HIGHER CRUSTACEANS (MALACOSTRACA) 833 



but here the eggs are attached to the pleopods and are carried 

 under the abdomen of the mother till the young are ready to hatch. 



Within these brood-pouches the embryonal development takes 

 place. After the young have reached a more or less advanced 

 stage, they leave the egg, but always remain a certain time in the 

 brood-room of the mother. In the Isopods (Asellus) the young 

 leave the egg at a rather early stage, and they have yet to undergo 

 considerable changes ; in the other groups the larva hatches in a more 

 advanced stage, and the subsequent changes are slight. In none 

 of our fresh-water crustaceans are free swimming larvae known, but 

 these might be present in the families Atyidae and Palaemonidae, 

 in which such have been observed in their allied marine forms. 



Of the life history of the Isopods, Amphipods, Mysidacea, and 

 most of the Decapods, practically nothing is known. However, in 

 the Decapod-genus Cambarus (crayfishes) more complete informa- 

 tion is at hand. 



After hatching, the young crayfishes remain for a short time 

 with the mother, but soon leave her, and grow in the beginning at 

 a rather rapid rate, each increase in size being connected with a 

 moulting of the shell. Later, they grow less rapidly, and, after the 

 first summer, we may distinguish, in general, a spring and an 

 autumn moult. The total length of life seems to be several years : 

 four, five, or even more. Sexual maturity may be reached within 

 the first year, at least in some species. Males and females attain 

 about the same size, but in most species (except the burrowing) 

 the male possesses much stronger chelae than the female. 



A very peculiar difference is found among the males, which at 

 first was believed to be dimorphism, but has now been recognized 

 as alternating conditions in the life of the same individual. Males 

 of the first form have been distinguished from males of the second 

 form; the former is the fully developed and sexually potent form, 

 while the latter is an impotent form. Generally speaking the first 

 form is assumed by the male in autumn, and lasts through the 

 winter (copulating season), while the other is assumed in spring, 

 and lasts through the summer. Young males, in their first summer, 

 are always of the second form. The difference between these two 

 forms is seen in the sexual organs: in the males of the second form 



