The Retina and Optic Gangiia in Decapods, especially in Astaci! s. 45 
type (PI. 1 Fig. 36, x) , and whence they proceed by a superficial 
course to pass obliquely over the dorsal face of the fourth ganglion 
tili they fìnally enter the dorsal part of the optic nerve and make 
their way to the brain. The fibres of the second decussation together 
with their appended ganglionic cells constitute the neurons of the 
third order. 
Third Optic Ganglion. This ganglion (PI. 1 Fig. 27, III) 
consists of a nearly spherical mass of »Punktsubstanz« almost iden- 
tical in composition with that of the second ganglion. The great 
majority of the neurons of the third order enter it distally and ter- 
minate in it; some few, however, as already mentioned, pass over 
it and enter the optic nerve directly. From its proximal face spring 
the fibres that connect it with the fourth ganglion. It is surrounded 
laterally, except on its posterior side, by small ganglionic cells 
(PI. 1 Fig. 36, cl.gn)^ which belong, as already mentioned, to the 
neurons of the third order. Near its posterodorsal angle is a group 
of large cells [x]^ to which are connected the fibres that pass directly 
from the second ganglion to the brain. 
Third Decussation. This decussation, as Grenacher (79, 
pag. 121) long ago pointed out, is by no means so regulär as the 
first and second, and, in fact, the fibres cross in such an erratic way 
that it is almost impossible to be sure that there is a true decus- 
sation present. The most that can be said about the character of 
the Crossing is that the fibres from the posterior end of the third 
ganglion pass over to the dorsal part of the fourth and that those 
from the anterior end of the third gain a ventral position in the 
fourth. Beyond this it is difficult to make any very definite State- 
ments, and this, as will be seen, involves only a partial crossing of 
the fibres. Excepting the few fibres already mentioned as passing 
directly from the second ganglion to the optic nerve, ali the fibres 
in the third decussation take their origin in the »Punktsubstanz« of 
the third ganglion and terminate in that of the fourth. They are 
connected with ganglionic cells of the small type, which fili the space 
between the third and fourth gangiia, and, together with these cells, 
constitute a system of neurons of the fourth order. 
Fourth Optic Ganglion. This ganglion, the largest of the 
four masses, differs from the others in several important particulars. 
Its surface, except about its distai and proximal ends, is irregularly 
covered with large instead of small ganglionic cells (PI. 2 Figs. 55, 
56, cl.gn)\ and the substance of the ganglion, instead of consisting 
