100 THE ORIGIN OF VERTEBRATES 
layer of ganglion-cells. In the crustacean, Berger in Squilla, Gren- 
acher in Mysis, and Parker in Astacus, look upon them as supporting 
Fig. 42._Iprat DIAGRAM OF THE LAYERS 
In A CRUSTACEAN EYE. 
The retina is divided into an epithelial 
part, C (the layer of retinular cells and 
rhabdomes), and a neurodermal or cere- 
bral part, which is formed of, A, the 
ganglion of the optic nerve, and, B, the 
ganglion of the retina. 1, optic nerve 
fibres which cross at itheir entrance into 
the retina; 2, int. molecular layer with 
its “two rows of cells; 3, int. nuclear 
layer; 4, Reichenbach’s double row of 
large lightly-staining cells; 5, layer of 
terminal retinal fibres; 6, ext. nuclear 
layer; 7, ext. limiting membrane; 8, 
layer of crystalline cones; 9, cornea. 
elements, while Viallanes in 
Palinurus considers them to be 
true ganglionic cells. 
Whatever the final interpre- 
tation of these cells may prove 
to be, we may, it seems to me, 
represent an ideal compound 
retina of the crustacean type by 
combining the investigations of 
Berger, Claus, Reichenbach, and 
Parker in the following figure. 
The comparison of this figure 
(Fig. 42) with that of the Pe- 
tromyzon retina (Fig. 41) shows 
how great is the similarity of 
the latter with the arthropod 
type, and how the very points 
in which it deviates from the 
recognized vertebrate type are 
explainable by comparison with 
that of the arthropod. The 
most striking difference between 
the retinas in the two figures is 
that the layer of terminal nerve 
fibres (5, Fig. 42), which, after 
all, are only the elongated termi- 
nations of the retinal cells be- 
longing to Parker’s neurones of 
the first order, is very much 
longer than in Petromyzon or in 
any vertebrate, for the external 
molecular layer (6, Fig. 41) 
(Miller’s layer of Nervenan- 
sitze) is very short and inconspicuous (in Fig. 41 it is drawn too 
thick). 
Turning from the retina to the fibres of the optic nerve we again 
find a remarkable resemblance, for in Ammoccetes, as pointed out by 
