ESTERLY: EUCALANUS. 9 
a. Location and General Features.— 'The organ is on the average 
about 440 « in front of the anterior end of the brain. It lies in the 
sagittal plane of the body and the ventral component rests upon the 
chitin of the ventral wall of the body, except for the intervention of 
an extremely delicate enveloping membrane (Plate 5, Fig. 44). 
It will be seen that the lateral paired eyes (Fig. 1) are oval and cup- 
like, the longer axis lying in the direction of the main axis of the 
animal’s body. ‘The lateral margins of the paired eyes extend beyond 
the lateral borders of the median ventral eye (Fig. 1, oc. m.), thus 
obscuring the latter when the organ is viewed as a whole from the 
dorsal side. When looked at in this direction, the ventral eye may, 
however, be seen between the paired eyes. ‘The anterior and posterior 
borders of the three divisions of the eye he in nearly the same trans- 
verse plane of the body. 
The cup of the ventral eye is about three-fourths as deep as.those of 
the lateral eyes, but it is about five-fourths as broad in the transverse 
plane of the body. ‘The whole eye is slightly flattened in the dorso- 
ventral direction (Fig. 7). 
The optic nerve (n. opt.) emerges from the eye directly behind, and 
dorsal to, the ventral ocellus (Plate 1, Figs. 1, 2, 3,6; Plate 5, Fig. 
46), between it and the postero-ventral curvature of the lateral ocelli. 
The nerves from the rostral organ (Figs. 1, 7, ». f.)— the frontal nerves 
of Claus and others — are conspicuous strands, which meet the optic 
nerve a short distance behind the eye, having passed, from their dis- 
tribution in the rostrum, over the outer and dorsal surface of the lateral 
eyes. ‘The rostral nerves do not innervate any of the cells of the eye. 
b. Pigment Mass.—'The central mass of the eye, between the 
optic cups, consists, I believe, of a single cell (Plate 1, Fig. 7; Plate 2, 
Fig. 23; Plate 5, Fig. 49, cl. c.). I have never seen more than a single 
nucleus (Plate 1, Fig. 2, nl.”) in this region whatever the method of 
preparation. Consequently it seems to me that the three optic cups 
may be said to rest in or upon a central cell (Plate 1, Figs. 7, 9; 
Plate 2, Fig. 23), which, as seen in cross-sections of the eye, is in 
general triangular. ‘That is, the region between the median walls of 
the lateral eyes and the dorsal surface of the ventral eye is three sided, 
and is occupied by a single cell. 
In Cyclops, Richard (’91, p. 207, Pl. 7, Fig. 23) found that the central 
pigment mass is composed of three cells separated from one another 
by two membranes, and Hartog (’88, p. 33) speaks of the division of 
the central mass by fine partitions separating the ‘blocks’ which 
receive the optic cups; the blocks contain nuclei, at least one, probably 
