esterly: eucalanus. y 



a. Location and General Features. — The organ is on the average 

 about 440 /x in front of the anterior end of the brain. It Hes 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 lie 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, n. f.) — the frontal nerves 

 of Claus and others — are conspicuous strands, which meet the optic 

 nerve a short ilistance 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, PI. 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 



