14 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The number is invariable. Grenacher ('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 wall 
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 Grenacher (’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, 
