CRUSTACFA. 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 Piyl/osoma. 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 variegatus, male. 
a, external antenna , 4, external foot-jaw ; ¢, 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, Carcinus, 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, Palemon, and Crangon.” 
HH 
