MUSEUM OF COMPARATIVE ZOOLOGY. 127 



lateral eyes present a highly modified condition. I have done this be- 

 cause I believe that the lateral eyes in many Copepods are degenerate, 

 and that therefore the evidence to be drawn from them cannot be as 

 trustworthy as that from other sources. That the lateral eyes in Cope- 

 pods are degenerate, is shown from the fact that in many members of 

 the group the eyes are entirely absent, and that in those in which they 

 do occur, their structure is subject to considerable variation. Thus in 

 Pontella the retina contains, besides one group of five retinular cells, three 

 isolated nervous cells, whereas in Sapphirina there is a group of three 

 retinular cells, and at least one isolated nervous cell. In Pontella, Sap- 

 phirina, Corycaeus, and Copilia each retina is provided with a single lens, 

 but in Ireuseus, according to Claus ('63, Taf. II. Fig. 1), there are two 

 lenses in each eye. These variations, including the total disappearance 

 of the organ in some members of the group, lead me to believe that the 

 lateral eyes in the Copepods are degenerated, and therefore are organs 

 in which the suppression of cells may have reduced them to even a 

 simpler condition than that presented by tlie ancestral ommatidium. 



The conclusion which I draw from the preceding argument is, that the 

 type from which the ommatidia in all living Crustaceans are probably 

 derived would exhibit the following structures : a corneal hypodermis in 

 which the cells are not regularly arranged, and consequently an un- 

 facetted corneal cuticula ; a cone composed of two cells ; a retinula com- 

 posed of five retinular cells and having a rhabdome which consists of 

 five rhabdomeres. The retina of the primitive eye, a simple thickening 

 in the superficial ectoderm, would be composed of ommatidia of this 

 type arranged upon the hexagonal plan. None of the Crustaceans 

 with which I am acquainted possess an eye of exactly this structure. 

 The one in which this condition is most nearly represented is perhaps 

 Gammarus. In this animal all the requirements of the hypothetical 

 eye are fulfilled, except that the form of the retina as a whole is some- 

 what disturbed by the separation of the corneal hypodermis from the 

 layer of the cones and retinulse by a corneo-conal membrane, and by the 

 partially disguised condition of the basement membrane. 



If my conclusions be correct concerning the structure of the primitive 

 ommatidium and the means by which it has been modified, it follows 

 that the principal types of ommatidia have been produced mainly by 

 increasing the number of cells in the primitive type, and that, of the 

 three means of modifying the structure of ommatidia, cell division has 

 been the most influential. 



Although the hypothetical ommatidium which has been described in 



