The Ketìna and Optic Ganglia in Decapods, especially in Astacus. 17 
separate, retaining on their axial faces the rows of half- or quarter- 
plates belonging to them. 
From the preceding account it must be evident that the rhab- 
domeres in Astacus and Homarus are not single continuous bodies, 
as, for instance, in Porcellio^ but consist of a series of separate 
pieces, the half- or quarter-plates, which project from the retinular 
celi like the teeth on a rack in a rack and pinion. Because of 
this resemblance I propose to cali such rhabdomeres toothed, to 
distinguish them from continuous simple rhabdomeres as in Porcel- 
lio. The same terms may be used to designate the rhabdomes 
formed by the union of one or other kind of rhabdomeres. 
The structure of the toothed rhabdome, as I bave described it, 
offers, I believe, 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 be divided into 
four rhabdomeres inclined Grenacher to believe that only four 
retinular cells were concerned in its production, though it is sur- 
rounded by seven such. From what I bave said of the two kinds 
of plates in the rhabdome and their alternation, it must be clear 
that ali seven cells produce rhabdomeres 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 piate, 
to divide the rhabdome into four rhabdomeres. 
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 piane 
of section passes through a dorsoventral piate, 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 celi 2. As is 
easily seen, this consists almost entirely of fine fibres, which extend 
from the piane of Separation between the two half-plates to celi 2. 
This celi itself dififers in appearance from the others in that its 
fibrillar axis is obscured by a deposit of Silver precipitate similar 
to that which colors the fibres themselves; nor is the coloration 
limited to this piane 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 
