CRUSTACEA AS FOOD FOR YOUNG FISH. 9 



which varies in length from 0.60 to 0.70 mm. (about 0.023 to 0.027 

 inch) in length and is usually abundant whenever algse are present. 

 It is a free-swimming form, occurring below the surface. 



Cypris vireois, 1.7 to 2 mm. (about 0.065 to 0.078 inch) in length, 

 is common in muddy ponds from April to July and occurs from 

 Massachusetts to Mexico. Cypris fuscata, about 1.30 mm. (about 

 0.051 inch) long, is common everywhere in shallow grassy ponds and 

 in swamps from April to June. 



MALACOSTRACA. 



This group of crustaceans comprises the generally larger forms 

 and in fresh water falls into four distinct groups or orders : Isopoda, 

 Amphipoda, Mysidacea, and Decapoda. 



Fresh- wat^r Malacostraca are found practically all over the world, 

 excepting in the antarctic regions. The various forms are adapted 

 to different surroundings, some pre- 

 ferring rivers, others creeks or 

 ponds, or pools or springs. 



The above - mentioned orders 

 differ much in their superficial ap- 

 pearance — shape of body, size, color, 

 and structure. All Malacostraca 

 have separate sexes and propaga- 

 tion is by eggs. The different 

 groups of Malacostraca differ 

 greatly in their economic value, but 

 all are of importance on account of 

 the service rendered as scavengers 

 and owing to the fact that they fig. 5.— isopod ; '• Asei." Aseiiuscom- 



p^ 1 J- ii • 1 munis. Enlarged twice. After Ward 



serve as rood tor other animals. and wwppie. 



ISOPODA. 



Isopods are represented by several species, one of the most common 

 of which is Asellus commiuiiis, about 20 mm. (0.78 inch) in length, 

 which is widely distributed, occurring in ponds, ditches, etc., and 

 living among decaying vegetable matter. It is related to the com- 

 mon terrestrial sow bug or pill bug, to which superficially it bears 

 but a remote resemblance. It feeds on water cress and other soft 

 plants, living or dead, and to some extent on animal matter. It 

 reproduces rapidly and, in spite of its cannibal habits when young, 

 often becomes exceedingly abundant. An adult Asellus coTrwiunis 

 produces about 60 eggs at a time and carries them in a brood pouch, 

 underneath her broad thorax, during incubation. There is a new 

 brood every five or six weeks during the early summer season (Need- 

 ham and Lloyd, 1916). 



AMPHIPODA OR SCUDS. 



The fresh-water amphipods or scuds are more commonly known as 

 fresh- water shrimp or just shrimp, although various forms of the sea- 

 shore and of brackish and sea water are more widely known as sand 

 hopper, water fleas, sea fleas, etc. Some of the scuds are of wide 

 distribution, the most common species being comprised in the genera 

 Gammarus, Hyalella, and Eucrangonyx. The amphipods generally 



51503°— 21 2 



