10 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
two. One would be led to expect similar conditions in the eye of 
Eucalanus, since it resembles so closely in general features the eye of 
Cyclops. But in the many series of sections studied, which have been 
cut in the frontal, sagittal and transverse planes, I have never seen 
traces of more than one nucleus which could by any possibility be 
related to the central mass of the eye,— that is, to that portion of the 
organ which lies entirely outside the true optic cups, and between the 
three. Moreover, there is no trace of what could be called cell walls 
or membranes in this region, and it seems unlikely that such would 
fail to appear in sections that show the boundaries of the retinal cells 
very clearly. 
The conclusion seems justified, therefore, that the median portions 
of the three optic cups in the eye of EKucalanus are embedded in a 
single cell, which corresponds to the “blocks” of Hartog (’88, p. 33) 
and to the “masse central de pigment” described by Richard ('91, 
p- 207) in the fresh-water Copepoda. 
That this cell contains the pigment of the eye, seems highly probable 
for a number of reasons, though this location is very different from that 
assigned to the pigment by Grenacher (’79, p. 64). The central cell 
of the eye (corresponding to Richard’s ‘central pigment mass”’) is 
in precisely the same position with reference to the rest of the eye as 
is the pigment mass in Cyclops. Moreover, its lateral and ventral 
margins form what Hartog (’88, p. 33) has named the “‘tapetum,” 
which he says “‘consists of fine reddish granules, lying on the face of 
the block.” I can confirm the portion of the statement relating to 
the position of what seems to be a reflecting layer in the pigmented 
portion of the eye of Cyclops, but I have not been able to satisfy myself 
that the tapetum consists of granules. It seems, rather, to be a differ- 
entiated margin of the pigment cell or cells. At any rate, the central 
cell of Eucalanus elongatus has when sectioned precisely the same 
appearance as in Cyclops and I believe that the structurés are homol- 
ogous in the two cases. (Plate 1, Figs. 2,3 tap.; Plate 2, Fig. 16; 
Plate 5, Fig. 49.) 
In my preparations the tapetal layer is colored yellowish in Mallory’s 
connective-tissue stain (Plate 5, Fig. 49), and intensely black in vom 
Rath’s (Plate 1, Fig. 5). In haematoxylin stains the tapetum has a 
vitreous appearance both in Clyclops and in Eucalanus, and it is this 
fact, as much as its position, that has led me to conclude that we are 
dealing with the same structure in the two forms, and that the single 
central cell in Eucalanus is the pigment cell of the eye. Such indirect 
evidence is all that is available, for the actual pigment is not demon- 
