26 MAIS^UAL OF THE MOLLUSCA. 



round orifice, on tlie right side of the animal, whicli opens and 

 closes at irregular intervals. The air in this cavity seems to 

 renew itself -with sufficient rapidity (by the law of difi'usion), 

 without any special mechanism. 



In the aquatic shell-fish respiration is T>erformed by the 

 mantle, or by a portion of it specialised, and forming a gill 

 {hranchia). It is afiected by the arms in all the Irachiopoda, 

 while the mantle serves as an auxiliary. In the ordinary 

 bivalves the gills form two membranous plates on each side of 

 the body ; the muscular mantle is still sometimes united, form - 

 ing a chamber with two orifices, into one of which the watei 

 flows, whilst it escapes from the other ; there is a third opening 

 in front for the foot, but this in no wise influences the branchial 

 circulation. Sometimes the orifices are drawn out into long 

 tubes or siphons, especially in those shell-fish which burrow in 

 sand (Figs. 19 and 7). 



Fig. 19. Bivalve with long siphons.* 



Those bivalves which have no siphons, and even those in 

 which the mantle is divided into two lobes, are provided with 

 valves or folds which render the respiratory channels just as 

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

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

 in moving, or in efforts to expel irritating particles.! 



In some of the gasteropoda the respiratory organs form tufts, 

 exposed on the back and sides (as in the nudihranchs), or "^vo- 

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

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



* Fig. 19. Psammohia vespertina, Chemn. after Poli, reduced one-half. The arrows 

 indicate the direction of the current ; r s, respiratoiy siphon ; e s, excuixent siphon ; 

 /, foot. 



t If a river-mussel fcs 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 aperture, whilst they enter the lower one rapidly. But as this kind of food is 

 not palatable, the creature will soon give a plunge with its foot, and closing its valves, 

 epirt the water (and with it the sand) from both orifices ; the motion of the foot is, of 

 course, intended to change its position. 



t Mr. Collingwood (Annals of Nat. Hist, for 1861), in discussing what function these 

 tufts or papillse perform, concludes thnt moq^hologically and physiologically they ai-e 

 not branchiae. 



