BIVALVE MOLLUSCANS. 255 



we shall soon see, increases the resemblance. 

 There is, however, a difference between the Con- 

 dylopes or annulose animals and the Molluscans, 

 which must strike every examiner, the latter 

 cannot be called symmetrical animals, while in 

 the former the most perfect symmetry, both as to 

 number of parts, and their structure, general 

 form, sculpture and painting, prevails ; in the 

 latter this general symmetry seems not to obtain ; 

 in the bimuscular bivalves, indeed, the two shells 

 are generally symmetrical both in form, size, and 

 sculpture, but this does not invariably take place. 

 In many of the unimusculars the upper shell 

 differs from the under, either in size or other 

 particulars; in the escallop shells it is much 

 flatter and more ornamented as to colouring ; 

 and in the animal itself it is not a general 

 principle that each part shall have its counter- 

 part, or, if single, that the two sides shall 

 exactly correspond. This furnishes some addi- 

 tion to the other proofs of the superiority of 

 the Insect over the Molluscan tribes ; symmetry, 

 especially of the external organs and parts, 

 distinguishes all the higher classes from man 

 downwards ; but is continued in the inverte- 

 brate sub-kingdom no further than the Condy- 

 lopes, when it is interrupted or altogether 

 ceases. It must be observed, however, that 

 in the animal of the univalves, a beginning of 

 symmetrical organs appears in the tentacles, 



