CRUSTACEA. 



349 



is round, having as yet no appearance of caudal appendages ; of 

 young animals in this condition Muller had formed a distinct 

 genus (Amymonc) :* in about a fortnight they get another pair 

 of legs, and form the genus Nauplius of the same author. They 

 then change their skin for the first time, and present the form of 

 the adult, but with antennae and feet smaller and more slender 

 than in the perfectly mature state. After two other changes of 

 skin they become capable of reproduction. 



Many of the Entomostraca, as for example Daphnia, do not 

 seem to undergo material alterations of form, but simply moult at 

 certain intervals, throwing off their old integument and acquiring a 

 new covering. Nevertheless, even in the Decapoda it is pretty 

 certain that great metamorphoses take place in the external ap- 

 pearance of the young animals, though many contradictory opi- 

 nions concerning their nature are entertained by naturalists. 

 Much confusion, indeed, still exists connected with this important 

 subject. Cavolini long since announced that the embryo of Can- 

 cer depressus exhibited at birth a singular and uncouth appear- 

 ance, of which he gave a very tolerable representation;")" and Mr. 

 Thompson, in a late number of the Philosophical Transactions, 

 has rendered it certain that even 

 in the developement of the com- 

 mon crab, so different is the out- 

 ward form of the newly-hatched 

 embryo from that of the adult, 

 that the former has been describ- 

 ed as a distinct species, and even 

 grouped among the ENTOMOS- 

 TRACA, under the name of Zoea 

 pelagica. On leaving the egg, 

 according to the author alluded 

 to, the young crab presents a cu- 

 rious and grotesque figure (Jig. 

 164) : its body is hemispherical, 

 and its back prolonged upwards 

 into a horn-like appendage ; the 

 feet are scarcely visible, with the 



exception of the two last pairs, which are ciliated like those of a 

 Branchiopod, and formed for swimming. The tail is longer than 



Fig. 164. 



* Latreille, Regne Animal, vol. iv. 



t Sulla Generazione del Pesci edei Granchi. 4to. Naples, 1787. 



