MOLLUSCA. 



CLASS IV. ACEPHALA.* 



ON comparing a bivalve with an univalve Mol- 

 lusk, we cannot but at once perceive the inferiority 

 of the former in the scale of existence, particularly 

 in the limited character of its motion from place 

 to place, and in the defect of those organs of sense 

 which are possessed by the latter. Indeed, one of the 

 most familiar bivalves, the common Oyster, has be- 

 come a proverb of insensate apathy, as if it were the 

 lowest form of animal life. Encased within heavy 

 stony plates, so formed as to shut out almost all 

 perception of what is going on without, acute sensi- 

 bilities or highly developed organs would have been 

 thrown away on them. " Numerous species 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 exemplified 

 in the case of the common Oyster; or else, as the 

 Mussels, anchor themselves securely and immove- 

 ably, by unyielding cables of their own construc- 

 tion. The Scallop, unattached, but scarcely better 

 adapted for changing its position, rudely flaps to- 

 gether the valves of its expanded shell, and thus, 

 by repeated jerks, succeeds in effecting a backward 

 movement ; while the Cockles, destined to burrow 



* A, without, and KttfiK^, kepliale, the head 



