Mollusca – Zoology Notes For – W.B.C.S. Examination.
কম্বোজ – প্রাণিবিদ্যা নোট – WBCS পরীক্ষা।
Mollusk, also spelled mollusc, any soft-bodied invertebrate of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. Along with the insects and vertebrates, it is one of the most diverse groups in the animal kingdom, with nearly 100,000 (possibly as many as 150,000) described species. Each group includes an ecologically and structurally immense variety of forms: the shell-less Caudofoveata; the narrow-footed gliders (Solenogastres); the serially valved chitons (Placophora or Polyplacophora); the cap-shaped neopilinids (Monoplacophora); the limpets, snails, and slugs (Gastropoda); the clams, mussels, scallops, oysters, shipworms, and cockles (Bivalvia); the tubiform to barrel-shaped tusk shells (Scaphopoda); and the nautiluses, cuttlefishes, squids, and octopuses (Cephalopoda).Continue Reading Mollusca – Zoology Notes For – W.B.C.S. Examination.
Size range and diversity of structure
Typical molluscan features have been substantially altered, or even lost, in many subgroups. Among the cephalopods the giant squids (Architeuthis), the largest living invertebrates, attain a body length of eight metres (more than 26 feet); with the tentacle arms extended, the total length reaches to 22 metres. Other cephalopods exceed a length of one metre. Many of the remaining molluscan classes show a large variation in size: among bivalves the giant clam (Tridacna) ranges up to 135 centimetres (four feet) and the pen shell (Pinna) from 40 to 80 centimetres; among gastropods the sea hares (Aplysia) grow from 40 to 100 centimetres and the Australian trumpet, or baler (Syrinx), up to 60 centimetres; among placophores the gumshoe, or gumboot chiton (Cryptochiton), achieves a length up to 30 to 43 centimetres; and, among solenogasters, Epimenia reaches a length of 15 to 30 centimetres. Finally, gastropods of the family Entoconchidae, which are parasitic in echinoderm sea cucumbers, may reach a size of almost 1.3 metres. In contrast, there are also minute members, less than one millimetre (0.04 inch) in size, among the solenogasters and gastropods.
Distribution and abundance
The mollusks have adapted to all habitats except air. Although basically marine, bivalves and gastropods include freshwater species. Gastropods have also adapted to land, with thousands of species living a fully terrestrial existence. Found on rocky, sandy, and muddy substrata, mollusks burrow, crawl, become cemented to the surface, or are free-swimming.
Some littoral bivalves, such as Tridacna, as well as some sea slugs, such as Aeolidia, share an obligatory symbiosis with zooxanthellae (a group of algae). Another metabolic association exists between certain bacteria and several bivalves and gastropods of deep-sea hot vents or other sulfidesystems. There are several parasitic mollusks.
Locomotion
Mollusks have a wide range of locomotory patterns. Solenogasters and various smaller gastropods glide upon cilia that beat rapidly against a pathway of mucus secretions. This pattern of movement is supported or replaced in larger mollusks by the propulsive waves that run along the surface of the foot and are controlled by the actions of the dorsoventral musculature (
). Burrowing occurs as an interaction between musculature and the hydrostatic skeleton (see below Internal features); it is performed in caudofoveates and several sea slugs by the whole anterior body but is restricted to the foot in scaphopods, bivalves, and some specialized gastropods.Various bivalves (e.g., cockles) and snails may perform rapid twists or jumps through violent flexion of the foot. Buoyancy floating and jet propulsion are found in cephalopods; floating is also known in gastropods, and swimming of a different kind is practiced by some opisthobranch and prosobranch gastropods as well as in scallops and related bivalves. Octopods use their arms to crawl or even to swim or float with the help of the body skin interconnecting the arms (interbrachiate web). Some bivalve groups bore into hard surfaces by secreting strong chemicals that dissolve the substrate or by drilling, using the shell and radula. A sedentary (sessile) way of life has been adopted by many bivalves and some gastropods.
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