132 ARTICULATED ANIMALS, 



sorts : the simple eyes, which present themselves under the 

 form of a very small lens, commonly three in number, and 

 disposed triangularly on the summit of the head ; and the 

 composite, or eyes with facets, whose surface is divided into 

 an infinity of different lenses, called facets, and to each of 

 which a thread of the optic nerve corresponds. These two 

 sorts may be united, or separated, according to the genera. 

 We do not yet know whether, when they exist simultane- 

 ously, their functions are essentially different or not ; but in 

 both vision is performed by means very different from those 

 by which it is caused in the eye of the vertehrata. 



Other organs which appear in this division for the first 

 time, and which are found in two of these classes, the Crus- 

 tacea and the insects, namely, the antennce, are articulated 

 filaments, infinitely diversified in form, often so even accord- 

 ing to the sexes, attached to the head, appearing eminently 

 adapted for the purposes of a delicate tact, and perhaps of 

 some other species of sensation of which we have no idea, 

 but which possibly may refer to the state of the atmosphere. 



These animals enjoy the sense of smell, and that of hear- 

 ing. Some place the seat of the former in the antennae. 

 Others, like M. Dumeril, at the orifices of the tracheae ; 

 others again, like M. Marcel de Serres, in the palpi. But 

 these opinions are by no means founded on positive and 

 conclusive facts. With regard to hearing, the decapod Crus- 

 tacea, and some orthoptera, are the only animals of this 

 division which have a visible ear. 



In the mouth of these animals there Is a great analogy, 

 which, according to the observations of M. Savigny, even 

 extends, relatively at least to the hexapod insects, to those 

 which can only suck fluid aliments. 



Those which are called grinders, because they have jaws 

 proper for the trituration of food, always have them in lateral 

 pairs, placed one before the other> The anterior pair are 



