PARKER: RETINAL PIGMENT CELLS OF PALZEMONETES. 291 
eyes that have been kept in the dark, namely, occasional single proxi- 
mal retinular cells which, instead of having their pigment granules 
transported to the retinal fibres, still hold them in their bodies, Such 
cells have probably suffered some pathological change by which their 
individual photomechanical functions have been interfered with. This 
independence in the action of parts of the retina has already been 
affirmed by Exner (91, p. 66) for the compound eyes of insects. 
PHOTOMECHANICAL CHANGES iN Exotsep EYES AND RETINAS. 
The extent to which the photomechanical changes in the retina are 
influenced by the central nervous organs has never been determined, I 
believe, for any arthropod. That some such influence is exerted is im- 
plied by several investigators ; thus Stefanowska (90, p. 156) states 
that, in preparing insects’ eyes, she cut the heads of the animals in two 
so as to prevent the nervous centres from affecting the retinal pigment 
cells, and Szczawinska ('91, p. 531) recommends as a fixing reagent a 
hot solution of eorrosive sublimate, because the action is so rapid that 
it is not necessary to use other means of intercepting the central ner- 
vous influences. This belief, that the central nervous organs can exert 
an influence on the retinal pigment cells, is not to my knowledge the 
result of direct oxperiment, but is the application to other groups of 
animals of a generalization firs& made by Engelmann and his followers for 
vertebrates. As is well known, Engelmann showed that, when one eye 
of a frog was protected from the light, the illumination of the other 
eye, or even of a portion of the surface of the body, sufficed to produce 
in the pigment cells of the protected eyo a condition characteristic for 
the light. This observation naturally led to the conclusion that the 
pigment colls of the retina were controlled in their movements by the 
central nervous organs, and that the optic nerve transmitted impulses 
centrifugally as well as centripetally., Fick (95, pp. 77 and 81), how- 
ever, has recently demonstrated that the same changes occur in a frog’s 
eye even after the optic nerve and sympathetic nerves have been cut, 
and that therefore the central nervous organs take no part in these 
changes. 
Before turning to the experimental evidence obtained from Palemone- 
tes, it will be well to consider some of the consequences of this question. 
In order that the central nervous organs should have any influence on 
the retinal pigment cells, the two sets of structures must be in nervous 
connection. So far as is known, the only structures in the retina of 
