376 



CONCHIFERA. 



with the external world, and deprived even of the means of com- 

 munication with each other, we might naturally expect their or- 

 ganization to correspond in its general feebleness with the cir- 

 cumscribed means of enjoyment and limited capabilities of loco- 

 motion allotted to them. Numerous species, indeed, are from the 

 period of their birth firmly fixed to the rock which gives them 

 support, by a calcareous exudation that cements their shells to its 

 surface, as is familiarly exemplified in the case of the common 

 Oyster ; or else, as the Mussels, anchor themselves securely and 

 immoveably by unyielding cables of their own construction. The 

 Scallop, unattached, but scarcely better adapted for changing its 

 position, rudely flaps together the valves of its expanded 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 which they dig the holes 

 wherein they lie concealed, and crawl, or even leap about, upon 

 the shore. Many, as the Pholades, penetrate the solid rocks and 

 stones, and excavate therein the caverns that they inhabit ; or, 

 in the case of the Teredo, with dangerous industry bore into the 

 bottoms of ships or submerged wood of any description, and 

 silently destroy by their insidious ravages the piers or dikes which 

 human labour has erected. 



(411.) Following our usual custom, we shall select for examina- 

 tion one of the most simply organized bivalves for the purpose of 

 illustrating the general structure which characterizes the class ; 

 and in the common Scallop (Pecten Jacobtea) we have a species 

 well adapted to exhibit the principal features of their economy. 

 On separating the two valves of the shell in the animal before us, 

 we at once perceive that each is lined internally with a thin and 

 semitransparent membrane (fig- 176, a, A), which, like the shells, 

 encloses the body of the Mollusk in the same way that the leaves 

 of a book are contained between its covers. The circumference 

 of these outer membranes, which form the mantle, is, in this case, 

 quite free and unconnected, except in the immediate vicinity 

 of the hinge that unites the two valves. The borders of the 

 mantle are thickened, and surrounded with a delicate fringe of 

 retractile filaments ; they moreover present a decided glandular 

 appearance, and secrete colouring matter of various tints, similar 

 to those seen upon the exterior of the shell : the glandular 

 margins of the mantle form in fact the apparatus by which 

 the extension of the shell is effected, and by them its outer 



