IS4 THE ANIMALS AND MAN 



feathery feet project from the opened shell when the animal 

 is undisturbed and, waving about in the water, catch small 

 animals which serve as the barnacle's food. The acorn 

 barnacle has no stalk but looks like a low bluntly pointed 

 pyramid, this appearance being due to tlie convergence of 

 the six calcareous plates in its body-wall. Barnacles have 

 no heart nor any blood-vessels and show other degenerate 

 features. This degeneration is due to their fixed life. The 

 young barnacles when hatched from the egg are free-swim- 

 ming larvae as with most other Crustaceans. 



Pill-bugs, wood-lice, or damp-bugs 

 (fig. 63), as they are variously called, 

 may be readily fouhd in concealed 

 moist places, under stones or boards, 

 on damp soil, etc. They run about 

 quickly, and feed chiefly on decaying 

 vegetable matter. They are night- 

 scavengers. Although commonly called 

 "bugs" and supposed to be insects, 

 they really belong to the Crustacea, 

 that class of animals which includes 

 Fig. 63. A damp-bug, the crayfish, lobster, and crabs. Ex- 

 Isopod. (Four times amine the body of a dead pill-bug. It 

 natura size.) j^ ^^^^j ^^^ convex above, rather pur- 



plish or grayish brown, and smooth. Note its division into 

 head, thorax, and abdomen. Find the eyes, the antennae, 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 the 

 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 membranes 

 may be kept damp. If these are not moist, they will not 

 permit the exchange of gases. 



