CHAPTER XVI. 



OF THE CLASSES OF PTEROPODA AND HETEROPODA. 



899. NOTWITHSTANDING the small size of the class PTEROPODA, 

 it is interesting in many particulars. It may be regarded as 

 representing, in the sub-kingdom Mollusca, the Birds of the Ver- 

 tebrated sub-kingdoms, and the Insects of the Articulated series; 

 and the inconsiderable number of distinct forms which it pre- 

 sents, may probably be accounted for by reverting to the wide 

 departure from the usual Molluscous type, which the animals of 

 this class exhibit in their structure and habits. They are par- 

 ticularly distinguished by the possession of a pair of fin-like 



Fio. 551 CLIO BOREAUS. 



organs, or wings, consisting of an expansion of the mantle on 

 each side of the neck, and furnished with muscular fibres; by 

 the aid of which instruments, they can be rapidly propelled 

 through the water. The body is uniformly symmetrical ; that 

 is, its two sides precisely correspond, a condition evidently 

 favourable for rapid movement. It is from the wing-like cha- 

 racter of these lateral appendages, that the name of the class, 

 which means wing-footed, is derived. Although the number of 

 species belonging to this Class is small, and their dimensions are 



