78 THE LIFE OF CRUSTACEA 



to the Lobsters, yet the difference in their mode of 

 development is even more pronounced. Instead of 

 beginning life as minute pelagic zoeae, they leave the 

 shelter of the mother's abdomen as perfectly- formed 

 little Crabs (Fig. 31). 



Amongst the Decapoda, instances of direct develop- 

 ment like those just described are exceptional, but in 

 some of the other orders of the Malacostraca direct 

 development is the rule. In the great division 

 Peracarida, as we have already seen, the females are 



FIG. 31 YOUNG SPECIMEN OF AN AFRICAN RIVER CRAB (Potamon 

 johnstoni), TAKEN FROM THE ABDOMEN OF THE MOTHER. 



MUCH ENLARGED 



The adult of an allied species is figured on Plate XXV 



provided with a pouch, or marsupium (from which 

 the name of the division is derived), in which the 

 eggs are carried. Within this pouch the young 

 undergo the whole of their development, and they 

 only leave it, as a rule, when they have attained the 

 structure of the adults. Among the more familiar 

 representatives of this division, the Sand-hoppers 

 (Amphipoda), the Woodlice (Isopoda), and the 

 Opossum Shrimps (Mysidacea), may be mentioned 



