2 CRUSTACEA, AKACHNIDES, INSECTA. 



and which is composed of two pieces(l), is called the coxa, 

 or hip ; the following one which is, usually, nearly in a hori- 

 zontal position, the femur, or thigh; and the third, generally 

 vertical, the tibia or leg. To these ensues a suite of small 

 ones which touch the ground, forming the true foot, or what 

 is denominated tiie tarsus. 



The hardness of the calcareous or horny(2) envelope of the 

 greater number of these animals, is owing to that of the ex- 

 cretion, which is interposed between the dermis and epider- 

 mis, or what is termed in man the mucous tissue. This ex- 

 cretion also contains the brilliant and varied colours with which 

 they are so often decorated. 



They are always furnished with eyes, which are of two 

 kinds : simple or smooth eyes(3), which resemble a very mi- 

 nute lens, generally three in number, and arranged in a tri- 

 angle on the summit of the head; and compound eyes, where 

 the surface is divided into an infinitude of different lenses called 

 facets, to each of which there is a corresponding filament of 

 the optic nerve. These two kinds may be either united or 

 separated, according to the genera. Whether their functions 

 be essentially different in those cases where they are found 

 to exist simultaneously, is a problem that remains to be solved ; 

 but vision is effected in both of them by means differing widely 

 from those which produce it in the eye of the Vertebrata(4). 



(1) In many of the Crustacea the second portion of the coxa seems to form 

 part of the thighs. The tibia, as in the Arachnides, is divided into two joints. 



(2) According to M. Aug. Odierj Mem. de la Soc. d'Hist. Nat., 182u, t. I, p. 29 

 et seq., the substance of this envelope is of a peculiar nature, which he calls Chi- 

 tine. He states that the phosphate of lime forms the g-reat mass of all the salts 

 contained in the teguments of Insects, while that in the shell of the Crustacea is 

 but trifling, though it abounds in the carbonate, which is not found in the prece- 

 ding animals. Other observations, those of M. Straus in particular, demonstrate 

 that the teguments here replace the skin of the Vertebrata, or that they do not 

 form a true skeleton. Those of M. Odier also militate against all the analogies 

 attempted upon this subject. 



(3) Ocelli stemmata. 



(4) See the Memoir of Marcel de Serres on the simple and compound eyes of 

 Insects, Montpcllier, 1815, Svo. Also the observations of M. de Blainville on the 

 eyes of the Crustacea, Bullet, de la Soc. Philomatique. We shall return to this 

 subject at another period. 



