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 
convineed 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 ommatidium 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. 86 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 retinule with five, six, or seven cells. These retinule 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 
