No. I.] EYES OF MOLLUSCS AND ARTHROPODS. 69 



nalia, produced by the ramification of the inter-cellular nerve- 

 fibres. Each cell, therefore, of these simple ommatidia is 

 capped with a double cuticular layer, which may be continuous 

 over all the cells, or divided more or less distinctly into hexago- 

 nal areas corresponding in size and shape with the outer ends 

 of the cells. 



Now we find that in Area the simple ommatidia described 

 above tend to collect in well-defined groups, forming, according 

 to their arrangement, optic cups or convex, faceted eyes. In the 

 formation of these eyes the ommatidia become more highly 

 developed, the nerve-supply is increased, while the inner cuticu- 

 lar layer thickens and divides into distinct blocks overlying 

 each cell. The retia terminalia extend into these blocks, which 

 are subsequently converted into hexagonal, cuticular columns, 

 or rods. These rods, which correspond to the rods found in 

 the retina of all other eyes, contain, therefore, a specialized 

 part of the rctia terminalia, or a retinidium. Since the 

 retinophora of the Molluscan ommatidium is always double, 

 its overlying rod is also double, and contains an axial nerve- 

 fibre like the retinophora itself, while the rods of the retinulae 

 are always single, and contain no axial nerves. 



The retia terminalia form an irregular network of very fine 

 fibrillae, continuous with each other in all directions ; the 

 fibrillae are most numerous around the outer ends of the 

 epithelial cells, and they are arranged so that most of the fibril- 

 lae are parallel with the surface of the cuticula. It is undoubt- 

 edly this network of nerves which gives the whole surface of the 

 body its sensitiveness to light. 



There is reason to believe that, in order to produce the 

 greatest effect upon the fibrillae, the rays of light must fall 

 upon them at right angles. This result is obtained by arranging 

 the fibrillae in superimposed layers, and by regulating the 

 direction of the rays of light. Axial nerves can give off radi- 

 ating fibrillae arranged in this way more easily than external 

 nerves. The double rods of the retinophoras, therefore, have 

 an advantage in the possession of axial nerves, in virtue of which 

 they gradually assume the most important r61e, while the retin- 

 ulae become modified in other directions. We therefore find 

 in the simpler eyes of the Mollusca, as in Haliotis and Patella, 

 that both single and double rods are present; while, in the more 



