190 Aquatic Organisms 



for climbing, and the abdominal appendages for swim- 

 ming and for jumping. The body is smooth and pale; 

 often greenish in color. The scuds are quick and active. 

 They dart about amid green water-weeds, usually 

 keeping well to shelter, and they swim freely and 

 rapidly when disturbed. The following figure shows 

 the three commonest genera of the eastern United 

 States. 



The scuds are herbivores, and they abound among 

 green water plants every~where. They are of much 

 importance as food for fishes. They are hardy, and 

 capable of maintaining themselves under stress of 



Fig. 97. Mysis stenolepis. (After Paulmier). 



competition. They carry their young in a 

 pectoral broodpouch until well developed ; and 

 altho they are not so prolific as are many 

 other aquatic herbivores, yet they have possibilities 

 of very considerable increase, as is shown by the fol- 

 lowing figures for Gammarus fasciatus, taken from 

 Embody' s studies of 191 1: 



Reproductive season at Ithaca, Apr. i8th to Nov. 3d, 

 includes 199 days. 



Average number of eggs laid at a time 22. Egg lay- 

 ing repeated on an average of 1 1 days. 



Age of the yotingest egg-laying female 39 days : num- 

 ber of her eggs, 6. 



Possible progeny of a single pair 24221 annually. 



Asellus is the commonest representative of the order 

 Isopoda; broad, dorsally -flattened crustaceans of some- 



