CRUSTACEANS. 615 
presents the smallest changes of form. The Amphipods, on the 
other hand, come from the egg in a form similar to that of the 
perfect animal. Many Isopods, also, undergo only slight changes of 
form ; many acquire another pair of feet in addition to those which 
they had on leaving the egg. The young Limuli, according to 
Mitne Epwarps, leave the egg without that ensiform appendage 
or pointed tail which so strikingly distinguishes the full-grown 
animal. 
The crustaceans cast their shell several times. In younger indi- 
viduals these moultings succeed each other at shorter periods, but 
in full-grown animals, at least in the decapods, the hard calcareous 
shell is cast off only once a year. In the River-cray the moulting 
occurs towards the end of summer. The hard shell begins to 
loosen itself from the body, which in the meantime is replaced 
by a new covering situated beneath the former. The animal makes 
many movements and contortions, until at last a fissure occurs on 
the back between the abdomen and the large cephalothorax, from 
which the animal with the fore part of its body and its feet makes 
its appearance. At last the hinder part of the body divests itself 
of its old covering.- For these observations we are indebted to 
ReEAumvr. In the short-tailed crabs the shell splits on each side 
along the epimera. The cast-off shell presents perfectly the form of 
the living animal?. 
As long as the new shell is still thin and flexible, the crays and 
crabs are very sensitive. ‘They then conceal themselves in holes, 
until the new shell has attained sufficient hardness, for which a few 
days only are requisite. 
With the shell the inner coat or the epithelium of the stomach 
is renewed in the crays and crabs. When a new internal tunic of 
the stomach has been formed the old one 1s cast off and dissolved in 
the cavity of the stomach. It has been supposed that the two 
round calcareous plates which are situated on each side of the 
contested the observations of THOMPSON, but afterwards admitted, with that upright- 
ness which always belongs to the inquirer after truth alone, his mistake respecting this 
important discovery, and that he had done the English naturalist a wrong ; Op. cit. 
s. 46. See also the observations of Du CANE on the metamorphoses of Cancer menas, 
Annals of nat. Hist. 11. pp. 438—440, Pl. x1. 
1 See Réaumur Mém. de UAcad. des Sc. 1718, p. 263 and foll., Cottinson Phil, 
Trans. 1746 and 1751, Minne Epwarps Hist. nat. des Crust. 1. pp. 53—57; comp. 
also RymeR Jones Animal Kingdom, 2d edit. 1855, pp. 434—436. 
