98 BULLETIN OF THE 
In accordance with the facts already presented, the number of cells 
contained in the ommatidium of Serolis can be stated as follows: cells 
of the corneal hypodermis, two, with possibly two others interomma- 
tidial in position ; cone cells, two; retinular cells,. six, two distal and 
four proximal ; hyaline cells, one or two; a variable number of small 
pigment cells of ectodermic (?) origin. 
Leptostraca, 
The histological structure of the ommatidia in the Nebalize has been 
investigated, so far as I am aware, only by Claus (’88, pp. 65-84). I 
have had no material for the study of the eyes in these Crustaceans, 
and I can therefore only present, in the form of a summary, the more 
important results of Claus’s exhaustive study. 
In Nebalia there is a corneal hypodermis (Claus, ’88, pp. 68 and 69), 
the cells of which are grouped in pairs. As in many of the higher 
Crustaceans, there is one pair of these cells for each ommatidium. The 
corneal cuticula is facetted ; the outlines of the facets are circulat, and ad- 
joining facets are separated from one another by a small amount of inter- 
vening cuticula (Claus, ’88, Taf. X. Fig. 10). The cones are composed 
of four segments (Claus, ’88, p. 69). The structure of the retinula is 
somewhat complex. The greater part of the rhabdome is surrounded 
by seven retinular cells. Distal to these cells, however, are seven pig- 
ment cells, which enclose the proximal prolongation of the cone cells and 
the distal end of the rhabdome. Such a relation between pigment cells 
and retinular cells is not of common occurrence among Crustaceans, and 
it is possible that the bodies which Claus has taken for pigment cells are 
really the distal ends of the retinular cells. Claus describes and figures 
what he believes to be the nuclei of both kinds of cells, but I think 
his figures fail to show that these nuclei are within the limits of the 
cells to which they are said to belong. It seems to me quite possible 
that what he has described as two circles of seven cells each may 
be merely one circle seen at two different levels, as the correspondence 
in numbers suggests. This single circle would be of course composed 
of retinular cells, the nuclei of which are probably the distal ones of 
the two sets described by Claus. The proximal nuclei, which, accord- 
ing to Clans, belong to the retinular cells, occupy positions not unfre- 
quently taken by the nuclei of accessory pigment cells, and I am inclined 
to think that such is their real nature. This interpretation would be 
more in accordance with the conditions found in ommatidia which have 
seven retinular cells than is the one given by Claus; but as I have not 
