10 
G. H. Parker 
The measurements upon which these Statements are based are 
contained in the preceding table. The length of the specimen, as 
recorded here, was measured from the tip of the rostrum to the 
extreme end of the telson when the chief axis of the animai was 
approximately straight. 
Since the ommatidia rest as a rule with their longitudinal axes 
vertical to the corneal surface, the divergence of the axes of any 
two adjoining ommatidia can be easily calciilated from the data 
just given. Thus, in an anteroposterior row, since 58 ommatidia 
occupy an are of 189° the axes of any two adjoining ommatidia 
must diverge from each other at an angle of about 3° 15'; calculated 
in a similar way, those in the dorsoventral rows diverge at a 
slightly greater angle, 4° 25'. It must be remembered, however, 
that both in the anteroposterior and in the dorsoventral rows, adjacent 
ommatidia are in contact with each other only at their angles and 
that consequently their axial divergence is greater than that between 
any two adjacent ommatidia in the oblique rows where these bodies 
are placed with their sides together, not corner to corner. The 
divergence of two adjacent ommatidia in the oblique rows was found 
by Observation to be about 3°, the smallest angle made by the axes 
of adjoining ommatidia. 
The total number of ommatidia in the adult retina is easily 
determined by counting the number of corneal facets; and, as is 
shown by the following enumerations taken from the right eyes of 
five crayfishes, the number may be placed approximately at 2500. 
Average 
Length of specimen in cm . . . 
10.1 
10.0 
10.6 
8.0 
7.9 
9.3 
Number of corneal facets . . . 
2696 
2872 
2456 
2378 
2375 
2555 
b. StruGture of Ommatidia. 
Notwithstanding the care and frequency with which the omma- 
tidia in Astacus have been studied, the results of even the later 
investigators are by no means in full agreement, and I therefore 
venture to redescribe these organs, dwelling at length, however, only 
upon those points where a difference of opinion exists or where I 
have gained by examination a clearer insight into their structure. 
According to my own observations, each ommatidium in Astacus 
contains, in addition to one or more accessory pigment cells, the 
