34 MANUAL OF THE MOLLUSCA. 



Those bivalves which have no siphons, and even those in 

 which the mantle is divided into two lobes, are pvo\ided with 

 valves or folds which render the respiratory channels just as 

 complete in effect. These currents are not in any way connected 

 with the opening and closing of the valves, which is oidy done 

 in moving ; or in efforts to expel irritating particles.* 



In some of the gasteropoda the respii-atory organs form tufts, 

 exposed on the back and sides (as in the nudib ranches), or pro- 

 tected by a fold of the mantle (as in the inferobranches and 

 tectibranclies of Cuvier). But in most the mantle is inflected, 

 and forms a vaulted chamber over the back of the neck, in which 

 are contained the pectinated or plume-like gills (fig. 61). In the 

 carnivorous gasteropods [sijjJionostomata) the water passes into 

 this chamber through a siphon, formed by a prolongation of the 

 upper margin of the mantle, and protected by the canal of the 

 shell ; after traversing the length of the gill, it returns and es- 

 capes through a posterior siphon, generally less developed, but 

 very long in ovulum volva, and forming a tubular spine in typhis. 



In the plant-eating sea-snails {holostomata), there is no true 

 siphon, but one of the " neck-lappets" is sometimes curled up 

 and performs the same office, as in paludina and ampullaria (fig. 

 84). The in-coming and out-going currents in the branchial 

 chamber, are kept apart by a valve-like fringe, continued from 

 the neck-lappet. The out-current is still more efiectually isolated 

 m Jissurella, haliotis, and dentallum, where it escapes by a hole in 

 the shell, far removed from the point at which it entered. Near 

 this outlet are the anal, renal, and generative orifices. 



The cephalopods have two or four plume-like gills, symme- 

 trically placed in a branchial chamber, situated on the under-side 



* If a river-mussel be placed in a glass of water, and fine sand let fall 

 gently over its respiratory orifices, the particles will be seen to rebound from 

 the vicinity of the upper apertm-e, whilst they enter the lower one rapidly. 

 But as this kind of food is not palatable, the creature wiU soon give a plunge 

 with its foot, and closing its valves, spirt the water (and with it the sand) 

 fi-om both orifices ; the motion of the foot is, of coui-se, intended to change 

 its position. 



