CONCHIFEfi-A. 491 



CHAPTER XX. 



CONCHIFEKA (Lamarck). 

 ACEPHALES TESTACES (Cuvier). 



(1288.) THE great majority of Mollusks which inhabit bivalve shells 

 constitute a very numerous and extensive class, distinguished by certain 

 characters possessed by them in common. Encased in dense and massive 

 coverings of such construction as to preclude the possibility of their 

 maintaining more than a very imperfect intercourse with the external 

 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, 

 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 im- 

 moveably 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 in- 

 habit ; 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 dykes which human 

 labour has erected. 



(1289.) Following our usual custom, we shall select for examination 

 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 Jacobcea) 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. 247, a, h\ 

 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 circum- 

 ference 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 ; 



