ON CRUSTACEA. 
251 
crust. None of their appendages, that is to say the palpi, 
the antenna), or the feet, appear to be modified for the exercise 
of tact. 
Nevertheless, we may, in this respect, admit of some 
shades between the divers Crustacea in proportion to the 
greater or less solidity of their testa. Thus the brachyurous 
decapods, and some of the macrouri, have their envelope ge- 
nerally thicker, more calcareous, and more solid than all the 
others. After them, come certain macrourous decapods, as 
Palaemon, Peneus, &c. and the stomapods, whose testa is 
flexible, corneous, semi-transparent ; and, finally, the ento- 
mostraca of the genera Apus and Branchipus, the softest of 
all these animals, which have a skin so fine that in all parts 
of the body it may prove a sufficiently delicate organ of tact. 
The male branchipoda have at the head two soft organs capa- 
ble of being rolled into a spiral form, like a sort of proboscis, 
and which may possibly be endued with a great degree of 
sensibility. 
At a certain period of the year, indeed, Crustacea, even 
the hardest, lose their old envelope, and are clothed with a 
new testa, extremely thin and very flexible. Then their sen- 
sibility is very great, and for fear of being wounded by the 
contact of external bodies, they remain concealed in the 
hollows of the rocks, until their new skin has acquired a 
sufficient consistence to protect them against accidents of 
this kind. 
The skin of the Crustacea is composed of many superposed 
layers, as has been ascertained by M. de Blainville. In 
Palinurus, we may distinguish, 1st. a first internal stratum, 
more fibrous than the others, translucid, evidently living and 
forming the interior lamina of the parts which do not become 
crustaceous ; 2d. a second stratum, more cartilaginous, of an 
opaline colour, a little thicker, and still appertaining to the 
