MUSEUM OF COMPARATIVE ZOOLOGY. 81 
the sections which pass through the retina. The most ventral section 
is shown in Figure 20, the most dorsal in Figure 29. 
Immediately below the lens the central part of the retina is occupied 
by a roundish granular mass (Fig. 18, con.), which in the living animal 
is the only part without pigment. In trausverse sections this mass is 
seen to consist of two bodies (el. con. 1, and cl. con. 2, Fig. 25), which 
extend as far as to the lens (compare Figs. 25-27). Each body con- 
tains a ‘nucleus (x/. con., Figs. 25 and 27) and consequently represents 
a cell. From the position which the mass occupies, and from the fact 
that it contains no pigment, it represents, I believe, a cone, and the two 
cells of which it is composed are its two segments. 
Claus (63, p. 47) states that in Pontella each retina is provided with 
six or more small crystalline cones, but my own observations do not 
confirm this statement. The body which, on account of its position, I 
have described as the cone in Pontella, is probably homologous with 
what Dana (’50, p. 133) first described as the inner lens in Coryczus, 
and with what subsequent investigators have called the crystalline cones 
in Sapphirina (Gegenbaur, ’58, p. 71) and Copilia (Leuckart, ’59, p. 252). 
Nothing, I believe, is known of the cellular composition of the cone in 
these genera. 
The arrangement of the elements in that portion of the retina which 
surrounds the cone in Pontella is not easily made out. The most con- 
spicuous structures in this region are rod-like bodies, which probably 
represent rhabdomeres. Eight of these, arranged in three groups, are 
present in each retina. The largest group, composed of five rods, lies 
directly beneath the cone. The rods of this group have been numbered 
from one to five in the retina to the left in Figures 21, 22, and 23. 
Posterior to this group, in the same retina, is the sixth rod, seen in 
Figures 24, 25, and 26. Anterior to it are the seventh and eighth 
rods, seen in Figures 26, 27, 28, and 29. 
The outlines of the cells to which these rods belong cannot always be 
distinguished ; that there is a cell for each rod is evident from the fact 
that near each rod there is a large nucleus. The nucleus belonging to 
the cell from which the eighth rod was produced is shown in Figure 28 
(nl. rtn.’) ; those belonging to the cells from which the sixth and seventh 
rods arose are indicated in Figure 26 (nd. rtn’.), and those belonging to 
the cells from which the central group of five rods came are seen, four in 
Figure 24 and one in Figure 25 (nl. rtn./). 
In addition to these nuclei, which judging from their positions and 
number are unquestionably the nuclei of the cells to which the rhab- 
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