More than 100,000 species of the phylum Mollusca are known to exist, belonging to 7 classes. The three largest of these are Gastropoda, which consists of snails, slugs and limpets; Bivalvia, which includes oysters, mussels, and clams; and Cephalopoda, containing squids, cuttlefish and octopuses.
The three main mollusk classes look very different, but all follow the same basic plan. The main bulk of the body contains the internal organs and is called the visceral mass. This mass lies above a muscular foot, and is surrounded by the mantle , a fleshy extension of the body wall, which hangs down on each side of the visceral mass. The space between the mantle and the visceral mass is called the mantle cavity. The mantle secretes the shell, which is present in most mollusks. Almost all aquatic mollusks breathe with the aid of cenidia (comb-like gills in the mantle cavity) which are covered with many small, hairlike structures called cilia, whose rhythmic movement draws water over the gill surface.
The left and right sides of the gastropod grow at different rate because of the spiral shape of the shell, the organs on one side do not develop, and the visceral mass, mantle and shell become spirally coiled; this development makes the long digestive system more compact. Most aquatic gastropods breathe by means of gills, but the land-dwelling snails and slugs lack gills, and have modified the mantle cavity into lung. Air is moving in and out of the mantle cavity by raising and lowering the mantle, which is moist and has a rich blood supply.
Ribbed neptuneRibbed neptuneNeptunea lyrata
Gastropods are generally considered to be inhabitants of damp places. Most gastropods move on a flat sole which in many species has a large pedal gland. This gland secretes mucus onto the surface over which the sole moves. Locomotion is achieved by waves of muscular contractions which pass down the foot. Aquatic gastropods have separate sexes, but snails and slugs are hermaphroditic. Some aquatic snails transmit human platyhelminth parasites such as the liver fluke and the blood fluke Schistosoma, genus of trematode flukes belonging to the family Schistosomatidae, the cause of bilharzia.
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The class Bivalvia includes clams, oysters, and mussels, and is characterized by the possession of a shell which is divided into two halves joined by an elastic ligament. Bivalves are mainly marine although some have invaded freshwater. The foot is adapted for burrowing into hard material; the shipworm, for example, drills into wood using the roughened edges of its shell, and other species even bore into rocks.
The body is long and laterally flattened, surrounded on each side by two lobes of the mantle. Each lobe secretes a shell called a valve. The gills hang from the roof of the mantle cavity and lie on each side of the body. They are used for collecting food and as well for breathing. Bivalves have no head, so the sense organs occur on the edges of the mantle. Most bivalves are filter-feeders. The gills are highly modified for this purpose and secrete mucus in which particles are trapped. A complex arrangement of cilia draws water into the mantle cavity, sorts the particles and carries food to the mouth and then to the stomach. Most bivalves are sluggish, but some, such as the razorshell clam have a thin, streamlined shape and large foot and can burrow very rapidly. Scallops use jet propulsion to swim, by clapping their two valves of the shell together.
The cephalopods represent the highest stage of the molluscan evolution. The three main groups belong to the subclass coleoides and are the squids, the cuttlefish, and the octopus. All cephalopods are adapted to a free-swimming life in the sea, but only one living species has an outer shell - the nautilus. Most other cephalopods have an inner chambered shell, which varies from species to species. All cephalopods are carnivorous. Cuttlefish and squid catch their prey with 2 long tentacles with suckers at the tips, and hold it near the mouth with the 8 shorter arms, which are equipped with suckers all along their length. The octopus, however, has 8 tentacles of equal length, any one of which is used to grab the prey. The prey is bitten with the 2 horny jaws and the octopus' poisoning saliva then enters the wound and kills the prey. Large pieces of flesh are torn off, pulled into the mouth by the radula and swallowed.
Cephalopods have the most highly developed nervous system of all mollusks. The brain is large, and, at least in the squid and octopus, can learn and remember information. Most cephalopods rely on their good eyesight to detect food and enemies. In addition, sensory cells on the octopus' tentacles detect minute concentration of chemicals which also alert it to the presence of prey or to possible danger. Many cephalopods can camouflage themselves in seconds, changing color by the expansion or contraction of small bags of pigment in the skin. Some species also have an ink sac - a pouch off the intestine - which releases a cloud of black liquid through the anus when the animal is alarmed, behind which it make its escape.