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13.28
Level IV life: Molluscs
 
  The word Mollusc is used as the common name for a particular class of soft-bodied invertebrate animals (Latin, mollus,"soft"), usually with a hard external shells. Familiar molluscs include the cockle, oyster, snail, slug, octopus, and squid. The mollusc category is the second-largest in the animal kingdom, after the Insects (arthropods) estimated at around 60,000 species.  
  Molluscs are highly successful in terms of ecology and adaptation, with representatives in marine, freshwater, and terrestrial 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 tonnes. Most molluscs, however, are about 1 to 20 cm (0.4 to 8 in) long, and some are scarcely visible.  
13.28.1 General characteristics  
  Molluscs are commonly divided into seven sub classes:  
 
Sub Class of Molluscs Examples
Aplacophora  
Polyplacophora  
Monoplacophora  
Bivalvia  
Scaphopoda  
Gastropoda which is divided into three subclasses Prosobranchia (mostly marine snails within three orders), Opisthobranchia (sea slugs and their allies within eight orders), and Pulmonata (lunged molluscs, largely freshwater and terrestrial within two orders);
Cephalopoda which is divided into the subclasses Nautiloidea (the nautilus, with four gills and other archaic traits such as external shells) and Coleoidea (octopuses, squid and cuttlefish, with two gills and other advanced traits).
 
  Although few features are common to all molluscs, 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 mollusc 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 molluscs, 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 molluscs 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 (rear) end of the idealized mollusc 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 tongue-like 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 (front) part of the gut, with one pair of nerve cords to the foot and another to the viscera. Ganglia (masses of nerve tissue) around the gut are usually 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.  
  The first mollusc fossils appear in early Cambrian rocks, about 600 million years old. All seven of the sub categories of molluscs have living representatives. In the class Aplacophora, the body is worm-like. No shell exists, only a tough mantle, and the foot has virtually been lost. The members of the class Polyplacophora (chitons) have a series of eight shell plates (valves) in a row and are well adapted to clinging to rocks. The mainly fossil Monoplacophora has one known living genus, Neopilina, discovered in deep water in 1952. The animal has a single flat shell and multiple gills. The members of the class Bivalvia, which includes cockles, clams, and mussels, have a shell divided into two halves, 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, no shell at all. The molluscs of the class Cephalopoda, which include octopuses, cuttlefish, and squid, are modified by reduction of the foot and shell and the development of arms around the mouth. Thousands of fossil species are also known.  
13.28.2 Behaviour  
  Although vision is poor in most molluscs, cephalopods such as squid have eyes with lenses, retinas, and other features remarkably similar to 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 molluscs, such as octopuses, cuttlefish, and squid, exhibit complicated courtship behaviour. Advanced cephalopods also possess considerable ability to learn from experience.  
13.28.3 Reproduction  
  The basic mollusc pattern is to have separate sexes, with sperm and eggs spawned into the water, where fertilization and early development occur. In most molluscs 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 for changing back and forth from being male to being female.  
 
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