126 BULLETIN OF THE 
segments persist is evidently more primitive than the one in which their 
outlines are obliterated. , 
Probably in Nebalia, in which the retinula is composed of seven cells, 
and certainly in Idotea, where it consists of six, the rhabdome shows 
no indication of being composed of rhabdomeres, but in Porcellio the 
seven retinular cells surround a rhabdome composed of a corresponding 
number of rhabdomeric segments. In Branchipus, the retinula consists 
of five cells, but the rhabdome is apparently not composed of separable 
rhabdomeres, whereas in Pontella, Argulus, Gammarus, Talorchestia, 
Hyperia, and Phronima the five retinular cells are each represented by 
arhabdomere. The more frequent occurrence of a primitive condition 
of rhabdome with the retinula having five cells than with that having 
seven, favors indirectly the idea that the retinula with the smaller 
number of cells is the more primitive of the two. The types of cones 
associated with the two kinds of retinulz offer almost no evidence on 
the question in hand. Thus, a retinula of seven cells is associated with 
a cone of four cells in Nebalia, and with one of two cells in Porcellio, 
and a retinula of five cells is combined with a cone of four cells in 
Branchipus and Argulus, and with one of two cells in Amphipods. The 
relation of the two kinds of retinule to the corneal hypodermis affords 
some slight evidence in support of the opinion that the retinula of five 
cells represents the more primitive type ; for although the differentiated 
type of corneal hypodermis —the one in which the cells are regularly 
arranged — may occur with either type of retinula, the undifferen- 
tiated hypodermis— in which the cells are not regularly grouped — is 
known to be associated only with retinule containing five cells (some 
Branchiopods, Argulus, and Amphipods). The evidence drawn from 
these various sources is obviously very slight ; but such as it is, it indi- 
cates that the retinula with five cells, rather than that with a greater num- 
ber, represents the more primitive condition. This conclusion receives 
some additional support from the fact that the retinula composed of 
five cells characterizes the ommatidia in a number of not otherwise very 
closely related Crustaceans (Pontella, Argulus, the Branchiopods, and 
Amphipods), whereas the type possessing seven cells occurs only among 
certain Isopods and in the Nebaliz. I believe, therefore, that all the 
evidence at present deducible from the condition of the simpler retinulz 
indicates that the one which contains five cells is more primitive than 
that composed of six or seven cells. 
In the present argument I have purposely omitted any mention of the 
condition of the retinula in the Coryczidz, those Copepods in which the 
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