The Ketina and Optic Ganglia in Pecapods, especially in Astacus. 9 
of the retina to its posteroventral one and at right angles to this; 
i. e., each facet is placed so that its four angles point one dorsally, 
one ventrally, one anteriorly, and one posteriorly (PI. 1 Fig. 29). 
The facets may also be regarded as arranged in rows extending in 
an anteroposterior and in a dorsoventral direction; but, in such 
rows, the adjoining facets, instead of having their sides together, 
are, of course, corner to corner. These rows give an accurate 
measure of the extent of the retina in their respective directions. 
As the result of a number of measurements (vid. table below), 
it was found that, while the rows in the anteroposterior direction 
through the centre of the retina contained about 58 ommatidia, those 
in the dorsoventral direction had only about 31 ; in other words, in 
the adult Astacus^ the retina is about twice as long as it is broad. 
Sepcimen 
Sex 
Lengtli 
of 
Specimen 
Eye 
Anteroposterior Curvature 
Dorsoventral Curvature 
Kadius 
Are 
Facets 
Radius 
Are 
Facets 
1. 
12.3 cm. 
R. 
mm 
1.64 
187° 
54 
mm 
1.25 
130° 
29 
L. 
1.63 
184° 
54 
1.25 
134° 
30 
2. 
11.7 cm. 
R. 
1.64 
190° 
58 
1.25 
128° 
29 
L. 
1.69 
190° 
58 
1.29 
127° 
30 
3. 
6 
10.6 cm. 
R. 
1.69 
188° 
58 
1.22 
147° 
32 
L. 
1.53 
187° 
57 
1.22 
142° 
32 
4. 
6 
Jl.l cm. 
R. 
1.55 
184° 
60 
1.19 
130° 
30 
L. 
1.55 
184° 
60 
1.25 
130° 
30 
5. 
Ó 
11.0 cm. 
R. 
1.64 
195° 
60 
1.16 
150° 
31 
L. 
1.63 
196° 
60 
1.19 
148° 
33 
1.62 
189° 
58 
1.23 
137° 
31 
Not only is the extent of the retina unequal in different direc- 
tions, but the curvature of its outer surface also varies. In an 
anteroposterior direction as measured on perfectly fresh eyes, the 
radius of curvature is about 1.62 mm. and the retina extends through 
some 189°, while the dorsoventral curvature with a shorter radius 
of only 1.23 mm. has also the smaller angular extent, 137°. 
