THE METAMORPHOSES OF CRUSTACEA 77 



(Fig. 30), when hatched, possess all the appendages 

 of the adult except the first pair of swimmerets and 

 the uropods, or outer plates of the tail-fan. The 

 carapace is almost globular, owing to the presence 

 inside the body of a large amount of food-yolk, 

 which supplies the nourishment necessary for the 

 young animal in the early stages of its development. 

 The chelae have hooked tips, by means of which the 

 young animal clings securely to the swimmerets of 

 the mother. After a time it moults, and the uropods 

 are set free, the chelae lose their hooked tips, the 

 carapace assumes nearly its final shape (the food- 

 yolk having been largely absorbed), and the young 

 Crayfish leaves the protection of its parent, to shift 

 for itself. The essential point of difference between 

 the development of the Crayfish and that of the 

 closely related Lobster (see Fig. 8, p. 28) is not so 

 much that the changes in structure which occur 

 after hatching are less profound in the former case, 

 but that there is no free larval stage. In the 

 Lobster the earlier stages are capable of indepen- 

 dent existence, and they differ from the full-grown 

 animal not only in structure, but also in habits, 

 swimming at the surface instead of creeping at the 

 bottom of the sea. 



A similar case to that of the Crayfishes is found in 

 the River Crabs of tropical countries, belonging to 

 the family Potamonidae. These Crabs are as closely 

 related to some marine Crabs as are the Crayfishes 



