MUSEUM OF COMPARATIVE ZOOLOGY. 125 



posed of five cells from those possessing four. Since there is no evidence 

 of degenerate cells in any of the cones composed of two segments, I am 

 convinced that cones with four cells are derived from those with two cells, 

 and not the reverse. On these grounds, I conclude that the most primi- 

 tive form of cone in living Crustacea is that consisting of two cells. 



The retinular cells in Crustaceans are subject to considerable varia- 

 tion. As I have previously shown, an onimatidium may contain one or 

 two kinds. When there is only one kind, all the cells are grouped 

 around the rhabdome, and are known simply as retinular cells. When 

 there are two kinds, one occupies a position around the rhabdome, and 

 the other around the cone ; the former I have called proximal retinular 

 cells, the latter distal retinular cells. Proximal and distal retinular 

 cells occur in Serolis, the Stomatopods, Schizopods, and Decapods ; 

 simple retinular cells apparently characterize the ommatidia of all other 

 Crustaceans. I have already presented reasons for considering the 

 distal retinular cells as modified simple retinular cells, which, in the 

 separation of the cone from the rhabdome by the elongation of the 

 ommatidium, have lost their connection with the nervous element, but 

 have retained their place next the dioptric one. A group of retinular 

 cells in which this differentiation has occurred is not so primitive in its 

 structure, therefore, as one in which all the retinular cells retain their 

 original position around the rhabdome, as in the groups of Crustacea 

 which possess simple retinular cells. 



The number of simple retinular cells in Crustacean ommatidia varies 

 from five to seven. In Nebalia, and some Isopods, the retinula con- 

 tains seven cells ; in other Isopods it is composed of six cells, and in 

 the Branchiopods, the Cladocera, some Copepods, and Amphipods it 

 consists of five cells. It is difficult to state which of these numbers 

 represents the primitive condition. In the Isopods, as I have previ- 

 ously indicated (pp. 8G and 87), there is considerable evidence to 

 show that a retinula composed of six cells has been produced from one 

 containing seven by the suppression of one cell. Possibly in this way 

 the retinula with five cells was derived from that with six, but I know 

 of no observations which favor this supposition. 



A small amount of indirect evidence on this question is to be ob- 

 tained from the other structural peculiarities of the ommatidia con- 

 taining retinulae with five, six, or seven cells. These retinulse occur 

 in connection with two kinds of rhabdomes, — one in which the rhab- 

 domeric segments are easily distinguishable, and the other from which 

 they are apparently absent. Of these two kinds, the one in which the 



