ANNULOSA. 531 



The organ of touch is not generally distributed over the 

 body, for there are few parts capable of receiving an im- 

 pression, or, at least, of being applied to the surface of 

 bodies. The antenna, organs peculiar to annulose animals, 

 are generally considered as appropriated to this sense. 

 These organs are two or more in number, and are present 

 in all the Crustacea and insects, but wanting in the arach- 

 nida. They are situate on the head, usually between the 

 eyes and the mouth. They consist of a number of joints, 

 determinate in the individuals of a species, and, in general, 

 capable, by their flexibility, of examining the condition of 

 the surface of a body. Those which have long setaceous 

 antennae, upon approaching a body, move them along its 

 surface with considerable rapidity, thrust them into its ca- 

 vities, and in this manner appear to become acquainted 

 with its form. Individuals of the same species meeting to- 

 gether, examine each other by means of their antennae. 



♦' This multitude of filaments, perpendicular to the cornea, have behind 

 them a membrane which serves them all as a base, and which is consequent- 

 ly nearly parallel to the cornea. This membrane is very fine, and of a black- 

 ish colour, which is not caused by a pigment, but extends to its most inti- 

 mate texture. We observe in it very fine, whitish lines, which are tracheae, 

 and which produce still finer branches, that penetrate between the hexagon- 

 al filaments as far as the cornea. By analogy, we may name this mem- 

 brane the ckoroides. 



" A thin expansion of the optic nerve is applied to the posterior part of 

 the choroides. This is a real nervous membrane, perfectly similar to the 

 retina of red blooded animals. It appears that the white filaments, which 

 form the particular retina of the different ocular surfaces, are productions of 

 this general retina, which perforates the membrane I have named choroides, 

 by a multitude of small and almost imperceptible holes, 



" To obtain a distinct view of all these parts, it is nscessary to cut off 

 the head of an insect that has the eyes large, and dissect it posteriorly ; each 

 part will then be removed in an order the reverse of that in which 1 have 

 described them." 



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