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A siphon is an anatomical structure which is part of the body of aquatic molluscs in three classes: Gastropoda, Bivalvia and Cephalopoda (members of these classes include saltwater and freshwater snails, clams, octopus, squid and relatives). Siphons in molluscs are tube-like structures in which water (or, more rarely, air) flows.
Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks [a] (/ ˈ m ɒ l ə s k s /). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda . [ 5 ]
In some mollusks the mantle cavity is a brood chamber, and in cephalopods and some bivalves such as scallops, it is a locomotory organ. The mantle is highly muscular. In cephalopods the contraction of the mantle is used to force water through a tubular siphon, the hyponome , and this propels the animal very rapidly through the water.
In the most primitive gastropods, however, the stomach is a more complex structure. In these species, the hind part of the stomach, where the oesophagus enters, is chitinous, and includes a sorting region lined with cilia. [1] In all gastropods, the portion of the stomach furthest from the oesophagus, called the "style sac", is lined with cilia.
The radula (US: / ˈ r æ dʒ ʊ l ə /; pl.: radulae or radulas) [1] is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. [2] It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus.
The mechanical properties of bivalve shells and their relatedness to microstructure was first published in 1969 by Stephen Wainwright at Duke University. [2] Following this, eight main categories of bivalve microsections were defined: simple prismatic, composite prismatic, sheet nacreous, lenticular, foliated, crossed-lamellar, complex crossed-lamellar, and homogenous.
Unlike other cephalopods, cuttlefish possess a unique internal structure called the cuttlebone, a highly modified internal shell, which is porous and is made of aragonite. Except for spirula , they are the only coleoid cephalopods with a shell with a phragmocone divided into chambers separated by septa. [ 17 ]
However, a growing body of molecular and biological data indicate that at least certain shell features have evolved many times, independently. [38] The nacreous layer of shells is a complex structure, but rather than being difficult to evolve, it has in fact arisen many times convergently. [ 38 ]