THE CRAYFISH AND OTHER CRUSTACEA 97 



Finally the larva becomes attached by its antennae to a 

 crab. Then it begins to lose its specialized organs, pushes 

 its branches into the host, and becomes a fleshy and almost 

 structureless mass. Knowledge of the early development 

 of an animal sometimes affords a clue to its true relation- 

 ship which could be ascertained in no other way. 



There are multitudes of species of the lower Crustacea 

 which are usually of small size. Some of the most common 

 are the water fleas (Daphnia and allied forms) which are 

 frequently found in fresh water. The minute copepods 

 which are abundant both in fresh water and in the sea 

 form an important factor in the food supply of aquatic 

 animals, especially fishes. 



