ESTERLY: EUCALANUS. 41 
just as little reason for regarding them as “intraepithelial.” Hesse’s 
definition of these two sorts of eyes is as follows: Entweder bleiben sie 
[Sehzellen] in dem Verband des Epithels wie die indifferenten Epithel- 
zellen, d. h. sie reichen mit ihrem distalen Ende ganz bis an die iussere 
Begrenzung des Epithels....dann sind sie selbst Epithelzellen 
geblieben, wir bezeichnen sie als epitheliale Sehzellen.... Wenn 
dagegen eine Sehzelle mit ihrem distalen Ende nicht bis an die distale 
Grenze des Epithels reicht, iibrigens aber zwischen den Epithelzellen 
liegt, so ist sie keine eigentliche Epithelzelle mehr, sie ist innerhalb 
des Epithels gleichsam versenkt, proximad verschoben: wir bezeich- 
nen sie als zntraepitheliale Sehzelle” (Hesse, :02, p. 618, 619). 
From these definitions, it is plain that the ventral portion of the 
median eye is to be regarded as composed of neither intraepithelial 
nor subepithelial, but simply of epithelial retinal cells. I have shown 
that it is continuous at its margins with the undifferentiated ectoderm, 
and that it is, therefore, merely a thickening in the ectoderm. It 
evidently is of the first type, which Parker (91, p. 59) has mentioned 
as characteristic of most of the groups of Crustacea. Moreover, 
Hesse (:02, p. 620) considers that the compound eyes of all arthropods 
have epithelial visual cells. But it is impossible to regard the sensory 
cells of the ventral part of the median eye as either intraepithelial 
or subepithelial. 
As regards the cells of the lateral portions of the median eye, they 
properly belong to those classed as subepithelial by Hesse or to the 
third of the types enumerated by Parker (’91, p. 60), if we are to judge 
from the conditions in the adult. But Figures 6 (Plate 1) and 44 
(Plate 5) show, especially at the anterior end, that the membranes 
around the lateral eyes are a part of the basement membrane of the 
hypodermis, though it cannot be claimed that the retinal cells and 
those of the hypodermis are now directly continuous as in the ventral 
portion. As regards the membrane which surrounds the optic nerve, 
its relation to are eye as a whole, and especially to the lateral portions 
of the eye, seems to me to argue for the subepithelial character of the 
retinal cells, for some of them certainly lie within the membrane, which 
is plainly subepithelial’ in position. 
If, then, we attempt to place the median eye (as known in Eucalanus) 
in the morphological classification proposed by Hesse (:02, p- 620), 
it 1s necessary to consider that the unpaired portion occupies one 
position in the system and that the paired portions occupy another. 
The ventral ocellus is epithelial; the lateral ocelli are subepithelial. 
And none of the cells of any part of the eye can be considered as in- 
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