52 BARKER : 



As stated above, the simple eye of the arthropod superfi- 

 cially resembles the vertebrate eye, but the anatomical details 

 are quite different. In the vertebrate eye the optic nerve pen- 

 etrates the retina and spreads out on the inside, forming the 

 fibrous layer which covers the rods and cones, whose free ends 

 are directed outward, and the space between the fibrous layer 

 and the lens is occupied by the vitreous humor ; but in the 

 simple eye of the arthropod the bacillary layer is lacking or 

 is represented by the visual rods or ommatidia Oetinophores 

 of Patten), occupying the greater part of the space between 

 the nerve layer and the lenses. Owing to the great convexity 

 of these simple eyes, their foci are short and the images 

 formed by them are small. 



The simple eye is the only kind possessed by Myriapods 

 (with a notable exception). Arachnids, the larvce of most 

 insects, and some of the Crustaccea. 



Tlie notable exception of the Myriapoda is Ccrniatia, which 

 has compound eyes only. 



One of the most interesting of the simple eye class is a 

 terrestrial crustacean which nearly everybody has seen, some 

 knowing it as the wood louse and others as the hog louse. 

 Its name in science, however, is Onisc'us. This creature 

 has two groups of simple eyes, with about twenty ocelli 

 in each group (Plate II, Fig. 6), a number which is exceeded 

 in few arthropods. 



Some species of Onisais live under the bark on rotting 

 windfalls, and others beneath stones in the open fields, always 

 shrinking from the light and running for cover when exposed. 



Many insects have ocelli in addition to the. compound eyes, 

 usually three in number, arranged in a triangle. They are, 

 however, unknown in butterflies with one exception, and are 

 possessed by few beetles. Amongst the moths, however, 

 there are many having two. 



Several authors, including Folsom, agree in the statement 

 that the arthropod with simple eyes must be very near sighted 

 owing to the great convexit\' of the lens and the absence of 



