The latter group has some members that have started to develop a tracheal 

 system for air-breathing and so have become able to leave the water and dwell 

 in moist places on hind. They are often common in damp cellars and under 

 bixirds or rocks in yards and gardens. Some of them can roll into a ball, when 

 disturbed, and have received the name of pillbugs. The others are commonly 

 called sovv'bugs or wood-lice. All are flattened dorso-ventrally, the aquatic 

 Isopoda extremely so. These usually hide in and feed upon water plants and 

 themselves are eagerly sought and eaten by fishes. Unlike the other groups 

 of Malacostraca the Isopoda have no thoracic gills, but instead have the ab- 

 dominal appendages especially modified to serve this function. 



The Aviphipoda are also flattened, but in the other dimension, so that 

 they appear to have been designed to slip between the stems of water plants. 

 The female amphipod (like the Isopoda) carries its eggs in a brood pouch 

 formed by outgrowths from the thoracic appendages. The amphipods also 

 make good food for young fishes. 



The Decapoda are the largest and most conspicuous of the fresh-water 

 Crustacea. The crayfishes look much like miniature lobsters except that their 

 chelae or large pinchers are alike. They are frequently miscalled crabs, but 

 the crabs have ver>^ small abdomens carried tucked under the rounded cephalo- 

 thorax and are to be found only along the coasts. The common name of cray- 

 fish is probably derived from the more descriptive term of "crevice-fish'". They 

 can retreat from danger with the greatest speed by flipping backwards, so that 

 to "crawfish out" of a situation has become a common phrase in our language. 



Crayfishes are of much interest ecologically, since the various species show 

 markedly ditFering preferences as to habitat, some living only in large streams, 

 some only in still ponds, and some forsaking even these to live in muddy bur- 

 rows often topped by five or six inch chimneys of excavated material. As 

 much diversity is shown in reproduction, some species mating only in the 

 autumn to produce young the next spring and some species apparently mating 

 freely at any time of the year. Even the body color varies with the locality 

 and the normal dull brown may be replaced by a decided blue or red. 



The identification of crayfish is complicated because the main distinguish- 

 ing characters must be based on the sexual differences of the mature males. The 

 sexually potent males, with horny tips on the first pair of abdominal appendages 

 and with well developed copulatory hooks on the third segments of the second, 

 third or fourth pair of legs, are said to be in the first form. In most crayfishes 

 this form occurs in the autumn and winter, when copulation takes place. In 

 spring and summer male crayfishes are in the second form, usually sexually 

 impotent and with their sexual characters suppressed. 



The eggs are carried by the female on her abdominal appendages or swim- 

 merettes, which she continually waves to aerate the eggs. She checks them 



175 



