In the great majority of marine Prosobranchiata the branchia is securely concealed within a chamber or pouch (the respiratory cavity), which is placed on the left dorsal side of the animal, generally near the back of the neck. For breathing purposes, water has to be conveyed into this chamber, and again expelled after it has passed over the branchia. In the majority of the vegetable-feeding molluscs (_e.g._ _Littorina_, _Cerithium_, _Trochus_) water is carried into the chamber by a simple prolongation of one of the lobes or lappets of the mantle, and makes its exit by the same way, the incoming and outgoing currents being separated by a valve-like fringe depending from the lobe. In the carnivorous molluscs, on the other hand, a regular tube, the _branchial siphon_, which is more or less closed, has been developed from a fold of the mantle surface, for the special purpose of conducting water to the branchia. After performing its purpose there, the spent water does not return through the siphon, but is conducted towards the anus by vibratile cilia situated on the branchiae themselves. In a large number of cases, this siphon is protected throughout its entire length by a special prolongation of the shell called the canal. Sometimes, as in _Buccinum_ and _Purpura_, this canal is little more than a mere notch in the ‘mouth’ of the shell, but in many of the Muricidae (_e.g._ _M. haustellum_, _tenuispina_, _tribulus_) the canal becomes several inches long, and is set with formidable spines (see Fig. 164, p. 256). In _Dolium_ and _Cassis_ the canal is very short, but the siphon is very long, and is reflected back over the shell. The presence or absence of this siphonal notch or canal forms a fairly accurate indication of the carnivorous or vegetarian tendencies of most marine Prosobranchiata, which have been, on this basis, subdivided into _Siphonostomata_ and _Holostomata_. But this classification is of no particular value, and is seriously weakened by the fact that _Natica_, which is markedly ‘holostomatous,’ is very carnivorous, while _Cerithium_, which has a distinct siphonal notch, is of vegetarian tendencies. In the Zygobranchiata the water, after having aerated the blood in the branchiae, usually escapes by a special hole or holes in the shell, situated either at the apex (_Fissurella_) or along the side of the last whorl (_Haliotis_). In _Pleurotomaria_ the slit answers a similar purpose, serving as a sluice for the ejection of the spent water, and thus preventing the inward current from becoming polluted before it reaches the branchiae (see Fig. 179, p. 266).