BREATHING ORGANS IN PROSOBRANCMIATA 



15 



grouped us Azygohranchiata. Even in Haliotis the right l)rancliia 

 is rather smaller than the left, while the great size of the attach- 

 ment muscle causes the whole branchial cavity to become pushed 

 over towards the left sidt\ In those forms wliich in other respects 

 most nearly approach the Zjgobranchiata, namely, the Trochidae, 

 Neritidae, and Turbinidae, the Ijranchia has two rows of filaments, 

 one on each sidt' of the long axis, while in all other I'roso- 

 1)ranchiata tliere is but one row (see Fig. 79, p. IG!)). 



In the great majority of marine Prosobranchiata tlu^ l)ranc]iia 

 is securely concealed ^\"ithin a chamber or pouch (the respiratory 

 cavity), which is placed on the left dorsal side of the animal, 

 generally near the back oi the neck. For l)reathing purposes, 

 water h;is to be conveyed into this chamber, and again expelled 

 after it has passed over the branchia. Tn the majority of the 

 vegetable- feeding molluscs (e.g. Lit- 

 torina, CcritJtiniii, TrocJ/ns) water 

 is carried into the chamljer by a 

 simple prolongation of one of the 

 lol)es or lappets of tlie mantle, and 

 makes its exit by the same way, 

 the incomini!,- and outgoing currents 

 l)eing separated 1-}' a valve -like 

 fringe depending from the lolie. 

 In the carnivorous molluscs, on tlie 

 other hand, a regular tul)e, tlie 

 hranchidl siphon, which is more 

 or less closed, has been developed 

 from a fold of the mantle surface, 

 for the sijccial ])urpose of con- 

 ducting water to the branchia. 

 After perforiniug its ])ur])ose there, 

 the s])eut \v;iter d»»cs not return 

 through the siphon, l)ut is cou- 

 ducte(l towiirds the iiuus by vibra- 

 tile cilia situated on the branchiae themselves, 



Fig. 62. — TiulJio lacri.ssuiiit (liucl., 

 sliowiiig liniiichial .siphon S ; F, F, F, 

 loot ; OP, operculum ; P, penis ; 

 Pr, proboscis ; T, T, tentacles. 

 (After Quoy ;ind Gaiinard.) 



In a large 



manlier of cases, this si])hoii is ])rotected throughout its entire 

 length by ii special prolongation of the shell called the ccmal. 

 Sometimes, as in Buvciuu m and J'tirpiira, this canal is little more 

 than a mere notch in the ' mouth ' of the shell, but in many of 

 the ]\Iuricidae ^e.g. J/! h<i nafell inn , fcnn.isphuf, trihaliis) the canal 



