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, Coryczeus, and Copilia each retina is’ provided with a single lens, 
but in Irenzeus, 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 the 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 onmatidia 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 retinulz 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 
