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 rhabdotne 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 Poiatella, Argulus, Gammarus, Talorchestia, 

 Hyperia, and Phronima the five retinular cells are each represented by 

 a rhabdomere. 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 retinulse 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 retinulee 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 diff'erentiated 

 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 retinulse 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 JSTebaliae. I believe, therefore, that all the 

 evidence at present deducible from the condition of the simpler retinula) 

 indicates that the one which contains five cells is more primitive than 

 that composed of six or seven cells. 



In the present argument T have purposely omitted any mention of the 

 condition of the retinula in the Corycaeidse, those Copepods in which the 



