466 



CHAPTER XVTI. 



PTEROPODS. 



"Natura non facit saltus." 



THE position of the Pteropoda is somewhat unsatisfactory Theii 

 organisation in some respects places them below the level of the 

 Gasteropods ; yet the general feeling amongst naturalists has been 

 to assign them a place between the Gasteropods and the most highly 

 organised of the molluscs, the Cephalopods. The number of genera 

 and species is very much less than that of the other great classes 

 of molluscs. 



The principal characteristic of the Pteropoda is a membranous 

 expansion situated on the right and left side of their head, from 

 which they take their name of Pteropoda, from -m-e^v, wing, and vovs, 

 gen. iroSbs, foot, winged feet. 



The wings or flappers with which they are provided enable them 

 to pass rapidly through the water, reminding us strongly of the 

 movements of a butterfly, or other winged insect, and like them, 

 their motions are long continued. They advance in this manner in 

 a given direction, while the body or the shell remains in an oblique 

 or vertical position. 



These little molluscs may be seen to ascend to the surface very 

 suddenly, turn themselves in a determinate space, or rather swim, 

 without appearing to change their place, while sustaining themselves 

 at the same height. If anything alarms them they fold up their 

 flappers, and descend to such a depth in their watery world as will 

 give them the security they seek. They thus pass their lives in the 

 open sea far from any other shelter, except that yielded by the gulf 

 weed and other algae. In appearance and habits, these small and 

 sometimes microscopic creatures resemble the fry of some other 

 forms of mollusca. They literally swarm both in tropical and 

 arctic seas ; and are sometimes so numerous as to colour the ocean 

 for leagues. They are the principal food of whales and sea-birds in 

 high latitudes, rarely approaching the coast. Only one or two 



