482 
THE OCEAN WOULD. 
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 PliyUosoma. 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' Prachyura. In the fresh- water cray-fish no 
change whatever takes place. Dissatisfied with the uncertainty of 
former experiments, Mr. Couch undertook a series of observations, 
which are recorded in the proceedings of the Cornwall Polytechnic 
Society, in which he established the fact that metamorphosis takes 
place in the following genera : Cancer, Zantho, Pelnmnus, Caeernus, 
Portumnus, Maia, 
Galathea, Homarus, 
and Palinurus. “ M e ' 
tamorpliosis has been 
demonstrated,” says 
Dr. Bell, “ in no l ftSS 
than seventeen genera 
of the Brachyurou® 
order of Deeapoda, 111 
which it is most de- 
cided and obvious > 
in the Leptopodiadie, 
Maiadae, Cancerid 08 ’ 
Portumnidae, Pinno- 
Fig. 334. Portumnus variegatus. male. flipriil-r. nnd Crap' 
a, external antenna; b, external jaw-foot ; c, tail or abdomen. tiiuj.iu.ct;, diiu 
sicUe. In the Anoin° u ' 
rous order it is seen in the Pagurus, Porcellana, and Galathea, and m 
the Macrouran order in Homarus, Palinurus, Palaanon, and Crangon- 
The swimming of these creatures is produced hy flexions and 
expansions of the tail, and hy repeated heating motions of the claws, 
the tail acting as a sort of vibratile oar, aided hy which they maintain 
themselves in the water and facilitate their progress. As the sh e 
becomes more solid they get less active, and finally return to tb e 
bottom to cast their shell and assume a new form. 
According to the observations of M. Coste, the young lobster cash 
its shell from eight to ten times in the first year, from five to seven 111 
