VI BREATHING ORGANS IN PROSOBRANCHIATA 1 55 



chiata, which have been accordingly grouped as Azygohranchiata. 

 Even in Rallotls the right branchia is rather smaller than the 

 left, while the great size of the attachment muscle causes the 

 whole branchial cavity to become pushed over towards the left 

 side. In those forms which in other respects most nearly 

 approach the Zygobranchiata, namely, the Trochidae, Neritidae, 

 and Turbinidae, the branchia has two rows of filaments, one on 

 each side of the long axis, while in all other Prosobranchiata 

 there is but one row (see Fig. 79, p. 169). 



In the great majority of marine Prosobranchiata the branchia 

 is securely concealed within a chamber or pouch (the respira- 

 tory 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, 

 TrochiLs) 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 pur- 

 pose of conducting water to the ^,^, Q^.-BulUa laevissima Gmeh, 

 branchia. After performing its showing branchial siphon S ; 



purpose there, the spent water does ^' J; ^^tr^^'^'^ 



not return through the siphon, but cles. (After Quoy and Gaimard.) 



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 



