IVORMS, CRAYFISH, CENT IP EDS, ETC. 



157 



that both crab and sea-anemones derive advantage from 

 this arrangement. The sea-anemone, which otherwise 

 cannot move, is carried from place to place by the crab, 

 and so may get a larger supply of food, while the crab is 

 protected from its enemies, the predaceous fishes, by the 

 stinging-threads of the sea-anemone, and also perhaps by 

 the concealment of the shell its presence affords. This 

 living together by two kinds of animals to their mutual 

 advantage is called commensalism or s)'mbiosis. 



Pill-bugs and water-fleas. — Pill-bugs, wood-lice, or 

 damp-bugs (fig. 1 14), as they are variously called, may be 

 readily found in concealed moist 

 places, under stones or boards, on 

 damp soil, etc. They run about 

 quickly, and feed chiefly on decay- 

 ing vegetable matter. They are 

 night-scavengers. Although com- 

 monly called " bugs " and supposed 

 to be insects, they really belong 

 to the crustaceans, that class of ani- 

 mals which includes the crayfish, 

 lobster, and crab. Examine the 

 body of a dead pill-bug. It is oval 

 and convex above, rather purplish or 



grayish brown, and smooth. Note its division into head, 

 thorax, and abdomen. Find the eyes, the antenna;, and 

 the mouth-parts. All the locomotory appendages are 

 adapted for walking or running, not swimming. How 

 many pairs of legs are there.' Find gills and gill-covers. 

 Although pill-bugs do not live in the water they breathe 

 partly at least by means of gills (though they may breathe 

 partly through the skin). It is therefore necessary for 

 them to live in a damp atmosphere, so that the gill mem- 

 branes may be kept damp. If these are not moist, the\' 

 will not permit the exchange of gases. 



Fig. 114. — A damp-bug, 

 Isopod, species not de- 

 termined. (Four times 

 natural size; iVom speci- 

 men. ) 



