:mollusca. 419 



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

 Those of the second group, including as well animals which 

 inhabit the land, as those which live in fresh water or in 

 the sea, executeprogressive motion by means of crawling 

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

 disc, termed a foot. 



Section 1. — Natantia. 



Organs of progressive motion Jitted for szoimming. 



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 the surrounding fluid in which they float about, 

 having their motions in a great measure regulated by its 

 changes. It is however probable, that, by means of some 

 contractile movements, they are capable of varying their 

 density, and ot rising or sinking in the water. They 

 swim slowly, even with their utmost efforts. M. Cuvier 

 has distributed the animals of this subdivision into two 

 classes, which he has termed Cephalopoda and Pteropoda. 



Class 1.-— Cephalopoda. 



Fins in the form of tentacula, surrounding the mouth. 



The cephalopoda, in reference to their external appear- 

 ance, may be regarded as consisting of two parts ; the 

 tunic or sac, which contains the viscera, and the head, sur- 

 rounded by the tentacula. The sac is, in some species, in 

 the form of a purse, destitute of any appendages, while in 

 others, it exhibits fin-like expansions. It varies consider- 

 ably in its consistence : in some, it is strengthened on the 

 back internally, by corneous ribs or testaceous plates, and 

 in others, it is protected externally by spiral shells. In 



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