14 bulletin: museum of comparative zoology. 



The number is invariable. Grenadier ('79, p. 64) found that there 

 were eight cells in the lateral eyes of Eucalanus attenuatus, but there 

 can be no doubt that in E. elongatus the number of cells is nine. 



These cells are always arranged in a definite way, viz., in three super- 

 posed layers, a dorsal, a middle and a ventral layer, three cells in each 

 layer (Plate 1, Fig. 1). This arrangement is most easily determined 

 by focusing upon the nuclei in entire preparations, as it is impossible 

 to follow the cell walls throughout their extent. In this way one can 

 readily see that there are three nuclei at a very high focus, if the eye 

 is viewed entire from the dorsal side. The nuclei in this stratum are 

 not at precisely the same level; the single one nearest the inner Avail 

 of the cup (Plate 1, Fig. 10; Plate 2, Fig. 22) is at the highest level; 

 this and the other two which belong to the same group are represented 

 in the same deep tone. It should be stated that in the figures men- 

 tioned the three nuclei in a given group have received the same tint, 

 but this should not be taken to mean that they lie at precisely the same 

 level in the eye. In all the groups of nuclei, the same arrangement 

 may be observed; that is, there is one nucleus nearer the median face 

 of the eye and two which are more lateral, though in the middle group 

 the nuclei are much more nearly in the same antero-posterior line than 

 in either of the other groups. The cell to which the median nucleus 

 of the upper group belongs, is triangular in outline, and does not extend 

 to the lateral border of the eye, while the remaining cells of the group 

 are elongated and extend from the inner or basal portion of the eye to 

 its outer margin (Plate 2, Fig. 22). The same may be said of the 

 remaining groups of nuclei also; the median cell is triangular and 

 does not extend to the exterior, while the anterior and posterior cells 

 reach from the base to the outer margin of the eye. It may be added 

 that the three anterior and the three posterior cells converge somewhat 

 from the inner (median) to the outer (lateral) face of the eye in the 

 manner described by Grenadier ('79, p. 64). 



It may be said, then, that the eye in Eucalanus elongatus contains 

 in all twenty-eight retinal cells. In Cyclops, according to Richard 

 ('91, p. 207), there are from 8 to 12 elements in each "Crystalline 

 sphere," while Hartog ('88, p. 33) states that the median eye possesses 

 "about eight peripheral and one central bacillus" and the lateral eyes 

 "at least 8 to 10 peripheral bacilli and three central ones." Claus 

 ('91, p. 246) was unable to determine with certainty the number of 

 retinal cells in the eye of Diaptomus. At the beginning of his work, 

 he found at least six nuclei, but later was led to believe that the number 

 of cells was still greater. In Pontella, a type with compound eyes, 



