192 CONCHIFERA. 



groups, all constructed upon the same plan as that described above, and all 

 actively employed in taking in and ejecting the currents that bring them nu- 

 triment. 



CHAPTER XVII. 



THIRD CLASS OF MOLLUSKS. 

 ORDINARY BIVALVES. CONCHIFERA.* 



' I ^HE inhabitants of bivalve shells constitute a very numerous 

 X and important class. Encased in dense and massive cover- 

 ings, of such construction as to preclude the possibility of their 

 maintaining more than a very imperfect intercourse with the ex- 

 ternal world, and deprived even of the means of communication 

 with each other, we might naturally expect their organization to 

 correspond in its general feebleness with the circumscribed means 

 of enjoyment and limited capabilities of locomotion allotted to 

 them. Numerous species are, from the period of their birth, firmly 

 cemented to the rock that gives them support, as is familiarly 

 exemplified by the common oyster, or else, as the mussels, anchor 

 themselves securely by unyielding cables of their own construc- 

 tion. The scallop, unattached, but scarcely better adapted for 

 changing its position, rudely flaps together the valves of its ex- 

 panded shell, and thus, by repeated jerks, succeeds in effecting a 

 retrogressive movement ; while the cockles, destined to burrow 

 in the sand, are furnished with a tongue-like foot, by means of 

 which they dig holes, wherein they lie concealed, or crawl, or even 

 leap about upon the shore. Many, as the Pholades, penetrate the 

 solid rocks and stones, and excavate therein the caverns they in- 

 habit, or, as in the instance of the Teredo, with dangerous industry, 

 bore into the bottoms of ships, or submerged wood of any de- 

 scription, and si 1 . ?ntly destroy, by their perfidious ravages, the piers 

 or dykes that human labour has erected. The general structure 

 of these animals may be readily understood by examining any of 

 the species common in our markets. We will select 



The Scallop (Pecten} for special description. On opening the shells of the 

 scallop, we see, inside each valve, first a thin and transparent membrane en- 

 closing the entire animal. This is the mantle. Its edges are thickened and 

 surrounded with a fringe of very sensitive fleshy filaments ; they are likewise 



* Concha, a shell ; fero, I carry. 



