488 



THE OCEAN WORLD. 



sion. As they increase in size they gradually approach the shore, 

 which they had for the moment abandoned, and return to the places 

 inhabited by the parent Crustaceans. 



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

 it would be difficult, except on the clearest evidence, to determine the 

 species from which they proceed. Former naturalists considered the 

 embryo cray-fish (Palinurus) to belong to a distinct genera, which 

 they designated Pliyllosoma. It is now known, however, that these 

 are the young of the higher forms of Crustaceans -undergoing meta- 

 morphosis. In the various forms of Macroura the metamorphosis is 

 less decided than in the Bracliyura. In the fresh- water cray-fish no 

 change whatever takes place. Dissatisfied with the uncertainty of 



former experiments, 

 Mr. Couch undertook 

 a series of observa- 

 tions, which are 

 recorded in the 

 proceedings of the 

 Cornwall Polytechnic 

 Society, in which he 

 established the fact 

 that metamorphosis 

 takes place in the fol- 

 lowing genera : Can- 

 cer, Xanthb, Plum- 

 nus, Carcinus, Port- 

 unus, Maja, Gralathea, 

 Hornarus, and Palinurus. " Metamorphosis has been demonstrated," 

 says Dr. Bell, " in no less than seventeen genera of the Brachyurous 

 order of Decapoda, in which it is most decided and obvious ; in Lep- 

 topodia, Majacea, Cancer, Portunidse, Pinnoteres, and Grapsus. In 

 the Anomourous order it is seen in the Pagurus, Porcellana, and 

 Galathea; and in the Macrouran order in Homarus, Palinurus, 

 Palaemon, and Crangon." 



The swimming of these creatures is produced by flexions and 

 expansions of the tail, and by repeated beating motions of the claws, 

 the tail acting as a sort of vibratile oar; aided by which they maintain 

 themselves in the water and facilitate their progress. As the shell 



Fig. 334. Pottunus variegatus, male, 

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



