G. H. Parker 
thus far proposed is fully satisfactory, I have not attempted to in- 
vent new ones, but have adopted such names from those already 
in use as seemed to me appropriate on account of their expressiveness 
or their more general acceptance. 
When a longitudinal section of an optic stalk is examined 
(PI. 1 Fig. 27), it will be observed that the cylindrical portion is 
covered with a layer of firm, thick cuticula [ctd) strongly impregnated 
with salts of lime and resembling that which covers the greater 
part of the animal’s body; proximally this cuticula becomes thin 
and flexible in the joint between the stalk and the body, and 
distally it passes rather abruptly into a thin transparent layer, the 
corneal cuticula [crn\ which covers the retina. The thick cuti- 
cula, as well as that forming the joint of the stalk, has its inner 
surface covered with a thin cellular layer, the hypodermis (Ä’cfrw), 
which in the region of the corneal cuticula becomes greatly thick- 
ened, forming the retina [r], From the proximal surface of the 
retina an immense number of nerve fibres, the retinal f ihres 
(fbr.r ) , make their way to the ganglionic mass that lies in the 
central part of the stalk. This mass consists of four ganglia, which 
may be distinguished , beginning with the one nearest the retina, 
as the first, the second, the third, and the fourth optic gan- 
glion (I, II, III, IV). From the last of these the optic nerve 
[n.opt] extends to the brain. The enlargement of the brain formed 
by the expansion of the optic nerve may be called the optic lobe. 
The advantages that these terms present over some already pro- 
posed will be seen in the sequel. 
4. Retina, 
a. Form. 
The retina in Astacus^ even upon superficial examination, is 
seen to be unsymmetrically developed, its anteroposterior extent 
being much greater than its dorsoventral one. The ommatidia 
composing it are very uniform in size and regulär in arrangement, 
and afford a convenient means of measuring its dimensions. Their 
positions are marked by the corneal facets, wh eh nve square in 
outline and arranged upon what I have called the tetragonal System 
(cf. Parker, 91, pag. 60); i. e., they are regularly placed side by 
side so as to form rows extending in two directions at right angles 
to each other. These rows run obliquely from the anterodorsal angle 
