Mollusk - biology.

Mollusk - biology. I INTRODUCTION Mollusk, common name for members of a phylum of soft-bodied animals (Latin mollus, "soft"), usually with a hard external shell. Familiar mollusks include the clam, oyster, snail, slug, octopus, and squid. The mollusk phylum is the second largest in the animal kingdom, after the arthropods. Earlier estimates of the number of mollusk species sometimes exceeded 100,000, but more recently this figure has been reduced to less than 50,000; the new estimates are incorporated here. Mollusks are highly successful in terms of ecology and adaptation, with representatives in virtually all habitats, but they are most diverse in the sea. Among them are some advanced animals, such as the octopus and squid. Giant squid are also the largest invertebrates, weighing up to 2,000 kg (4,000 lb). Most mollusks, however, are about 1 to 20 cm (about 0.4 to 8 in) long, and some are scarcely visible. The first mollusk fossils appear in early Cambrian rocks, about 600 million years old. Seven of the phylum's classes have living representatives: the wormlike, shell-less aplacophorans, with 250 species; the chitons, with 600 species; the monoplacophorans, with 10 species; the bivalves, such as clams, with 7500 species; the scaphopods, or tusk shells, with 350 species; the gastropods, such as snails and slugs, with 37,500 species; and the cephalopods, such as octopuses and squid, with 600 species. Several fossil classes and thousands of fossil species are also known. II GENERAL CHARACTERISTICS Although few features are common to all mollusks, the animals are not readily mistaken for anything else, and all may be treated as variants on a common theme (not to be confused with a common ancestor). A theoretical, idealized mollusk would crawl on a single flat, muscular foot, and the body would have at least a suggestion of a head at one end and an anus at the other. Above the body would be an external shell mounted on a visceral hump containing internal organs. This shell, secreted by a sheet of tissue called the mantle, is complicated in mollusks, being made up of calcium carbonate and other minerals in an organic matrix produced in layers by the mantle at the edge of the shell and under it. It is also generally covered by an outer layer without minerals, called the periostracum. The shell may be multiple, as in chitons, or paired, as in bivalves. In various mollusks the shell is reduced in size and is sometimes lost completely; in aplacophorans there is no direct evidence that a shell ever existed. At the posterior end of the idealized mollusk would be a groove or depression called the mantle cavity, with gills to each side of the anus, and openings to the kidneys and reproductive structures. A single pair of gills is common, but many gastropods have only one gill. The cephalopod nautilus has two pairs, and monoplacophorans and chitons have several to many pairs. Generally the molluscan gut is equipped with jaws and a tonguelike structure, called a radula, with teeth on it. Also present are a stomach and a pair of digestive glands. The nervous system consists of a ring of nerves around the anterior part of the gut, with one pair of nerve trunks to the foot and another to the viscera. Ganglia around the gut usually are developed into a brain with various sense organs; the nervous system of cephalopods is as complex and as highly organized as that of fishes. The heart is located at the posterior end of the body; it sends blood into an open system that forms the main body cavity. Associated with the heart is a complex of organs that includes the kidneys and gonads and sometimes other reproductive structures. III BEHAVIOR Although vision is poor in most mollusks, cephalopods such as squid have eyes with lenses, retinas, and other features remarkably like those of vertebrates. Some gastropods have a well-developed sense of smell and can locate food in the water at a considerable distance. Predators may similarly be detected by the chemical senses and are sometimes evaded by leaping or swimming. Some mollusks exhibit complicated courtship behavior. Advanced cephalopods possess considerable ability to learn from experience. IV REPRODUCTION The basic mollusk pattern is to have separate sexes, with sperm and eggs spawned into the water, where fertilization and early development occur. In most mollusks a larval stage follows, in which the larvae swim about for a while and then settle on the bottom and mature; this stage is often modified or absent, however. Fertilization may also be internal, with glands secreting protective coverings around the eggs. Slow-moving creatures such as snails often evolve into hermaphrodites (both male and female), because this doubles the number of appropriate mates. Sometimes the mother protects the developing eggs. Some oysters are remarkable in caring for the young inside the mantle cavity and switching back and forth from being males to being females. V ECOLOGY AND IMPORTANCE Mollusks are abundant and hence important in food chains in many habitats. A large number are herbivores or grazers, especially the chitons and many gastropods. Tusk shells and some other mollusks feed on matter deposited on the bottom, whereas most bivalves filter suspended materials from the water. Many gastropods are carnivorous, most of them preying on slow-moving or attached animals. Cephalopods are active predators on larger animals such as crabs. Numerous mollusks are important food sources for humans, but some gastropods damage crops, and others harbor disease-causing parasites. Scientific classification: Mollusks make up the phylum Mollusca. In the class Aplacophora, the body is wormlike. No shell exists, only a tough mantle, and the foot has virtually been lost. The three orders of the class Polyplacophora (chitons) have a series of eight shell plates (valves) in a row and are well adapted to clinging on rocks. The mainly fossil Monoplacophora is now known to have one living genus, Neopilina, discovered in deep water in 1952. The animal has a single flat shell and multiple gills. The class Bivalvia have a shell divided into two valves, and they feed with their gills. As a consequence the head is poorly developed. Members of the class Scaphopoda (tusk shells) have a long, tapered, slightly curved shell and live on sandy bottoms. Members of the class Gastropoda (snails and slugs) are asymmetrical and have only one shell or, as in slugs, are shell-less. The three subclasses of the Gastropoda are the Prosobranchia (mostly marine snails, with three orders), Opisthobranchia (sea slugs and their allies, with eight orders), and Pulmonata (lunged mollusks, largely freshwater and terrestrial, with two orders). The class Cephalopoda are modified by reduction of the foot and shell and the development of arms around the mouth. The two subclasses are Nautiloidea ( Nautilus, with four gills and other archaic traits such as an external shell) and Coleoidea (octopuses, squid, and cuttlefish, with two gills and other advanced traits). See Bivalve; Cephalopod; Chiton; Gastropod. Contributed By: Michael T. Ghiselin Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.