G. H. Parker 
thus far proposed is fully satisfactoiy, I bave not attempted to in- 
vent new ones, but bave adopted sueb names from tbose already 
in use as seemed to me appropriate on account of tbeir expressiveness 
or tbeir more general acceptance. 
Wben a longitudinal section of an optic stalk is examined 
(PL 1 Fig. 27), it vrill be observed tbat tbe cylindrical portion is 
covered witb a layer of firm, tbick cuticula [cta] strongly impregnated 
witb salts of lime and resembling tbat wbicb covers tbe greater 
part of tbe animal's body; proximally tbis cuticula becomes tbin 
and flexible in tbe Joint between tbe stalk and tbe body, and 
distally it passes ratber abruptly into a tbin transparent layer, tbe 
come al cuticula [crn]^ wbicb Covers tbe retina. Tbe tbick cuti- 
cula, as well as tbat forming tbe Joint of tbe stalk, bas its inner 
surface covered witb a tbin cellular layer, tbe bypodermis [h'drm]^ 
wbicb in tbe region of tbe corneal cuticula becomes greatly tbick- 
ened, forming tbe retina (r). From tbe proximal surface of tbe 
retina an immense number of nerve fibres, tbe retinal fibres 
(fbr.r) , make tbeir way to tbe ganglionic mass tbat lies in tbe 
central part of tbe stalk. Tbis mass consists of four ganglia, wbicb 
may be distinguisbed , beginning witb tbe one nearest tbe retina, 
as tbe first, tbe second, tbe tbird, and tbe fourtb optic gan- 
glion (I, II, III, IV). From tbe last of tbese tbe optic nerve 
[n.opt] extends to tbe brain. Tbe enlargement of tbe brain formed 
by tbe expansion of tbe optic nerve may be called tbe optic lobe. 
Tbe advantages tbat tbese terms present over some already pro- 
posed will be Seen in tbe sequel. 
4. Retina, 
a. Form. 
Tbe retina in Astacus^ even upon superficial examination, is 
Seen to be unsymmetrically developed, its anteroposterior extent 
being mucb greater tban its dorsoventral one. Tbe ommatidia 
composing it ^ are very uniform in size and regulär in arrangement, 
and afford a convenient means of measuring its dimensions. Tbeir 
positions are marked by tbe corneal facets, wb cb are squai'e in 
outline and arranged upon wbat I bave called tbe tetragonal system 
(cf. Parker, 91, pag. 60); i. e., tbey are regularly placed side by 
side so as to form rows extending in two directions at rigbt angles 
to eacb otber. Tbese rows run obüquely from tbe anterodorsal angle 
