(326 William Patten 



of all eyes. How far that has proved true will be seen in the foUow- 

 ing pages. A mere accident led me to choose Arthropods as most 

 convenient for testing this supposition. I eutered upon this subject 

 with that respect for the researches of Grenacher (which have been 

 so steadily gaining in general acceptancy since their publicatiou) that 

 one must feel for honest work, whenever brought in contact with it. 

 But my first observations , conducted under other methods. raised 

 suspicious that, at least in that case, Grenacher's explanations would 

 not work, and against my desire I was drawn away from my in- 

 tended course into a boundless field of Observation, with the feeling 

 that neither time, nor existing circumstances, would allow me to 

 treat the subject in a satisfactory manner. The evidence against 

 Grenacher's theory appeared so over-whelmiug, that I could not resist 

 the temptation of making a few observations, with the double objeet 

 of testing my own conception of the structure of the ommatidia, and 

 of studying the nerve endings there. The limits of this paper would 

 not allow me to carry my observations any farther in this direction, 

 but I hope in the near future to extend them, in order, if possible, to 

 clear up many difficulties, which I am at present unable to explain. 

 They are difficulties, however, which lie in those places where the 

 least satisfactory work has been done , and it is not improbable that 

 they too will disappear, as I have seen others do, upon a more careful 

 study. The favorable conditions, offeredby the eyes oi Peìiaeus^ have 

 been utilized in order to obtain as complete a knowledge as possible 

 of one form, which might then serve as a type for comparison with the 

 eyes of other genera. 



In Penaeus^ the cornea is divided into Square faeets, the outer 

 surfaces of which show mere traces of convexity. Beneath a thin, struc- 

 tureless and refractive, outer layer, is a thicker, less dense, and lamin- 

 ated one (PI. 31 , fig. 69 c^). Below the cornea is a thin, continuous 

 layer — the corneal hypodermis — to which the corneal cuticula 

 owes its origin. Each facet rests upon two flattened, oblong cells, reduc- 

 ed in the centre to a delicate membrane. In the thickened, abaxial 

 edges, are the faint, ovai nuclei, so arranged that those of the adjacent 

 faeets are situated near each other , on either side of the dividing line 

 (PI. 31, figs. 69 and 76). It is not difficult to obtain examples in which 

 the entire ommatidial hypodermis has been removed, leaving the cor- 

 neal layer intact, so that the number and arrangement of the nuclei, 

 as shown in fig. 76, and the median di vision between the two cells 

 may be easily observed. The delicate outlines of these very thin cells, 



