Even among the Decapoda, however, there are many species that are hatched from the egg in a form that does not differ essentially from the adult, and are therefore said to have a direct development. This is often the case with species which live in fresh water or in the depths of the sea. For example, the young of the fresh-water Crayfish (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 chelæ 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 chelæ 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 independent 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 Potamonidæ. These Crabs are as closely related to some marine Crabs as are the Crayfishes to the Lobsters, yet the difference in their mode of development is even more pronounced. Instead of beginning life as minute pelagic zoëæ, they leave the shelter of the mother's abdomen as perfectly-formed little Crabs (Fig. 31). [Illustration: 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 XXIII]