CRUSTACEA, ARACHNIDES, INSECTA. 289 



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 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 with eyes, Avhich are of two kinds : simple 

 eyes or ocelli, which resemble a very minute lens, generally 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 dif- 

 ferent 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 genus. 



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 antennae, 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, others, in the orifices 

 of the tracheae, and Marcel de Serrcs, &c. in the palpi; neither of these 

 opinions, however, is corroborated by positive and conclusive facts. As 

 to the second, it is only in the Crustacea Decapoda and some few of the 

 Orthoptera, that we can find a visible ear. 



The mouth of these animals presents a great analogy, which extends 

 to those which can only feed by the suction of liquid aliment. 



Those called Tritores or Grinders (brayeurs*), on account of their 

 having jaws fitted for triturating their food, always present them in 

 lateral pairs, placed one before the other; the anterior pair are especially 

 called mandibles ; the pieces which cover them before and behind are 

 named labia, and the front one, in particular, labrum. The palpi are 

 articulated filaments attached to the jaws or to the lower lip, and appear 

 to be employed by the animal in recognising its food. The form of 

 these various organs determines the nature of the regimen with as 

 much precision as the teeth of quadrupeds. The ligula, or tongue, 

 commonly adheres to the lower lip. Sometimes, in the Bees and other 

 Hymenopterous Insects, it is considerably elongated, as are also the 

 jaws, forming a. sort of false proboscis (promuscis) at the base of whicli 



