96 BULLETIN OF THE 
The change which would convert an ommatidium like that in Sphee- 
roma into one like that in Serolis is easily imagined. It would consist 
in the complete abortion of one of the three small retinular cells, and the 
conversion of the other two into the pigment cells surrounding the cone. 
In addition to the elements which have already been described in the 
ommatidium of Serolis, there are certain small pigment cells which oc- 
cur for the most part in the region of the retinule. Beddard (84%, 
p. 21) describes these as long branching “ connective-tissue cells,” a 
name which might imply that they originated from the mesoderm, and 
were therefore intrusive. Watase (’90, p. 293, Plate XXIX. Fig. 1) has 
also described and figured these cells, but distinctly states his belief that 
they are reduced ectodermic cells. In the adult I have observed in the 
region of the cones, as well as near the retinule, certain small nuclei 
which are usually surrounded with more or less black pigment. These, 
I believe, represent the cells described by Beddard and Watase. In the 
embryo certain scattered nuclei (nl. h’drm., Figs. 65 and 70) oceur in 
the spaces between the ommatidia. It is probable that these nuclei are 
ectodermic in origin, and I am at a loss to know what has become of 
them in the adnlt, unless they form the pigment cells already men- 
tioned. I am therefore inclined to believe, with Watase, that the small 
additional pigment cells are reduced ectodermic cells. 
The presence of the hyaline cells in the ommatidium of Serolis is, as 
Beddard has pointed out, almost a unique feature. These cells, usually 
two in each ommatidium, fill the space immediately below the rhabdome. 
They are bladder-like (Fig. 62, cl. hyl.) and contain each a large gran- 
ular nucleus. Although it is stated that there are usually two of these 
cells in each ommatidium, I never found more than one to an ommatid- 
ium in the several eyes of S. Schythei which I examined. This circum- 
stance, however, is not surprising; for, as Beddard (’84*, p. 22) has 
remarked, the number of these cells is subject to variation, there being 
sometimes one, sometimes two, for each ommatidium. In S. Schythei 
the single hyaline cell envelops more or less completely the distal part 
of the fibrous portion of the cone cells, so that this part seems to pierce 
the hyaline cell. A closer inspection, however, will usually show two 
lines extending from the fibre to the periphery of the hyaline cell (com- 
pare Fig. 62), and these lines indicate, I believe, the two walls of the 
cell which have been infolded by the presence of the fibre during the 
growth of the hyaline cell. 
The source of the hyaline cells is not definitely known. Their nuclei 
(Fig. 65, ni. hyl.), as Beddard (’88, p. 450) has observed, are present 
