PAKKER: KETINAL PIGMENT CELLS OF PAL^MONETES. 291 



eyes that have been kept iu 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 Excised 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 Stefauowska ('90, p. 150) 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 corrosive 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 experiment, but is the application to other groups of 

 animals of a generalization first made by Engelmann and his followers for 

 vei'tebrates. As is well known, Engelmann showed that, when one eye 

 of a frog Avas 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 eye a condition characteristic for 

 the light. This observation naturally led to the conclusion that the 

 pigment cells 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 Palaemone- 

 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 



