Mollusca
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Mollusk is a common name for members of a phylum of soft-bodied animals (Latin mollus, “soft”), usually with a hard external shell. The mollusks represent a diverse group of marine, freshwater, and terrestrial invertebrates, including such varied forms as snails, chitons, limpets, clams, mussels, oysters, octopuses, squid, cuttlefish, tusk shells, slugs, nudibranchs, and several highly modified deep-sea forms. They all have one anatomical feature in common, the presence of a shell at some stage in the life cycle. Although most mollusks have a shell as adults, the octopus, squid, and deep-sea forms do not. They do however have a small, shell-like structure, called a shell gland, present for a short time during embryonic development. The mollusk phylum is the second largest in the animal kingdom, after the arthropods.
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).
Mollusk is a common name for members of a phylum of soft-bodied animals (Latin mollus, “soft”), usually with a hard external shell. The mollusks represent a diverse group of marine, freshwater, and terrestrial invertebrates, including such varied forms as snails, chitons, limpets, clams, mussels, oysters, octopuses, squid, cuttlefish, tusk shells, slugs, nudibranchs, and several highly modified deep-sea forms. They all have one anatomical feature in common, the presence of a shell at some stage in the life cycle. Although most mollusks have a shell as adults, the octopus, squid, and deep-sea forms do not. They do however have a small, shell-like structure, called a shell gland, present for a short time during embryonic development. The mollusk phylum is the second largest in the animal kingdom, after the arthropods.
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).
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