PTEROPODA. 67 



44. The CARINARI^E have the abdomen (that is to say, a 

 kind of sac enclosing the heart, the liver and some other organs) 

 covered by a symmetrical and conical shell, the apex of which 

 is curved backwards, and the anterior edge covers the base of the 

 branchiae (fg. &6, br}. There is one species in the Mediterra- 

 nean, but three have been discovered. 



The FIOKOL.E have no shell, but, in other respects, very much 

 resemble the Carinariee. 



CLASS OF MOLLUSCA PTEROPODA. 



45. The mollusks composing this group 

 are organized for swimming only: they 

 possess no organ by means of which they 

 can crawl, or even attach themselves to 

 submarine bodies, but continually float in 

 the sea, and move by the assistance of fins, 

 placed like wings on each side of the mouth 

 (fg. 87). They are all of small size, 

 and most of them inhabit the seas of warm 

 countries ; but they are also found in the 

 neighbourhood of the poles. The Clio 

 borealis, for example, abounds so exten- 

 Fig. 87 HYALEA. sively in the arctic regions, that, in spite 

 ol their very small size (scarcely an inch long), they become, 

 in certain seasons, the ordinary food of whales. Their form 

 varies much : some of them are naked ; others are provided with 

 a shell. They constitute several genera, the principal of which 

 are the Clio, Hyalea, Cleodora, Cymbulia, &c. 



44. What are the characters of the genus Carina'ria ? (Carinaria ; from 

 the Latin, cati'na, a keel.) 



45. How are the Pte'ropoda characterized ? (Pteropoda ; from the Grek, 

 Vteron, wing, and pous, foot.) 



