PTEROFODA. 



dense leatliery mantle of an oval form, and considerably more 

 extensive than the cavity containing the viscera. Where not 

 covered by the calcareous laminae, the exterior of the mantle forms 

 a broad edge variously sculptured in different species : but along 

 its central part the shelly plates, generally eight in number, are 

 partially imbedded in its substance ; being, no doubt, secreted by 

 the surface whereunto they are attached. These mollusks, not- 

 withstanding the singularity of their covering, which almost re- 

 minds us of the armour of many ARTICULATA, in their internal 

 anatomy conform exactly to the type of structure common to the 

 Gasteropod orders, and offer no peculiarities of organization worthy 

 of special notice. 



CHAPTER XXIV. 



PTEROPODA* (Cuv.) 



(464.) NEARLY allied to the Gasteropods in their internal or- 

 ganization, but differing from them remarkably in the character 

 and position of their locomotive apparatus, are the PTEROFODA ; 

 a class of mollusks of small dimensions, but met with in astonish- 

 ing quantities, at certain seasons, in various parts of the ocean. 

 So numberless indeed are these little beings in those regions where 

 they are common, that the surface of the sea seems literally alive 

 with their gambolings ; and thus the store of provisions necessary to 

 render the waters of the ocean habitable to animals of higher grade 

 in the scale of life is still further increased. The great character 

 that distinguishes the members of the class upon the investigation 

 of which we are now entering, is derived from the structure of their 

 organs of locomotion. These are only adapted for swimming, and 

 consist of two broad and fleshy expansions, attached like a pair of 

 wings to the sides of the neck, and forming moveable fins ; enabling 

 the little beings to dance merrily among the foamy waves, now 

 sinking, and again rising to the surface, until some passing whale, 

 opening its enormous jaws, engulphs multitudes of such tiny vic- 

 tims, and hence derives the materials for its subsistence. 



(465.) Several distinct genera of Pteropoda have been esta- 



a wing ; vovs, vobos, a foot. 



