarization vision.

Respiration and Circulation

Excluding cephalopods, mollusks have an open circulatory system, meaning the blood does not circulate entirely within vessels but is collected from the gills, pumped through the heart, and released directly into spaces in the tissues from which it returns to the gills and then to the heart. Such a blood-filled space is known as a hemocoel ("blood cavity"). In the mollusks, the hemocoel has largely replaced the coelom, which is reduced to a small area around the heart and to the cavities of the organs of reproduction and excretion. Cephalopods, whose vigorous activities require that the cells be supplied with large quantities of oxygen and food molecules, have a closed circulatory system of continuous vessels and accessory hearts that propel blood into the gills. The presence of discrete respiratory and circulatory systems has led to an improved capacity for oxygen uptake and distribution, and hence an increase in body mass. Molluscans include the largest and most advanced of living invertebrates with the cephalopods, including the monstrous Giant Squid Architeuthis, which can reach a total length, including tentacles, of 18 meters.

Locomotion

Locomotion amongst the molluscans varies considerably, and is dictated by the structure of the foot. Herbivorous forms are commonly gliders, moving on waves of muscular contraction. However, many carnivorous forms have achieved more advanced forms of locomotion. Cephalopods swim actively by a type of jet propulsion, in which water is rapidly expelled from the mantle cavity via the siphon. In the streamlined squid this technique has enabled to animals to achieve the fastest speeds of any aquatic invertebrate. The Cuttlefish and the Sea Hares rely upon undulating lateral fins for highly maneuverable locomotion. In the bivalves the foot has developed into a tool for burrowing, which can be remarkably rapid for example in the common Razor Shells.

Mollusks exhibit a tremendous diversity of form and behavior. The three major classes range from largely sedentary or sessile filter-feeding animals, such as clams and oyster (class Bivalvia), through aquatic and terrestrial snails and slugs (class Gastropoda), to the predatory cuttlefish, squids, and octopuses (class Cephalopoda).

Class Bivalvia

The approximately 7,500 living species of bivalves include such common animals as clams, oysters, scallops, and mussels. They derive theirname from the two parts, or valves, into which the shell is divided. One or two large, well-developed adductor muscles (the edible part that so many people consider a delicacy) are used to close the shell swiftly and tightly in times of danger. These muscles are so strong in scallops that the animals can move swiftly through the water by clapping their shells together, thereby eluding predators such as starfish.

Most bivalves are dioecious (have separate sexes; are not hermaphroditic) and fertilization can be either internal or external, depending on the species. Freshwater clams differ in that their fertilization is internal, with brooding taking place in the gill chamber. Specialized "brood pouches", in which the young are protected during their early development, are found in hermaphroditic species as well as in the females of some species with separate sexes. The bivalves have sensory cells for discrimination of touch, chemical changes, and light. The scallop has quite complex eyes; a single individual may have a hundred or more eyes located among the tentacles on the fringe of the mantle. The lens of this eye cannot focus images, however, so it does not appear to serve for more than the detection of light and dark, including passing shadows cast by other moving organisms. Abundant in both salt and fresh water, most adult bivalves are sedentary, herbivorous filter feeders, using currents set up by cilia on their gills to bring in food particles, usually microscopic algae. A few bivalves, such as scallops, are able to move rapidly by clapping their valves together. Others are able to move slowly by muscular action of their foot. Still others are completely sessile.

Class Gastropoda

The gastropods, which include the snails, whelks, periwinkles, abalone, and slugs, are the largest group of mollusks (at least 37,500 living species). They have either a single shell or no shell. Gastropods are common in both salt and fresh water and on land. Class Gastropoda includes herbivores, omnivores, a wide variety of specialized carnivores, scavengers, and even some parasites.

Land-dwelling snails do not have gills, but the area in their mantle cavities once occupied by gills is rich in blood vessels, and the snail's blood is oxygenated there. Thus the mantle cavity has become, in effect, a lung. Some snails that were probably once land dwellers have returned to the water, but they have not regained gills. Instead, they bob up to the surface at intervals to entrap a fresh bubble of air in their mantle cavities.

Gastropods, which lead a more mobile, active existence than bivalves, have a ganglionated nervous system with as many as six pairs of ganglia connected by nerve cords. There is a concentration of nerve cells in the tentacles, at the anterior end. In some of the animals, the eyes are quite highly developed.

In some gastropods, the primitive form of reproduction--separate sexes with external fertilization--is retained. In most gastropods, however, fertilization is internal, and hermaphroditism has evolved repeatedly in this group. The simultaneous hermaphroditism found in many snails and slugs probably resulted from the difficulties of such slow-moving animals in finding mates. In some species, the animals are males when they are younger, then females when they are older and larger.

Sea slug

Class Cephalopoda

The cephalopods (the “head-foots”) are, in many respects, the most evolutionarily advanced animals to be found among the invertebrates. The 600 living species in this strictly marine class rival the vertebrates in complexity and, in some cases, in intelligence. Active predators, they compete quite successfully with fish.

Although obviously mollusks, the cephalopods have become greatly modified. The large head has conspicuous eyes and a central mouth surrounded by arms, some 90 in the chambered Nautilus, ten in the squid, and eight in the octopus. The octopus body seldom reaches more than 30 centimeters in diameter (except on the late late show), but giant squids sometimes attain sea-monster proportions. One caught in the Atlantic in 1861 was about six meters long, not counting the tentacles.

cuttlefish

Nautilus, as the only modern shelled cephalopod, offers an indication of some of the steps by which this class disposed of the shell entirely. The animal occupies only the outermost portion of its elaborate and beautiful shell, the rest of which serves as a flotation chamber. In the squid and its relative, the cuttlefish, the shell has become an internal stiffening support, and in the octopus, it is lacking entirely.

Freedom from the external shell has given the mantle (the outermost layer or body wall, or a soft extension of it) more flexibility. The most obvious effect of this is the jet propulsion by which cephalopods dart through the water.

Usually, water taken into the mantle cavity bathes the gills and is then expelled slowly through a tube-shaped structure, the siphon; but when the cephalopod is hunting or being hunted, it can contract the mantle cavity forcibly and suddenly, thereby squirting out a sudden jet of water. Contraction of the mantle-cavity muscles usually shoots the animal backward, head last, but the squid and the octopus can turn the siphon in almost any direction they choose. Cephalopods also have sacs from which they can release a dark fluid that forms a cloud, concealing their retreat and confusing their enemies. These colored fluids were at one time a chief source of commercial inks. Sepia is the name of the genus of cuttlefish from which a brown ink used to be obtained.

The cephalopods have well-developed brains, composed of many groups of ganglia, in keeping with their highly developed sensory systems and their lively, predatory behavior. These large brains are covered with cartilaginous cases. The rapid responses of the cephalopods are made possible by a bundle of giant nerve fibers that control the muscles of the mantle. Many of the studies on conduction of nerve impulses are made with the giant axons of squids, which are large enough to permit the insertion of electrodes.

Cephalopods are always diecious, and fertilization is internal. Courtship and mating behavior are complex, and males often fight for access to females. Fertilization occurs when the male uses one of his arms to transfer the packets of sperm, called spermatophores, from his mantle cavity to the mantle cavity of the female. The female produces an intricate, gelatinous egg mass in which the developing embryos are protected until they hatch as miniature adults. In Octopus and some other cephalopod genera, the mother guards the egg masses, cleaning and aerating them.