94 THE HAUNTS OF LIFE 



Some of the drifters, instead of having the 

 whole body made light, have some special 

 part of it adapted to serve the same end. We 

 can best understand these adaptations if we 

 compare a pelagic animal with one of its own 

 relatives which lives under different conditions. 

 For instance, in many parts of the ocean, there 

 are often to be seen swarms of what are 

 popularly called "sea-butterflies," or, not quite 

 so prettily but more accurately, " winged 

 snails." These little animals are Gasteropod 

 Molluscs, and some of them for there are 

 many different kinds have shells, in one 

 case spirally twisted like that of the snail. 

 But whatever be the form of the shell it is 

 always small and light so as not to add much 

 weight to the body. In place of the fleshy 

 walking "foot" of so many land and shore 

 snails the sea-butterflies have "wings," not 

 in the least like those of a butterfly, but simply 

 outstretched lobes or leaves of muscle which 

 buoy them up and catch the wind so that 

 they seem to be skimming lightly over the 

 surface of the water. Most of the sea- 

 butterflies inhabit warm latitudes, but one 

 kind, with a shell no larger than a pin- 

 head, occurs in such numbers in the Arctic 



