The Eetina and Optic Ganglia in Decapods, especially in Astacus. 4 5 
type (PI. 1 Fig. 36, x), and whence tbey proceed by a superficial 
course to pass obliquely over tbe dorsal face of tbe fourtb gangliori 
tili tbey finally enter tbe dorsal part of tbe optic nerve and make 
tbeir way to tbe brain. Tbe fibres of tbe second decussation togetber 
witb tbeir appended ganglionic cells constitute tbe neurons of tbe 
tbird Order. 
Tbird Optic Ganglion. Tbis ganglion (PI. 1 Fig. 27, III) 
consists of a nearly spberical mass of » Punktsubstanz (c almost iden- 
tical in composition witb tbat of tbe second ganglion. Tbe great 
majori ty of tbe neurons of tbe tbird Order enter it distally and ter- 
minate in it; some few, however, as already mentioned, pass over 
it and enter tbe optic nerve directly. From its proximal face spring 
tbe fibres tbat connect it witb tbe fourtb ganglion. It is surrounded 
laterally, except on its posterior side, by small ganglionic cells 
(PL 1 Fig. 36, clgn), wbicb belong, as already mentioned, to tbe 
neurons of tbe tbird order. Near its posterodorsal angle is a group 
of large cells [x], to wbicb are connected tbe fibres tbat pass directly 
from tbe second ganglion to tbe brain. 
Tbird Decussation. Tbis decussation, as Grenacher (79, 
pag. 121) long ago pointed out, is by no means so regulär as tbe 
first and second, and, iq fact, tbe fibres cross in such an erratic way 
tbat it is almost impossible to be sure tbat tbere is a true decus- 
sation present. Tbe most tbat can be said about tbe cbaracter of 
tbe Crossing is tbat tbe fibres from tbe posterior end of tbe tbird 
ganglion pass over to tbe dorsal part of tbe fourtb and tbat tbose 
from tbe anterior end of tbe tbird gain a ventral position in tbe 
fourtb. Beyond tbis it is difficult to make any very definite State- 
ments, and tbis, as will be seen, involves only a partial Crossing of 
tbe fibres. Excepting tbe few fibres already mentioned as passing 
directly from tbe second ganglion. to tbe optic nerve, all tbe fibres 
in tbe tbird decussation take tbeir origin in tbe »Punktsubstanz« of 
tbe tbird ganglion and terminate in tbat of tbe fourtb. Tbey are 
connected witb ganglionic cells of tbe small type, wbicb fill tbe space 
between tbe tbird and fourtb ganglia, and, togetber witb tbese cells, 
constitute a System of neurons of tbe fourtb order. 
Fourtb Optic Ganglion. Tbis ganglion, tbe largest of tbe 
four masses, differs from tbe otbers in several important particulars. 
Its surface, except about its distal and proximal ends, is irregularly 
covered witb large instead of small ganglionic cells (PI. 2 Figs. 55, 
56, cLgn); and tbe substance of tbe ganglion, instead of consisting 
