292 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY 



Palsemonetes that have nervous counections are the proximal retinular 

 cells, the accessory cells and the distal cells not being supplied with 

 nerves. Since photomechanical changes occur in both the accessory and 

 the distal cells, the inference might be drawn that in these instances the 

 changes were necessarily independent of the central nervous organs. 

 But it might also be argued that these very changes indicate nervous 

 connections that have escaped the eye of the anatomist. To this it 

 might be replied that, as the nervous connections of the proximal cells 

 are so very obvious, it is highly improbable that the distal and accessory 

 cells have a hitherto undescribed nerve supply. So far, then, as the 

 purely anatomical relations are concerned, they indicate that the photo- 

 mechanical changes, in the accessory and distal cells at least, are inde- 

 pendent of central nervous influences. 



In the case of the proximal retinular cells, where each cell possesses a 

 single nerve fibre, the central nervous organs might control the pigment 

 changes. However, if they do, the retinal fibres afford, so far as I know, 

 the first good instance of normal double conduction. Since each retinal 

 nerve fibre is the one nervous process from some proximal retinular cell, 

 and since all these cells show photomechanical changes, it follows that, 

 if these changes are controlled by the central nervous organs, all retinal 

 fibres must transmit central impulses peripherad. As these same fibres 

 ai'e the only nervous connections between the retina and the central ner- 

 vous organs, some at least must also transmit retinal impulses centrad. 

 Therefore, if it can be shown that the central nervous organs influence 

 the photomechanical changes in the proximal retinular cells, it is like- 

 wise demonstrated that double conduction is a natural occurrence. As 

 Fick ('95, p. 73) justly remarks, the solution of this problem involves 

 one of the most fundamental principles concerning the transmission of 

 nervous impulses. 



The method by which I proceeded to test this matter in Palsemonetes 

 consisted in examining the changes that went on in eyes after their con- 

 nection with the central nervous organs had been severed. These con- 

 nections were conveniently cut in one of two places ; either the whole 

 optic stalk was excised, in which case the optic nerve was cut between 

 the brain and the optic ganglia, leaving the latter in normal connection 

 with the retina, or the retinal end of the stalk was cut off, thus separat- 

 ing the retina in the region of the retinal nerve fibres from the optic 

 ganglia as well as from the brain. 



To ascertain whether the brain had any influence over the retinal pig- 

 ment, the following experiments were tried. Four live shrimps, whose 



