730 William Patten 



tective ones, or retinulae; the arrangement of the cells is necessarily 

 such that there is no room for ganglionic ones. This applies te the 

 compound eyes of Arca as well as those of Crustacea and Insects. 

 In the ommateum, there is a very marked efifort to from a single 

 layer, since each of the retinulae and retinophorae, although greatly 

 enlarged at varying levels, invariably extend from the inner to the 

 outer surface of the ommateum. But stili in the str^ctest sense it can- 

 not be regarded as a single layer , since it contains also the nerve 

 fibres, which must be considered as the outer ends of ganglionic cells 

 originally situated between the ommateal cells. 



We shall attempt to follow the ommatidia in their various phases 

 of modification. 



Among those heliophagous organs less specialized than the omma- 

 tidia, may be mentioned Chlorophyll, the red eye spots of zoospores and 

 of Protozoa; the ill-defined red, yellowish or black pigment spots found 

 upon the general epithelium, or upon the tentacle tips of larvai Echino- 

 derms, Worms etc.; the red, orange, or l)lack, star-shaped chromato- 

 phores of mauy Crustacea and Coelenterates ; the chromatophores of 

 Cephalopods, and the beautifully radiating pigment cells so commonly 

 found in the larvae of pelagic fishes. Although the highly complicated 

 Cephalopod chromatophores, the development of which shows that 

 they bave several points of resemblance to the ommatidia, may be 

 useful as protective organs , it does not follow that this is their only 

 function, neither could it bave been their principal, or originai one, for 

 on this supposition we can give no satisfactory explanation of their ori- 

 gin. It is more than probable that they stili act as heliophags, and that 

 their protective function is an incidental and secondary one. 



There are often very extensive deposits of pigment at the outer 

 ends of epithelial cells in Molluscs, and there f ore in just 

 those places where the retia terminalia are most abun- 

 dant. When the pigment serves a purely protective purpose, it is 

 deposi ted at the inner ends of the cells, as in the iris o{ Pecten. A stili 

 farther differentiation is produced by the development, at certain, limited 

 areas, of a clear space beneath the very outer layer of the cuticula; 

 this space contains the greater part of the retia terminalia, while just 

 beneath is the Stratum of pigment. 



Beginning with a simple pigmented layer, ommatidia arise by the 

 formation of pigmented cells around a centrai colorless one provided 

 with a nucleolated nucleus. Such organs undergo the following changes: 

 (1) they collect at certain points to form ommateal tracts; (2) the color- 



