The Retina and Optic Ganglia in Decapods, especially in Astacus. 27 
must, on the one haud, be connected witb the exterior by means of 
structures capable of transmitting light to them, and, on the other 
hand, they must have nervous Connections with the brain. Of the 
various views known to me concerning their location, that suggested 
by Lowne (84) seems least successfully in meeting the requirements 
mentioned above. According to this investigator the perceptive ele- 
ments do not occur in what is usually termed the retina, but in 
what I have designated the first optic ganglion, where they are 
represented by a layer of bodies called by Viallanes (92 a, pag. 391) 
the neurommatidia. This layer, however, as I shall presently show, 
is not accessible to light nor does it represent the distal ter- 
mination of the optic fibres, two facts that appear to me fatal to 
Lowne’s view. So, too, Wagner’s (35, pag. 372) opinion that the 
pigmented sides of the cone form a perceptive surface, as well as 
Leydig’s (55, pag. 416) and Patten’s (86, pag. 692) belief that the 
cone itself is the perceptive body, seems to me erroneous, since these 
structures, though accessible to light, have been shown to be without 
nervous Connections. Objections can also be raised against Vial- 
lanes’ opinion that the pigmented distal ends of the proximal reti- 
nular cells are the perceptive structures and that the pigment itself 
plays an essential part in the production of the visual impulse. If 
these cells act as Viallanes believes they do, it is not easy to 
understand why the axis cylinder extending through each of them 
does not terminate in their distal ends instead of in the rhab dorne 
as admitted by Viallanes himself. Moreover the fact that albino 
animals, such as rabbits (cf. Angelucci, 78, pag. 377) are able to 
see, though the postretinal epithelium of their eyes is devoid of 
pigment, shows conclusively that this pigment does not play the 
essential part in vision that Viallanes ascribes to it; and, further, 
since this view, like that of Wagner, Leydig and Patten, offers no 
explanation for the presence of the rhabdome nor for the migration 
of the pigment in the retinular cells, I believe that it, like the others, 
must be abandoned. 
Unsatisfactory as these different opinions are, there is still one 
against which it seems difficult to raise serious objections and that 
is the opinion first clearly expressed, I believe, by Max Schultze 
(68, pag. 12) and afterwards defended by Grenacher (79, pag. 157) 
that the rhabdome is the perceptive organ. The fact that the diop- 
tric apparatus of the eye transmits light to the rhabdome but no 
further, that there are nervous Connections between rhabdome and 
