128 BULLETIN OF THE 
the preceding paragraphs has been spoken of as ancestral, it is not to be 
supposed that the condition which it presents must be regarded as necessa- 
rily its simplest form. I feel tolerably confident, however, that the prim- 
itive ommatidium must have been at least as simple as I have assumed 
it to be. Possibly its retimula may have been composed of less than five 
cells, as is that seen in some Copepods ; although, as I have previously 
remarked, the condition of the lateral eyes in these Crustaceans is 
probably influenced by degeneration, and therefore may not represent a 
primitive stage. What might be regarded, however, as a more primitive 
form of ommatidium than that which I have described, may be seen 
in the eye of the Chetopod Nais (Carriere, ’85, pp. 28, 29)... In this 
worm the eye lies in the hypodermis on the side of the head, and con- 
sists of a few relatively large transparent cells, the proximal faces of 
which are in part covered by pigment cells. It is probable that the 
transparent cells are merely dioptric in function, and that the pigment 
cells are neryous.. The transparent cells may therefore be looked upon 
as the forerunners of cone cells, and the pigment cells at their bases as 
retinular cells not yet differentiated into a retinula. It is not difficult 
to imagine the origin of an ommatidium from a single one of the trans- 
parent cells and its accompanying pigment cells, and, by an increase in 
the number of such groups, the production of a retina like that of the 
compound eye of Arthropods. 
This view of the origin of the ommatidia in Arthropods is irreconcila- 
ble with that recently advanced by Watase (’90), according to whom 
each ommatidium is to be regarded as a pit formed by an involution of 
the hypodermis. The supposed cavity of this pit occupies nearly the 
* whole length of the axial portion of the ommatidium, and is filled by 
the secretions of the cells constituting its wall. The secretion in the 
deeper part of the pit forms the rhabdome; that which is produced 
nearer its mouth, the cone. During the formation of the pit, the hypo- 
dermal cells are believed to retain such mutual relations that their mor- 
phologically distal ends lie next its cavity ; hence the secretions produced 
by these ends, the rhabdome and cone, are to be regarded as modifica- 
tions of the chitinous cuticula of the outer surface of the body. 
Ingenious as this theory is, I have not been able to convince myself of 
its tenability. It may be urged against the assumption that the retinu- 
lar cells occupy a proximal position and the cone cells a distal one on the 
wall of a hypodermal pocket, that in Gammarus the retinular cells extend 
from the distal to the proximal face of the retina, and that in Homarus 
the cone cells have a corresponding extent ; these conditions show that 
—— rll 
