MOHLUSCA. 1 1 9 



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, and 

 admit of the others being included under them as subordi- 

 nate sections. In the first of these, the animals are all in- 

 habitants of the water, and perform their progressive mo- 

 tion through that element by organs fitted for swimming. 

 They are destitute of any ventral disc on which to crawl. 

 In the second group, including animals which inhabit the 

 land, as well as those which live in fresh water and in the 

 sea, progressive motion is performed by means of crawling 

 along the surface of objects, the body resting on a ventral 

 disc termed a foot. 



SECT. I. NATANTIA. Organs of progressive motion 

 fitted for swimming*. 



The organs of motion are situate near the anterior ex- 

 tremity of the body, and consist either of flexible tentacula 

 or membranaceous expansions. All the species reside in 

 the sea. They are nearly of the same specific gravity with 



