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 condi- 
tions. 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 
