148 CRUSTACEA, ARACHNIDES, INSECTA. 



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 the tarsus. 



The hardness of the calcareous or horny * envelope of the greater 

 number of these animals, is owing to that of the excretion, which 

 is interposed between the dennis and epidermis, or what is termed 

 in man the mucous tissue. This excretion 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 eyesf, which resemble a very minute lens, gene- 

 rally three in number, and arranged in a triangle 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 J. Other 

 organs which for the first time are here presented to us, and which 

 are found in two of these classes, the Crustacea and the Insecta, 

 the antenna, are articulated filaments varying greatly in form, and 

 frequently according to the sex, attached to the head, appearing to be 

 peculiarly devoted to a delicate sense of touch, and perhaps to some 

 other kind of sensation of which we have no idea, but which may 

 refer to the state of the atmosphere. 



These animals enjoy the sense of smell and that of hearing. Some 

 authors place the seat of the first in the antennae |J, others, M. Dumeril 



* According to M. Aug. Odicr, Mem. de la Soc. d'Hist. Nat., 1823, t. I, p. 29 

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

 He states that the phosphate of lime forms the great 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 preceding 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. 



f Occelli stemmata. 



$ See the Memoir of Marcel de Serres on the simple and compound eyes of 

 Insects, Montpellier, 1815, 8vo. Also the observations of M. de Blainville on the 

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

 subject at another period. 



And even in the Arachnides, but under different forms, and with different func- 

 tions. 



|| As regards insects, and when they are claviform, or terminate in a club more 

 or less developed, or furnished with numerous hairs. According to M. Robineau, 

 Desvoidy, the intermediate antennte of the Crustacea Decapoda are the olfactory 



