ON CRUSTACEA. 253 



state of exhaustion which lasts many days, until the most ex- 

 ternal part of the dermis is filled with calcareous molecules, 

 which re-establish its solidity. 



In the entomostraca, whose growth is much more rapid than 

 that of the Crustacea properly so called, and in which the 

 duration of life is extremely short, the moultings succeed each 

 other quickl}'. Thus, M. de Jurine, who observed the daphnia 

 from the moment of their birth tp that of their first ovideposi- 

 tion, counted, in an interval of seventeen days, eight moultings, 

 which took place pretty nearly within two days' interval of 

 each other. He did not pursue his observations on those 

 changes of skin beyond this point, as they succeed each other 

 in the same manner in summer up to the death of the animal. 

 In winter the moultings are very much retarded, and it is not 

 uncommon for eight or ten days to elapse between them. 



In cypris, apus, branchipus, lynceus, limnadia, and poly- 

 phemus, the moultings are also very frequent. 



In all the Crustacea and entomostraca, it is remarked that 

 the old skin is composed of all the principal or accessory parts 

 which belong to the animal, and that often each spine or each 

 hair there is hollow, and covers another spine or another hair. 

 The chemical analysis of the old testa demonstrates that it is 

 formed of carbonate and phosphate of lime, united to gelatine 

 in various proportions, which are generally in relation to the 

 solidity of the testa. 



We proceed to a few observations on the function of Nu- 

 trition in the Crustacea. Most of these animals feed upon 

 solid substances, and generally on animal matters more or 

 less in a state of decomposition. There are, however, some 

 among them that live on fluids, which they suck from the ani- 

 mals to which they are parasitically attached. 



The former are all provided with a mouth more or less com- 

 plicated, and composite, as we have already seen, and fur- 

 nished with a variable number of organs for mastication or 



