Mollusks (phylum Mollusca) are the second largest group of invertebrates (the arthropods being the largest), with over 100,000 species. They are characterized by a head with sense organs and mouth, a muscular foot, a visceral hump containing the digestive and reproductive organs, and an envelope of tissue (the mantle) that usually secretes a hard, protective shell. Practically all of the shells found on beaches and prized by collectors belong to mollusks. Among the more familiar mollusks are snails, whelks, conchs, clams, mussels, scallops, oysters, squid, and octopuses. Less conspicuous, but also common, are chitons, cuttlefish, limpets, nudibranchs, and slugs.
The largest number of species of mollusks are in the class Gastropoda, which includes snails with a coiled shell, and others lacking a shell. The next largest group are the bivalves (class Bivalvia), the chitons (class Amphineura), and octopus and squid, (class Cephalopoda). The other classes of mollusks are the class Scaphopoda (consisting of a few species of small mollusks with a tapered, tubular shell) and the class Monoplacophora, a class once regarded as extinct, but now known to have a few living species restricted to the ocean depths.
Fossil shells recognizable as gastropods and bivalves are present in rocks from the Cambrian period, about 570 million years ago. Present classifications based on the evolutionary relationships of mollusks are derived from studies of embryonic development, comparative anatomy, and RNA nucleotide sequences. The findings suggest affinities of mollusks with sipunculid, annelid, and echiurid worms.
Mollusks provide a clear example of adaptive radiation. The gastropods and bivalves which were originally marine, subsequently radiated into freshwater habitats. Without much change in gross appearance, these animals developed physiological mechanisms to retain salts within their cells and prevent excessive swelling from water intake in freshwater. Several groups of freshwater snails then produced species adapted to life on land. Gills adapted for the extraction of oxygen from water were transformed in land snails into lungs which extract oxygen from air, and the ammonia excretion typical of aquatic mollusks became uric acid excretion typical of birds and reptiles. A small squid, Onycoteuthis, moves so rapidly through the water that it often becomes airborne. It does not nest in trees, but it may help to explain why some authors ascribe to the squid Loligo a "parrot beak" and a "gizzard." Restaurant menus often include bivalve mollusks (oysters on the half-shell, steamed mussels, fried clams), cephalopod mollusks (fried squid), or gastropod snails (escargots).
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