8 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the cells rather than with the structure of the eye as a whole. Most 
investigators have based their studies upon the fresh-water forms, such 
as Cyclops or Diaptomus, while but three, so far as I am aware, have 
had an opportunity to study the marine forms. 
Claus (’63) mentions no features of the eye of Eucalanus (E. atten- 
uatus Dana) except its general form, the optic nerve, and the relation 
of the frontal nerves to the eye. Grenacher (’79), working upon the 
same form, made a closer study, but employed only isolation prepara- 
tions or entire ones. His observations were confirmed in some respects 
by Claus (91). Hesse (:01) was the first to make sections of the eye 
of Eucalanus. 
Grenacher (’79, p. 63) has given an accurate account of the optic 
organ. He found that it conformed to the well-known tripartite type, 
or “‘x-shaped pigment spot,” consisting of a ventral unpaired portion 
and two dorsal paired parts, each of the three divisions possessing a 
special pigment plate. To each of the pigment plates belongs a very 
constant number of cells, all of which, though not uniform in shape, 
turn an attenuated end toward the point of exit of the optic nerve, 
which hes between the three divisions of the eye. Each of the paired 
portions of the eye, or the “‘lateral eyes,” contains eight cells arranged 
in two layers.. The upper layer contains five cells and the lower layer 
three. ‘The median, ventral or unpaired eye contains ten cells, eight 
of which are arranged in pairs, two being unpaired. The optic nerve 
fibres continue into the cells, entering them at their pointed ends. 
Grenacher states that the fibres could be traced within the cells in 
some cases. Claus ('91, p. 229) states that Grenacher failed to notice 
~ that the nerve fibres enter from the outer side, thus giving the eye the 
nature of an inverted “‘ Becherauge,”’ but (p. 246) confirms Grenacher’s 
observations as to the number and position of the cells. Hesse (:01, p. 
350) mentions the eye of Eucalanus very briefly, merely pointing out 
the character of the nerve endings and the presence of the “interior 
bodies” (Binnenk6rper). 
I have studied the eye of Eucalanus both by means of sections and 
from preparations of the entire organ. ‘The general form of the eye 
is shown in Plate 1, Figure 1, and has already been described by Claus 
(63, Taf. 7, Fig. 9) and by Grenacher (’79, Taf. 5, Fig. 36; Taf. 6, 
Fig. 37, 38) in E. attenuatus. J have been unable to obtain any speci- 
mens of this species, but, so far as I can judge from these descriptions, 
the shape and more general features of the eye are the same as in E. 
elongatus. 
