274 The Evolution of Optics. 



which has probably arisen, as we have already said, as a 

 modification of dermal structures and a nerve of common 

 sensation, and in which the nerve is distributed at the back 

 of the retina and the terminal filaments extend forward 

 toward the light ; the other, the vertebrate eye, which in 

 its development is a portion of the brain, and in which the 

 nerve is distributed upon the anterior surface of the retina 

 and the tips are turned down into the substance of the retina 

 and away from the light. Many explanations of this curious 

 phenomenon have been attempted, but none are altogether 

 satisfactory. It has been suggested that the primitive ver- 

 tebrate must have been a transparent animal, and 'that the 

 eye or eyes were situated inside the brain, as in the ascidian 

 tadpole, which would explain the fact that the light-receiv- 

 ing portion of the eye is, in its development, an offshoot of 

 the brain. This whole subject is extremely interesting, but 

 it has not been as fully and definitely elaborated and ex- 

 plained as we could wish. 



Among the invertebrates the greatest variety of optical 

 apparatus is met with. In most lower forms the eye-speck 

 is found with or without a condensing lens, and vision 

 probably amounts to little more than ability to differentiate 

 between different degrees of illumination. The usual type 

 of eye is the one found on the invertebrate plan, as we have 

 already mentioned ; but to this the eyes of Pecten and a few 

 other classes form exceptions, for in them the optic-nerve 

 terminals are turned away from the light as in the verte- 

 brate eye. The nerve does not, however, pierce the retina 

 from behind, as in the latter type, but bends over from the 

 side. In certain Southern snails we find an eye still more 

 closely resembling the eyes of the vertebrata, for not only 

 is the distribution of the retinal elements the same, but the 

 nerve also enters from behind, as in the vertebrate eye. But 

 this is seen to be only an arriving at the same goal by differ- 

 ent routes and not an evidence of relation, for the eye in its 

 development is a dermal structure, a true invertebrate eye. 



We may take the eyes of insects as typical examples of 

 the invertebrate eye, for in this class the eyes have obtained 

 great perfection. The eyes of insects are of two kinds the 

 simple eye or ocellus, and the compound eye. Ocelli differ 

 somewhat in structure, but consist essentially of a corneal 

 lens, either spherical or cylindrical, a mass of transparent 

 cells, a retina, optic nerve, and pigment. This eye sees as 

 does the human eye that is, by a reversed image but is very 



