CRUSTACEA. 



513 



The form of the larva differs so much from that of the adult, 

 that it would be difficult, except on the clearest evidence, to deter- 

 mine the species from which they proceed. Former naturalists 

 considered the embryo cray-fish {Palinurus) to belong to a distinct 

 genus, which they designated Phyllosoma. It is now known, how- 

 ever, that these Phyllosoma are but the young of the higher forms 

 of Crustacea undergoing metamorphosis. In the various forms of 

 Macrura the metamorphosis is less decided than in the Brachyura. 

 In the fresh-water cray-fish no marked metamorphosis whatever 



Fig. 342. Portunus vanegatus, male. 

 a, external antenna ; b, external foot-jaw ; c, tail or abdomen. 



takes place. Dissatisfied with the uncertainty of former experiments, 

 Mr. Couch undertook a series of observations, which he has re- 

 corded in the proceedings of the Cornwall Polytechnic Society, in 

 which he established the fact that a metamorphosis takes place in 

 the following genera : Cancer, Xantho, Pilumnus, Cardnus, Portunus, 

 Maja, Galathea, Homarus, and Palinurus. " Metamorphosis has 

 been demonstrated," says Dr. Bell, " in no less than seventeen genera 

 of the Brachyurous sub-order of Decapoda, in all which it is most 

 decided and obvious ; in Leptopodia, Maja, Cancer, Portunus, 

 Pinnotheres, and Grapsus. In the Anomourous sub-order it is seen 

 in Pagurus, Porcellana, and Galathea ; and in the Macrourous sub- 

 order in Homarus, Palinurus, Palamon, and Crangon" 



