26 MANUAL OP THE MOLLTJSCA. 



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

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

 renew itself with sufficient rapidity (by the law of diffusion), 

 without any special mechanism. 



In the aquatic shell-fish respiration is performed by the 

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

 (branchia). It is affected by the arms in all the hrachiopoda, 

 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 water 

 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 (Pigs. 19 and 7). 



Fig. 19. Bivalve vrith 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 effect. 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. f 



In some of the gasteropoda the respiratory organs form tufts, 

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

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

 tectihranchs of Cuvier)4 But in most the mantle is inflected, 



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

 indicate the direction of the current ; r s, respiratory siphon ; e s, excurrent siphon ; 

 /, foot. 



t 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 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, 

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

 course, intended to change its position. 



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

 tufts or papillae perform, concludes that morphologically and physiologically they are 

 not branchise. 



