ON CRUSTACEA. 251 



crust. None of their appendages, that is to say the palpi, 

 the antennae, 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, imtil 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 



