832 FRESH-WATER BIOLOGY 



The typical pleopods consist of a simple basal part, with two sub- 

 equal, terminal, articulated branches. But in many cases differen- 

 tiations and reductions are observed, Jthe most important being 

 that of the uropods, referred to above, and the transformation of 

 certain pleopods into copula tory organs in the male. 



In certain forms (Mysidacea and Decapoda) the dorsal shell of 

 the most anterior part of the body (head) is produced backward, 

 and covers more or less the thoracic somites in the shape of a shield, 

 curved down over the sides, which is called the cephalothorax or 

 carapace (car). Very often the carapace has a median anterior 

 projection, called the rostrum (r). 



The branchial apparatus of the Isopods is formed by the pleo- 

 pods. In all other groups special appendages (gills) of the thoracic 

 somites assume this function; they may be attached to the sides 

 of the thorax, or to the basal parts of the thoracic limbs. 



The genital openings of the male are always originally on the 

 coxopodite of the eighth trunk-leg (or fifth peraeopod), those of the 

 female on the sixth (or third peraeopod), but in certain cases either 

 one of these may shift to the sternite. 



All Malacostraca of the fresh water have separate sexes, and 

 very often the males are distinguished by secondary sexual char- 

 acters (size, color, development of claws). Copulation, or rather 

 conjugation, seems to take place in all of them, although this has 

 been observed in detail only in very few forms : it is best known in 

 the crayfishes. 



Propagation is by eggs. In the smaller forms (Isopoda, Amphi- 

 poda, Mysidacea), very little is known about propagation and 

 development, and with regard to the North American forms of 

 these groups investigations are altogether lacking. But from what 

 is known of exotic, chiefly European, forms it is probable that in all 

 the eggs are carried by the female for a certain period, before the 

 young are set free. In the Isopods, the female develops during 

 the breeding season peculiar lamellae at the base of some thoracic 

 legs (four pairs in Asellus), which serve to cover and to hold the eggs. 

 In the Amphipods and Mysidacea similar, but greatly variable, de- 

 vices are present. In the Decapods, no such apparatus is known, 



