The Ketina and Optic Ganglia in Decapods, especially in Astacus. 33 
one impression than many. Owing, however, to the peculiar over- 
lapping of the quarter- and half-plates of adjoining rhabdomes as 
previously described, even this assumption seems to me inadmissible; 
for, in conformity with this overlapping, there can in reality be 
only three retinal elements in each rhabdome. This number is 
obviously too small to constitute any serviceable retina, and I, there- 
fore, believe that, both from the character of the Image thrown on 
the rhabdome and from the structure of this body itself, we are 
justified in concluding that each rhabdome is a perceptive 
Organ for a single impression. 
To this conclusion the objection might be raised that the function 
ascribed to the rhabdome is altogether too simple to require such 
a complicated structure as this organ possesses. But this objection 
seems to me falsely grounded, for, though each rhabdome cannot 
receive more than a single impression at a time, it does not follow 
that the impressions themselves cannot be complex ; in fact, I believe 
we must assume them to be so, since the animals give every evidence 
of distinguishing between lights of different intensity as well as of 
different colors. It, therefore, seems to me possible that the complex 
structure of the rhabdome is necessitated by its complex function. Thus, 
as was previously stated, diffused daylight enters only the distal 
portion of the rhabdome, while strongest light is required to pene- 
trate to its proximal end. Since diffused daylight can, therefore, 
stimulate only the more distal fibrillae, while light of greater intensity 
effects not only these but also fibrillae in more proximal positions, 
it is conceivable that the difference in intensity between any two 
impressions might be expressed by the difference in numbers of 
fibrillae thrown into functional activity in the two cases. If this be 
true, an efficient rhabdome would consist of numerous fibrillae placed 
at different levels in respect to the light. Why, as is actually the 
case, there should also be many fibrillae at the same level is not 
easily understood, unless we assume that there are different ones for 
the perception of different colors, in which case the series might be 
repeated at successive levels. But whether this be true or not, 
it must be remembered that on the sides of our own retinas, the 
elements are more numerous than the impressions we receive, and, 
though I am as unable to explain this excess as I am that in the 
rhabdome of Astacus^ the facts that such an excess really exists, 
proves, I believe, that the objection mentioned in this paragraph is, 
after all, without weight. 
Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. 12 3 
