VISION IN INSECTS, 137 



as their radiated disposition will permit ; but they are 

 much smaller in diameter than the cylinders, and, 

 notwithstanding their slenderness, appear, under the 

 microscope, somewhat opaque and of a fibrous texture. 

 Surrounded by a dark choroid secretion (i), these fila- 

 ments, on account of their great tenuity, cause the 

 pigment to appear much thicker and darker, when re- 

 garded en masse, than that portion of it represented as 

 passing between the cylinders. These latter are al- 

 most in immediate contact with each other : the ner 

 vous filaments, on the contrary, are separated by spaces 

 much exceeding in size their own diameter. 



' In the centre of the eye is the optic ganglion (J), 

 which, however pulpy and homogeneous it may appear 

 at first sight, exhibits nevertheless a fibrous and radi- 

 ated structure when submitted to moderate compres- 

 sion. Indeed, it may in some degree be regarded as 

 the optic nerve passing into the filamentary arrange- 

 ment observed a little farther from the centre. 



^ Such are the anatomical details exhibited in the 

 eye of the gray Lihcllida and of other insects, with 

 some modifications to be hereafter noticed. In exam- 

 ining each of these parts, we may, to a certain extent, 

 refer them hypothetically to the structures forming the 

 simple eye of the vertebrated animals. In fact, we 

 find in these compound eyes a nervous filament at- 

 tached to the extremity of a transparent body repre- 

 senting the vitreous humour and crystalline lens ; a 

 transparent cornea covering externally this apparatus; 

 and a choroid membrane, represented here by a co- 

 loured pigment, which surrounds, as in the vertebrat- 

 ed animalsj these minute organs of refraction and sen- 

 sation. We may still further remark that the pigment, 

 continuous in all parts, although varying in thickness, 

 forms between the cornea and the transparent or crys- 

 talline cyhnder, an iris (/), or at least a uvea, which 



VOL. XII. 12* 



