esterly: eucalanus. 13 



of the eye. It seems to me, however, that indirect evidence points 

 toward the central cell as containing the ])igmented 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 Grenadier 

 ('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 Grenadier ('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 Avith a few exceptions the conditions are not favorable for 

 counting the cells in the ventral eye. Figure 21 (Plate 2) is drawn 

 from a voiii Rath preparation which had been decolorized in H^Oj. 

 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 

 % (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. 



