PTEROPODA. 219 



around them ? They are created to live and are born amidst the 

 fields of sea-weed, which voyagers describe with amazement as 

 covering leagues of sea within the tropics ;* and to enable them 

 to attach themselves to the narrow leaves of this " Gulf-weed " 

 (Sargassuin], they are furnished with a small sucker, which, like 

 a cupping-glass, applied against the surface of the leaf, suspends 

 them without exertion. This little sucking disk is situated on 

 the upper and hinder margin of the fin. DR. JOHNSTON. 



CHAPTER XX. 

 SIXTH CLASS OF MOLLUSKS. 



PTEROPODA.f 



THE Pteropoda are of small dimensions, but met with 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 with their gambollings ; 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 in- 

 creased. 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 ex- 

 pa.nsions, attached like a pair of wings to each side of the neck, 

 forming moveable 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 externally appear 

 to be separate instruments, are in reality but one organ, being 

 made up entirely of muscular bands that' pass right through the 



* The Gulf-weed forms a floating continent. In steering towards the equator it is 

 usually first observed in fields and islands near the coast of Madeira, whence it spreads 

 to the Gulf of Mexico and the Carribbean Sea. After sweeping round these shores, 

 it escapes by the Gulf of Florida, whence it progresses towards the Azores. There is 

 a similar circulation of Gulf-weed in the southern hemisphere. 



f- TTTepov, pteron, a iring ; irovs, iroSos, pous, podos, afoot: ti'ing-J 



