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 infgeneral 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 Eucalanus 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. IMoreover, 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 structures 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 Mallorv'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- 



