SURFACE-SWIMMING FAUNA (INVERTEBRATES). 89 



foundly changed that they might at first sight 

 be relegated to another class of organisms al- 

 together. 



Now we must remember that animals that live 

 in the surface waters must be prepared to keep 

 afloat for the whole period of their lives from 

 the time they are hatched until they fall a prey 

 to some voracious enemy. Under ordinary cir- 

 cumstances, they never find an opportunity of 

 resting, either on the sea-bottom or on any float- 

 ing substance. 



If a Copepod is watched in a tumbler of water 

 it will be seen to give a number of strokes with 

 its long antennae and then to rest suspended for 

 a few seconds; a few more strokes follow and 

 then another pause, and so on. During the 

 period of rest the body sinks slowly, sometimes 

 almost imperceptibly, but never so much that it 

 cannot recover its position in the water after the 

 first few strokes. 



It must be clear to the reader that the less 

 it sinks during the pause the less will be the 

 muscular activity required to regain its position, 

 and that, consequently, every mechanical con- 

 trivance that its body possesses to diminish its 

 tendency to sink will be a saving of muscular 

 and nervous energy. 



A very simple experiment will demonstrate 

 that a body which presents a considerable sur- 

 face to the water, sinks more slowly than one 

 of the same weight that is round and compact. 

 If we take two equal pieces of silver paper and 

 roll one of them into a tight little ball, leaving 

 the other as a flat sheet, and then let them sink 



