544 William Patteu 



alone, I was led into other fields where I should bave been working stili, 

 if circumstances had not oblìged me to bring this paper to a dose. 



The arrangement of these pages does not follow the order in which 

 the observations were made, uor any system, except that which appeared 

 to lead up to a better understanding of the more eomplicated eyes. 



My studies upon the Arthropods bave led me to concliisions as 

 widely different from those of Grenacher as could possibly be the case, 

 even in such eomplicated structures as the Arthropod eyes. I bave the 

 more faith in the observations , since they lead to the reduction of the 

 essential parts of ali visual organs to one structural pian, which can be 

 follo wed through the whole animai kingdom, from the lo west to the 

 highest. 



I bave endeavored, first of ali, to obtaiu as accurate a histological 

 knowledge as possible of any one eye ; upon this knowledge I bave 

 based my conclusions concerning those of other groups or genera. 



GtRenacher's ideas bave received such universal acceptance, that it 

 is almost entirely with bim I bave to deal. But my own observations, 

 and the Interpretation I put upon them, dififer so widely from his, that 

 it is impossible to accept bis terminology without great confusion. When- 

 ever it was possible, I used old names ratber than invent new ones; but in 

 many cases, I was obliged to adopt the latter course. In the follow- 

 ing, the meaning of the new terms, and the added or modified signi- 

 fication attached to the old, will be explained. 



In many places upon the Molluscan hypodermis , especially those 

 parts exposed to the light, the cuti cui a is divided into two layers, an 

 outer structureless one, the corneal cuticula, and an inner layer, 

 the retinidial cuticula, filled with the retia terminalia, or 

 ultimate ramifications of the hypodermic nerves. 



The ommatidia (PI. 32, figs. 128, 132 etc.), or structural Cle- 

 ments of ali eyes, consist of from 2 to 4 colorless cells, the retino- 

 phorae [n. tf.), surrounded by a circle of pigmented ones, or re- 

 tinulae [pg.]. 



The cuticular secretion of each celi forms a rod, containing a spe- 

 cialized part of the retia terminalia, or re tini di um. 



In the more specialized ommatidia, the rods of the retinulae dis- 

 appear, leaving the double (Molluscs, Worms etc.) and quadruple 

 (crystalline cone of Arthropods) rods of the retinophorae. 



The apposed walls of the retinophorae disappear to a greater or 

 lesa extent, so that the nerve fibres between the cells come to lie in 



