PR ee 
ESTERLY: EUCALANUS. ao 
Thus far in this paper, I have described the median or tripartite eye 
of Eucalanus elongatus as a type of this structure among Copepoda. 
I have tried to show that none of the parts of the eye may be considered 
as inverted in the sense that the nerves leave the retinal cells from 
their distal ends. The nerves pass through the basal plates of the 
optic vesicles and, in part, traverse the entire extent of the “central 
cell” of the eye, on their way to the brain. It is immaterial, in con- 
sidering the relation of optic fibres to sensory cells, whether the “cen- 
tral cell” or the basal plates are held to be the pigment bearers of the 
eye. he relations of axis cylinders and cells remains the same. 
There can be no reasonable doubt that there is one nerve fibre, and 
only one, to each retinal cell, for in several instances such a nerve 
fibre has been traced from its emergence from a single cell which gives 
rise to no other fibres; and, furthermore, the number of fibres in the 
optic nerve and the number of sensory cells in the eye is precisely the 
same in all cases. It also seems very probable that only the unpaired 
portion of the-eye retains, in the adult, its original relation to the 
hypodermis; the lateral eyes are without doubt no longer directly 
continuous with the hypodermis as is the ventral eye. ‘The foregoing 
are the principal facts which it will be necessary to consider in a general 
discussion. 
29. Tue “INVERTED” Eves or ‘‘ORGANS OF CLAUS.” 
It remains, now, to describe certain other structures which are proba- 
bly optical in function. I shall call these “‘the organs of Claus,” from 
their discoverer. Claus (’63, p. 56) was the first to describe the organs 
in question, and since that time no one has investigated them in any 
way, so far as I know. Richard ('91, p. 209) states that they are not 
found in Cyclops, and Hartog (’88, p. 33) located similar “concretions” 
at the base of the fifth feet in Cyclops brevicornis. Claus’s observa- 
tions were made upon Eucalanus attenuatus Dana (Calanella medit- 
terranea); his description of the organs is as follows “‘Gehérorgane 
wurden nicht mit Sicherheit beobachtet, méglicher Weise aber gehort 
in die Kategorie dieser Organe eine eigenthiimliche Bildung im 
Gehirnganglion von Calanella. Es sind zwei kugelige, Gehorblasen 
ihnliche Raume, in deren hellem Inhalte ein Ballen von Concre- 
tionen bemerkt wurde. Ob diese Differenzirung regelmissig auftritt 
oder nicht, habe ich leider unterlassen zu entscheiden.” 
a. Location— The organs are symmetrically located within the 
brain at its anterior end (0. Claus, Plate 2, Fig. 24; Plate 3, Figs. 25, 
