418 PHILOSOPHY OF ZOOLOGY. 



Molluscous animals divide themselves into two great 

 divisions, distinguished from each other by well defined 

 characters. In the first, the presence of a head may be 

 recognised, together with eyes and even ears, in some of 

 the groups. In the other, containing animals much less 

 perfect in their organization, there is no head, nor vestiges 

 of eyes or ears in any of the groups. The former have 

 been termed Mollusca cephala, the latter Mollusca acepJia- 

 la. This arrangement was first employed by the celebrat- 

 ed CuviEK, and afterwards by Lamark, and other mo- 

 dern systematical writers. In the last work of the former 

 naturalist, this method is departed from, and six orders are 

 now constituted of equal rank, instead of being placed sub- 

 ordinate to the two primary divisions, under which, not- 

 withstanding, they can be suitably distributed. 



Division I. 

 MOLLUSCA CEPHALA. 



Head distinct from the body, bearing the lips or jaws. 



The head, or the anterior part of the body on which 

 zoologists have bestowed that denomination, possesses more 

 or less freedom of motion, and, on the dorsal aspect, sup- 

 ports either tentacula or eyes, frequently both. The ani- 

 mals of this division exhibit so many modifications of form 

 and structure, in all the series of organs, that the positive 

 characters which they possess in common are few in num- 

 ber. They easily admit, therefore, of subdivision into in- 

 ferior groups, which exhibit well marked characters of dis- 

 tinction. Two of these groups occupy a primary rank, 

 the others being included under them as subordinate sec- 

 tions. The animals of the first of these groups are 

 all inhabitants of the water, and execute their progressive 

 motion through that element by organs fitted for swimming. 



