74 NATURE STUDY REVIEW [9 :3— Mar., 1913 



amphipods are flattened as if they had been placed with their 

 sides against the jaws of a vise and pressure appHed. They also 

 differ from isopods in the manner of locomotion ; instead of crawl- 

 ing, th,ey usually swim. This swimming is accomplished as in 

 the crayfish by bending the abdomen against the under surface 

 of the thorax and then straightening it. The color varies in 

 these animals more than in isopods. Th,e older animals are a 

 dull brown, while the younger ones are usually a pale whitish 

 green. They may be an inch long, but are more often taken 

 about a quarter of an inch in length. 



Nine different species of amphipods occur near Chicago. Two 

 are present only in the deeper parts of Lake Michigan; four 

 other species are found in wells, so that only three species occur 

 in the surface waters. There is no easy way to distinguish these 

 species as was true of the isopods of the region, but the following 

 general characters may h,elp give an idea of the species at hand: 



1. Gammarus fasciatus is a stream inhabiting species and is 

 most abundant in small swift-flowing streams. This species usu- 

 ally swims on its side, although at times some individuals will 

 swim ventral side down. 



2. Hyallella knickerbockerii occurs most often in slow-flow- 

 ing but not stagnant streams, as in the Calumet River at Clark 

 Junction, Ind., and in young ponds such as those near Pine, Ind. 

 About fifty per cent of these swim on their side. 



3. Eucrangonyx gracilis is found in ponds, especially in old 

 forest ponds such as contain isopods. Thus they are present in 

 the ponds near Hessville, Ind., and in pools in tamarack swamps. 

 These very often swim ventral side downward when first taken 

 and may be distinguished in this way. 



The manner of swimming is easily modified by external con- 

 ditions and is only of value as a suggestion of the species at hand 

 if observed when the amphipods are first taken. 



Gammarus is abundant in the city park lagoons, especially 

 among water plants. They are best collected by means of a tow 

 net. This kind of a net is made by fastening a tapering bag 

 made of a good grade of cheese cloth to a round or square wire 

 support. Strings are fastened to the wire in three or four differ- 

 ent plac,es and united to one long cord by means of which the 

 net may be drawn. For collecting from a bank it is better to 

 place a stone in the apex ; with the aid of this weight the net may 

 be thrown som,e distance from the bank. 



The breeding season of these animals begins as soon as the ice 

 is out of the water in the spring and two or three broods are 



