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The siphon is part of the mantle of the mollusc, and the water flow is directed to (or from) 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.
The term "siphon" is used for a number of structures in human and animal anatomy, either because flowing liquids are involved or because the structure is shaped like a siphon, but in which no actual siphon effect is occurring: see Siphon (disambiguation). There has been a debate if whether the siphon mechanism plays a role in blood circulation ...
A diagram of a hypothetical ancestral mollusc (HAM) with osphradia indicated on the far right side of the image inside the posterior mantle cavity
Simplified structure and mechanism of cephalopodic siphuncle. The siphuncle is used primarily in emptying water from new chambers as the shell grows. [2] To perform this task, the cephalopod increases the saltiness of the blood in the siphuncle, and the water moves from the more dilute chamber into the blood through osmosis.
In certain groups of carnivorous snails, where the siphon is particularly long, the structure of the shell has been modified in order to house and protect the soft structure of the siphon. Thus the siphonal canal is a semi-tubular extension of the aperture of the shell through which the siphon is extended when the animal is active.
The siphon is additionally innervated by about 30 peripheral motor neurons. [2] Kandel and colleagues used preparations of Aplysia californica where individuals were restrained in small aquariums in a manner that the gill was exposed. A tactile stimulus was administered to the siphon and elicited the gill and siphon withdrawal reflex.
The siphonal notch is a noticeable notch situated at the most posterior part of the aperture of the shell, through which the siphon is extended when the animal is active. The notch at the posterior end of the aperture is also called the anal notch, anal sinus, anal canal or posterior canal.
Siphonophorae (from Greek siphōn 'tube' + pherein 'to bear' [2]) is an order within Hydrozoa, which is a class of marine organisms within the phylum Cnidaria.According to the World Register of Marine Species, the order contains 175 species described thus far.