226 



Discoveries of Muller and others in the 



this covering. Towards the bulb of the optic nerve it again 

 appears. (Jig, 39.) 



40 



In the iucanus Cervus, the cornea (Jig, 40. a) is of extra- 

 ordinary thickness, and its facets are accordingly so much 

 elongated as to appear like prisms. The cones have their 

 bases nearly in contact with the cornea, and at that part are 

 apparently without pigment ; towards their apices, where 

 they are attached to the nervous filaments, they are sur- 

 rounded with pigment of a violet colour. The nervous fila- 

 ments, also, in the greater part of their course from the optic 

 nerve, are without any investiture of coloured matter. 



In the Meloe majalis, the cornea is thick, and provided 

 with a small number of large facets. Its inner surface is 

 covered with very convex and almost parabolic eminences, 

 which appear to be the crystalline bodies; and in contact 



1 ^^ ^ ^ ^ ^ ^ ^ with them are the nervous 



filaments, which are coated 

 with a dark brown pigment 

 (Jig. 41.). In the ^schia 

 grandis, the crystalline bo- 

 dies, if they can be said to be present, are extremely small 



(Jig' 42.) ; and such also seems 

 to be the case with a few other 





42 



insects. 



In the Peneus sulcatus, the eyes 

 are very large. Behind the very 

 thin cornea, which has quadran- 

 gular facets, is a layer of short 

 crystalline bodies, placed close to 

 each other, with their lateral sur- 

 faces covered with a whitish or whitish green and opaque pig- 

 ment. They are quadrangular, 

 and only about as long again as 

 they are wide. The filaments of 

 the optic nerve are invested 

 ^ IBHlllill throughout their whole length 



with dark-coloured pigment. 

 (Jig, 43.) In many of the other 

 Crustacea, the facets of the 



