page_title: Siphon (mollusc)
section_title: Paired siphons of bivalves
hierarchical_section_title: Siphon (mollusc) / Paired siphons of bivalves
caption_attribution_description: Diagram of the internal view of the left valve of a bivalve shell resembling a venerid.
context_page_description: A siphon is an anatomical structure which is part of the body of aquatic molluscs in three classes: Gastropoda, Bivalvia and Cephalopoda.
  Siphons in molluscs are tube-like structures in which water flows. The water flow is used for one or more purposes such as locomotion, feeding, respiration, and reproduction. The siphon is part of the mantle of the mollusc, and the water flow is directed to the mantle cavity.
  A single siphon occurs in some gastropods. In those bivalves which have siphons, the siphons are paired. In cephalopods, there is a single siphon or funnel which is known as a hyponome.
context_section_description: Those bivalves that have siphons, have two of them. Not all bivalves have siphons however: those that live on or above the substrate, as is the case in scallops, oysters, etc., do not need them. Only those bivalves that burrow in sediment, and live buried in the sediment, need to use these tube-like structures. The function of these siphons is to reach up to the surface of the sediment, so that the animal is able to respire, feed, and excrete, and also to reproduce.
  The deeper a bivalve species lives in the sediment, the longer its siphons are. Bivalves which have extremely long siphons, like the geoducks pictured here, live very deeply buried, and are hard to dig up when clamming.
  Many bivalves that have siphons can withdraw them completely into the shell when needed, but this is not true of all species. Bivalves that can withdraw the siphons into the shell have a "pallial sinus", a sort of pocket, into which the siphons can fit when they are withdrawn, so that the two shell valves can close properly. The existence of this pocket shows even in an empty shell, as a visible indentation in the pallial line, a line which runs along parallel to the ventral margin of the shell.
  The bivalve's two siphons are situated at the posterior edge of the mantle cavity. There is an inhalant or incurrent siphon, and an exhalant or excurrent siphon. The water is circulated by the action of the gills. Usually water enters the mantle cavity through the inhalant siphon, moves over the gills, and leaves through the exhalant siphon. The water current is utilized for respiration, but also for filter feeding, excretion, and reproduction.
caption: Diagramatic drawing of the inside of one valve of a bivalve such as a venerid: pallial sinus on the lower left, at the posterior end of the clam
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