The Ketina and Optic Ganglia in Decapods, especially in Astacus. 17 
separate, retaining on their axial faces the rows of half- or quarter- 
plates helonging to them. 
From the preceding account it must he evident that the rhah- 
domeres in Astacus and Homarus are not single continuous hodies, 
as, for instance, in Porcellio^ but consist of a series of separate 
pieces, the half- or quarter-plates, which project from the retinular 
cell like the teeth on a rack in a rack and pinion. Because of 
this resemblance I propose to call such rhahdomeres toothed, to 
distinguish them from continuous simple rhahdomeres as in Porcel- 
lio. The same terms may he used to designate the rhabdomes 
formed hy the union of one or other kind of rhahdomeres. 
The structure of the toothed rhabdome, as I have descrihed it, 
offers, I helieve, an answer to a question that Grenacher (79, 
pag. 124) raised concerning the origin of this organ and that, so 
far as I am aware, has never been satisfactorily settled. The fact 
that in transverse sections the rhabdome seems to he divided into 
four rhahdomeres inclined Grenacher to helieve that only four 
retinular cells were concerned in its production, though it is sur- 
rounded by seven such. From what I have said of the two kinds 
of plates in the rhabdome and their alternation, it must be clear 
that all seven cells produce rhahdomeres and that what Grenacher 
took for the planes of Separation between the four supposed rhab- 
domeres are really the division-planes between the half-plates. 
These planes, since they are at right angles to each other in ad- 
joining plates, seem, in transverse sections thicker than one plate, 
to divide the rhabdome into four rhahdomeres. 
The structure of the half-plates in the rhabdome is most clearly 
seen in preparations made by Golgi’s method. A retinula prepared 
in this way and cut transversely is shown in figure 24; the plane 
of section passes through a dorsoventral plate, i. e., components of the 
rhabdome belonging to retinular cells 1, 2, 5, and 6, and, as often 
happens with this method, only a portion of the preparation is col- 
ored, in this case the quarter-plate belonging to cell 2. As is 
easily seen, this consists almost entirely of fine fibres, which extend 
from the plane of Separation between the two half-plates to cell 2. 
This cell itself differs in appearance from the others in that its 
fibrillär axis is obscured by a deposit of silver precipitate similar 
to that which colors the fibres themselves; nor is the coloration 
limited to this plane only, but, as I demonstrated to my satisfaction 
by studying successive sections, it extends distally and proximally 
Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. 12. 2 
