PTEROPODA. 293 



CHAPTER XIX, 



Sixth Class of Mollusc a. 

 Pteropoda.* 



The Pteropoda are of small diinensions, but met 

 witli in astonishing 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 Avith their gambolings ; and thus the 

 store of provisions necessary to render the waters of 

 the ocean habitable, for animals of higher grade in 

 the scale of life, is still further increased. The great 

 character which distinguishes this class is derived 

 from the structure of their locomotive apparatus. 

 This is only adapted for swimming, and consists of 

 two broad fleshy expansions, attached like a pair of 

 wings to each side of the neck, forming moveal:)le 

 fins, by the aid of which these little beings dance 

 merrily among the foaming waves, now sinking, and 

 again rising to the surface, until some passing whale, 

 opening its enormous jaws, engulfs multitudes of 

 such tiny prey, and terminates their brief existence. 



The two wing-like expansions, although they ex- 

 ternally appear to be separate instruments, are in 

 reality but one organ, being made up entirely of 

 muscular bands that pass right through the neck, 

 and spread out on each side, in the substance of 

 the wing, forming an apparatus exactly comparable 

 to tlie double-]3addled oar with which the Green- 

 lander so dexterously steers his kajac or canoe 

 through the very seas inhabited by the little crea- 

 tures we are describing. We select as examples of 

 this order 



* Trrep6v, jitevon, a wing : r.ovs, iroSo's, pous, polos, afoot — n-ing- 

 footed. 



