



THE ANIMALS OF THE FRESHWATER AQUARIUM 109 



which the eyes are mounted on stalks. As in wood-lice, breathing 

 is effected by means of delicate outgrowths on the under surface 

 of the abdomen, which are covered over by two protective plates. 

 The abdomen is very small, and the legs are numerous ; if poss- 

 ible the mode of action of the breathing plates should be observed 

 in the living animal, the way in which their covering plates are 

 raised and lowered being interesting. 



The freshwater shrimp is very common in fresh water, where 

 it swims about actively by means of its numerous appendages. 

 Its body is flattened from side to side, a marked point of dis- 

 tinction from Asellus. 



Another group of Crustacea which are copiously represented 

 in fresh water are the water-fleas, of great importance as part of 

 the food of many of the larger animals. With the unaided eye, 

 however, these are hardly more than specks, which swim through 

 the water with a characteristic jerking movement. The water- 

 fleas are often extraordinarily numerous in fresh water, so much 

 so that a glassful of water may seem turbid owing to their abund- 

 ance. A neglected aquarium will also sometimes swarm with 

 them. 



Far more numerous and interesting are the insects of fresh 

 water. Some of these are permanently aquatic, alike as larvae 

 and adults. In this case, as for example in the water-beetles, 

 the adults generally possess wings by means of which they may 

 make their escape if the pool dries up, or may seek mates at a 

 distance from their native pool. Others, like the larvae of caddis- 

 flies and may-flies, of gnats and of dragon-flies, are only aquatic 

 when young, and leave the water at the time of the advent of the 

 winged stage. In many of these cases the larvae can very easily 

 be reared up to the time of the metamorphosis, and every child 

 should be given an opportunity of watching the changes of the 

 commoner forms, just as of watching the progressive changes of 

 the tadpole and its final conversion into a frog. - As some of 

 these larvae are exceedingly voracious, it is in many cases advis- 

 able to rear the different kinds separately, and as many others 

 form very acceptable food for fishes, etc., it is easily seen that 

 the general aquarium is not the most suitable place for rearing 

 the animals. 



