256 ORGANS OF THE SENSES. 



they have a cup-like form, each is covered with its convex 

 cornea, having a transparent axis, and behind this is the 

 little hard spherical crystalline lens, together with the vitreous 

 humour, the chorid and its pigment, and the optic nerve. 

 Each ocellus receives a branch of a trachea along with its 

 nervous filament, and the tracheae have been traced also into 

 the compound eyes of perfect insects. Most generally there 

 are three ocelli, in perfect insects, behind each compound 

 eye, on the sides of the head. Some insects, as the claviger, 

 appear to possess neither simple nor compound eyes, nor any 

 other organ of vision, like many of the helminthoid animals 

 beneath them. The eyes of the higher forms of insects are 

 thus numerous, and varied in their directions, from the ses- 

 sile and immoveable character of these organs, and to suit 

 the rapid and varied movements of these animals \ and they 

 compensate for the want of the external protecting apparatus 

 of higher classes, by the density and insensibility of the out- 

 ward exposed parts of these organs, which they cleanse from 

 adhering matter by the brushes of hair developed on their 

 tarsi, or some other moveable parts. Hairs are often deve- 

 loped from the surface of the compound eyes, originating from 

 the depressions between the hexagonal cornese, and some- 

 times the margins of the corneae are themselves extended 

 outward like the hexagonal tubes of a honey-comb, as in the 

 sty fops. The eyes of the arachnida are the largest and most 

 perfect forms of the ocelli met with in the articulated classes ; 

 like those of the myriapods they are simple and sessile organs, 

 while those of Crustacea are compound, like those of insects, 

 and are generally pedunculated. From two to twelve of 

 these smooth eyes are found in the arachnida, and the largest 

 forms of the organs are presented by the scorpions where 

 their structure has consequently been most satisfactorily ex- 

 amined. They are commonly arranged symmetrically in one 

 or two transverse rows on the upper and fore part of the ce- 

 phalo-thorax, as we observe those, generally eight in number, 

 arranged on the back of the spiders. Beneath the prominent 

 convex cornea in the large ocelli of the scorpion there is a 

 spherical firm transparent lens, more like that of a molluscous 

 or of a vertebrated animal than that commonly found in the 

 entomoid articulata. There is a considerable vitreous humour 

 filling half of the eye-ball, placed behind the crystalline lens, 



