64 
G. II. Parker 
8. Deriyation of Ommatidia. 
The mimerical relations of the cells in the ommatidia of crus- 
taceans, as I have already pointed out (cf. Parker, 91), have such 
a remarkable uniformity that the idea of ommatidia! types definable 
by the number and arrangement of their cells almost naturally 
suggests itself. As an example of the most complex of these types, 
the ommatidium in Astacus may be taken. The numbers of cells 
that it contains, omitting the accessory pigment cells, are as follows : 
corneal hypodermal cells, two; cone cells, four; distal retinular cells, 
two; proximal retinular cells, seven functional and one rudimentary. 
The arrangement of these Clements as seen in the longitudinal and 
transverse aspect of the ommatidium is shown in the diagrammatic 
figures 64 to 66. One of the simplect ommatidia is that in Bran- 
chipus in which there are two corneal hypodermal cells, four cone 
cells, and fiye retinular cells. In some ommatidia, notably those 
in schizopods, the cones consist of only two cells, a condition which 
suggests that the ommatidia in Branchipus^ at least so far as their 
cones are concerned, probably do not represent the simplest type. The 
least number of cells that we should be justified in assuming for the 
simplest type of ommatidium would then be two corneal hypodermal 
cells, two cone cells, and fiye retinular cells. The arrangement 
of these Clements is indicated in figures 61 and 62. 
These two types of ommatidia, the simplest and the most com- 
plex, are without doubt genetically connected, and, as there is no 
evidence to show that the former is a degenerate product of the 
latter, it is probable that the simplest type is a primitive one from 
which the other has been derived. The numerical relation of the 
ommatidial cells indicates the way in which this derivation could 
have been accomplished. Thus a cone composed of two cells could 
be easily converted into one formed of four cells by a division of 
each original cell into two, and the simpler group of five indifferen- 
tiated retinular cells could by differentiation and division give rise to 
the two distal and eight proximal retinular cells of the more complex 
type (cf. Parker, 90, pag. 56). 
The necessary steps in the conversion of an ommatidium of the 
simpler type into one of the more complex type are indicated in 
the diagrammatic figures 61 to 66. These changes effect only the 
retinular and cone cells, the corneal hypodermal cells being essen- 
tially alike in both types. The cone cells, originally two in number 
