Sis? ae _ 
ESTERLY: EUCALANUS. 13 
of the eye. It seems to me, however, that indirect evidence points 
toward the central cell as containing the pigmented background of 
the eye, rather than to the plates on the inner faces of the optic cups, 
which partially envelope the retinal cells. 
d. Number and Arrangement of the Retinal Cells—— 'The optic 
cups themselves (Plate 1, Figs. 1, 2, etc.) are more or less globular 
masses of retinal cells, as Hartog (88), Richard (91) and Grenacher 
(79) have shown. The last named author determined accurately, 
as already stated, the number of cells in each part of the eye of Euca- 
lanus attenuatus (Calanella mediterranea). In Eucalanus elongatus 
the cells in the ventral eye (Plate 1, Figs. 8, 10; Plate 2, Figs. 21, 22) 
are arranged on precisely the same plan as Grenacher (’79, p. 65) has 
described for the other species. ‘That is, there are in all ten cells; 
of these, one is in almost the exact centre of the eye, lying slightly 
anterior to the mid-transverse plane (Plate 1, Fig. 10). There is one 
cell directly anterior to it, and, on each side of the longitudinal axis 
of the eye are four others, which are paired, each meeting its mate in 
the median plane. ‘There are, then, two unpaired and four pairs of 
cells in the ventral eye. This number can be very readily determined 
either in frontal sections, where the whole number may be seen in a 
single favorable section (Plate 1, Fig. 8), or by reconstructions from 
sagittal or transverse series of sections. It is not so easy to count the 
nuclei in entire preparations, but when that is possible there is no 
doubt as to the number. I have very many preparations of the whole 
eye, but with a few exceptions the conditions are not favorable for 
counting the cells in the ventral eye. Figure 21 (Plate 2) is drawn 
from a vom Rath preparation which had been decolorized in H,O,,. 
The preparation is viewed from the ventral surface, consequently 
the ventral eye is uppermost in the drawing. It was impossible to see 
the most anterior nucleus of the ten in the ventral eye, yet the cell 
walls were perfectly distinct. Likewise the posterior nucleus of the 
four on the right side (left in the drawing) was invisible, yet from 
the arrangement of the other nuclei there can be no doubt of its 
occurrence, especially when sections such as that shown in Figure 
8 (Plate 1) exhibit precisely the same arrangement of the elements. 
In Figure 22 (Plate 2) all the nuclei of each of the three divisions of 
the eye can be seen, though the eye as a whole has been somewhat 
distorted by pressure. 
In each of the lateral eyes there are nine cells. This has been deter- 
mined by careful reconstruction drawings from series of cross, sagittal, 
and frontal sections, and by counting the nuclei in entire preparations. 
