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 Avhat 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 dermis 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 Avith eyes, which are of two kinds ; 
simple or smooth eyesf, Avhich 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 corresj^onding 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 l)y means differing widely 
from those which produce it in the eye of the Vertebrata^. Other 
organs Avhich for the first time are here presented to us, and Avhich 
are found in two of these classes, the Crustacea and the Insecta§, 
the antenncp, 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 pei’haps to some 
other kind of sensation of Avhich Ave have no idea, but Avhich may 
refer to the state of the atmosphere. 
Tliese animals enjoy the sense of smell and that of hearing. Some 
authors jdace the seat of the first in the antennae |1, others, M. Dumeril 
* According to M. Aug. Odier, Mem. de la Soc. d'Hist. N^al., 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 animalg. 
Other observations, those of M. Straus in particular, demonstrate that the teguments 
here replace the skin of the Vertebi’ata, or that they do not fornr a true skeleton. 
Those of M. Odier also militate against all the analogies attempted upon this 
subject. 
f Occelli siemmafa. 
J See the Memoir of Marcel de SeiTes 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. Fhilomatique. We shall return to this 
subject at another period. 
§ And even in the Arachnides, but under different forms, and with different func- 
tions. 
11 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 anteunre of the Crustacea Deeapoda are the olfactory 
