240 INTRODUCTION TO CONCHOLOGY. 



These inollusks are conjectured to exist in greater or less 

 abundance in all pelagic waters, and, like the Carinaria, 

 they generally swim in company. It is unusual to see them 

 during the day, or in stormy weather, as they delight in 

 calm seas and the stillness of the evening twilight. 



With regard to their distinctive features of organization, 

 they are intermediate between the Cephalopoda and Gaste- 

 ropoda : the foot of the former, being modified into a kind of 

 swimming fin, ofi^ers a transition as it were to the locomotive 

 tentacula of the others. Their structure is somewhat gela- 

 tinous and soft, and their hinder parts are enclosed within a 

 glassy shell; their heads are rather indistinct, and almost 

 or altogether destitute of eyes, but their mantles are large 

 and thin, and capable of great dilatation and contraction. 

 Their mouths are most curiously constructed, being subter- 

 minal, and provided on either side with one or more mem- 

 branaceous wing-like swimming fins; their breathing organs 

 are pectinated and internal, similar to those of the Gastro- 

 pods, but in some species so exceedingly minute, as scarcely 

 to be discerned by even a strong magnifying power. They 

 are not all provided with shells, but when these occur, they 

 are either globose or cylindrical, and for the most part 

 partially or altogether enveloped by the mantle ; in some 



