MUSEUM OF COMPARATIVE ZOOLOGY. 95 
vanced embryos (compare zi. dst., in Figs. 65 and 69). In transverse 
sections at this stage (Fig. 69) each cone is surrounded by a circle of 
six nuclei. Each nucleus, however, participates in three adjoining cir- 
cles, consequently there are only twice as many nuclei as ommatidia. 
In the adult the nuclei of these pigment cells (Fig. 60, ni. dst.) occupy 
the same relative positions as in the embryo; in the latter, however, they 
are usually somewhat hidden by the pigment which surrounds them. 
In the embryo the nuclei of the pigment cells surrounding the cone 
resemble very closely, except in point of size, the nuclei of the retinular 
cells (compare nl. dst. and nl. px. in Fig. 65). In the nuclei of the 
retinular cells there is usually one distinct nucleolus, sometimes two, but 
as a rule no finer particles. This condition also obtains in the nuclei of 
the pigment cells. Not only are the nuclei of these two kinds of cells 
similar in the embryo, but they are also much alike in the adult (com- 
pare nl. dst. in Fig. 60 with x. rtn.! in Fig. 63). 
Because of this resemblance, I believe that the pigment cells which 
surround the cone can be fairly considered to be modified retinular cells, 
which have lost their sensory function in precisely the same way as in the 
case of the distal retinular cells in Decapods (see Parker, ’90°, p. 57). If 
this interpretation of the pigment cells be accepted, it follows that in 
Serolis, as in Decapods, two kinds of retinular cells are present, proximal 
and distal, and that the primitive ommatidium from which that of Serolis 
was derived probably contained six retinular cells functional as nervous 
structures. It need scarcely be added, that this number is characteristic 
for the ommatidia of many Isopods. 
The retinula in the species of Spheroma which I studied presents 
an appearance which suggests the differentiation of simple retinular cells 
into proximal and distal cells. In Sphzeroma there are seven retinular 
cells (Plate V. Fig. 58) ; three of these are considerably reduced ; the 
remaining four are large, and recall the four retinular cells of Serolis. 
In transverse sections it can be shown that the four large cells in Sphe- 
roma not only resemble in appearance the four proximal cells in Serolis, 
but that they occupy the same relative positions in the ommatidium. 
In Serolis the plane which separates the two cone cells of any given 
cone, when extended, separates the four proximal retinular cells into two 
groups of two cells each (compare Plate VI. Fig. 68 with Figs. 71 and 
72). The plane of separation in the cone of Spheeroma divides the retin- 
ula by passing through the single small retinular cell shown in the lower 
part of Figure 58 (Plate V.) and between the two small cells on the oppo- 
site side, thus separating the four large retinular cells into two groups, 
as in Serolis. 
