40 



MOLLUSC A. 



admitting the water into the gill-chamber, and passing 

 it out again across the end of the anus in order to sup- 

 ply food sifted out of the water, and also to carry off 

 feculent matter, as in the oyster. The little muscular 

 junction of the flaps of the mantle at the termination 

 of the gills, the fleshy bar, in the oyster being here, 

 however, prolonged so as to form the partition between 

 the tubes. The mantle edges are joined, and also pro- 

 longed so as to complete a tube on either side of the bar. 



The foot is a little muscular nub, not usually — ex- 

 cept in specimens allowed to die slowly m cold fresh 

 water — so large as in the figure. When alive, this is 

 capable of great extension, and can pass out to a con- 

 siderable distance through the hole (Fig. 3, op) pre- 

 viously described. 



If now the symmetrical distribution of the parts are 

 observed, as was done in the oyster, it may be at once 

 seen that the forward part of the animal is not near the 

 beaks, but near the larger end, and the hinder part at 

 the funnel or smaller end, and the beaks or young 

 shells situated on the back. 



To finish up these comparisons, place the oyster and 

 clam side by side on their convex edges, which coin- 

 cide in both, and observe, not only these contrasts, but 

 the equality of the valves of the shell in the clam, 

 which, unlike those of the oyster, are as symmetrical 

 as the internal parts (Fig. 16). 



The young of the clam is also free-moving and sym- 

 metrical, and we can account for its continuing so only 

 on the supposition that the possession of a foot enables 

 it to maintain an upright position favorable to the sym- 

 metrical growth of the parts, instead of being forced 

 either to lie or to swim on one side, like Pecten. 



