Mollusks (phylum Mollusca) are the 2nd biggest group of invertebrates (the arthropods getting the most important), with above 100,000 species. They’re characterized by a head with sense organs and mouth, a muscular foot, a visceral hump that contains the digestive and reproductive organs, and an envelope of tissue (the mantle) that usually secretes a tough, protecting shell. Basically most of the shells located on beaches and prized by collectors belong to mollusks. Among the many far more acquainted mollusks are snails, whelks, conchs, clams, mussels, scallops, oysters, squid, and octopuses. A lot less conspicuous, but in addition frequent, are chitons, cuttlefish, limpets, nudibranchs, and slugs.
The biggest range of species of mollusks are in the class Gastropoda, which incorporates snails using a coiled shell, and Other folks missing a shell. The subsequent premier group will be the bivalves (class Bivalvia), the chitons (course Amphineura), and octopus and squid, (course Cephalopoda). One other courses of mollusks are the class Scaphopoda (consisting of some species of little mollusks that has a tapered, tubular shell) and The category Monoplacophora, a category after viewed as extinct, but now recognised to have a number of residing species restricted towards the ocean depths.
Fossil shells recognizable as gastropods and bivalves are existing Mollusk in rocks within the Cambrian period of time, about 570 million decades ago. Current classifications determined by the evolutionary interactions of mollusks are derived from studies of embryonic advancement, comparative anatomy, and RNA nucleotide sequences. The conclusions suggest affinities of mollusks with sipunculid, annelid, and echiurid worms.
Mollusks give a apparent example of adaptive radiation. The gastropods and bivalves which were being at first maritime, subsequently radiated into freshwater habitats. Devoid of Significantly improve in gross overall look, these animals designed physiological mechanisms to keep salts within their cells and prevent excessive swelling from h2o ingestion in freshwater. Various groups of freshwater snails then generated species tailored to existence on land. Gills tailored for that extraction of oxygen from water ended up transformed in land snails into lungs which extract oxygen from air, and the ammonia excretion normal of aquatic mollusks became uric acid excretion standard of birds and reptiles. A little squid, Onycoteuthis, moves so promptly from the water that it usually results in being airborne. It does not nest in trees, but it might assist to explain why some authors ascribe into the squid Loligo a “parrot beak” plus a “gizzard.” Cafe menus normally contain bivalve mollusks (oysters to the fifty percent-shell, steamed mussels, fried clams), cephalopod mollusks (fried squid), or gastropod snails (escargots).