26 MANUAL OF THE MOLLUSCA. 
round orifice, on the right side of the animal, which opens and 
closes at irregular intervals. The air in this cavity seems to 
renow 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 brachiopoda, 
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 (Figs. 19 and 7). 
Fig. 19. Bivalve with long siphons.* 
Those bivalyes 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 moying, 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 nudibranchs), or pro- 
tected by a fold of the mantle (as in the inferobranchs and 
tectibranchs of Cuyier).{ But in most the mantle is inflected, 
* Fig. 19. Psammobia vespertina, Chemn. after Poli, reduced one-half. The arrows 
indicate the direction of the current; 7 s, respiratory siphon ; e s, excurrent siphon; 
J; foot. 
{ If ariver-mussel 63 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 rapidJy. 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. 
¢ Mr. Collingwood (Annals of Nat. Hist. for 1861), in discussing what function these 
tufts or papille perform, conclndes that morphologically and physiologically they are 
not branchiz, 
