538 THE SENSES AND SENSORY ORGANS. 



insect is far the largest nerve in its body, that it is connected 

 with enormous ganglia, and that the whole of the nerve fibres 

 they imagine they have seen would not make up one-thousandth 

 part of its bulk. 



No doubt the optic nerve appears much less bulky in 

 specimens made by imbedding the object in paraffin ; when, 

 however, the optic nerve has been treated with alcohol, chloro- 

 form, often with ether, and soaked for hours in hot paraffin, it 

 is no longer opaque, but transparent, and all but its proteid 

 constituents have been dissolved. The optic nerve seen in the 

 recent condition is large and opaque, is quite white, and its 

 fibres are by no means fine. Whilst the recent dioptron is 

 transparent the whole of the nervous structures are opaque 

 with the exception of the outer ends of the retinal rods. 



The Tracheae of the Retina. Two sets of tracheal vessels may 

 be said to belong to the retina. One set, which are very fine 

 and which penetrate between the fibres of the optic nerve, 

 form a delicate network all over each fasciculus of retinal end 

 organs, as far as the outer extremity of their inner segments. 

 The other, which consists of moderate-sized vessels, form a 

 reticulum in the basilar membrane between the retinulae and 

 give off the tracheal vessels of the dioptron, which perforate 

 the cuticular layer of the basilar membrane and course out- 

 wards between the great rods. 



In Eristalis the vessels of this reticulum are very much 

 larger than in the Blow-fly, and the corresponding vessels of the 

 dioptron are also very large. Exner speaks of such tracheae as 

 a tapetum. 



6. THE DEVELOPMENT OF THE COMPOUND EYE. 

 a. In Arthropoda generally. 



It has long been known that the number of ommatea in the 

 compound eye increases with each ecdysis in many Crustacea, 

 new ommatea appearing around the circumference of the eye. 

 The same kind of increase takes place in the ametabolic 



