PARKER: CHANGES IN RETINAL PIGMENT OF GAMMARUS. 145 



however, were almost entirely devoid of pigment, while the proximal 

 portions were as densely filled with pigment as the distal portions. 



Obviously the changes induced by the presence or absence of light 

 affect the pigment of only the middle and proximal parts. When an 

 animal that has been kept in the dark is exposed to the light, the pig- 

 ment that is massed in the proximal parts of the retinular cells (Fig. 3) 

 migrates distally and fills the middle portions, without however entirely 

 abandoning the proximal parts, especially around the nucleus (Fig. 1). 

 When an animal that has been kept in the light is placed in the dark, 

 the pigment in the middle portions (Fig. 1) migrates into the proximal 

 parts till almost no pigment is left in the middle portions. In other 

 words, the presence of light induces a distal migration of much of the 

 pigment from the proximal parts and the absence of light brings about a 

 proximal migration of almost all the pigment in the middle parts. 



In none of my observations was there any evidence of photomechanical 

 changes in the accessoi-y pigment cells. 



Aside from the dispai'ity due to the different anatomical descriptions 

 of the eyes, the physiological results given in this paper confirm in the 

 main those given by Szczawinska ('91, p. 548). In one respect only is 

 there a significant difference. Szczawinska claims that in G. roeselii the 

 pigment in what I have called the distal parts of the retinular cells shows 

 photomechanical changes. In G. ornatus no evidence of such changes 

 could be found, and since in G. roeselii, according to the figures given 

 by Szczawinska (Planche XVI. Figs. 1, 2), the supposed evidence of 

 these changes may be entirely the result of a slight difference in the 

 planes at which the sections have been cut, it may fairly be doubted if 

 these changes occur at all. 



The relations that the photomechanical changes, described above, bear 

 to the physiology of the eye in G. ornatus are not far to seek. Light pass- 

 ing through the axis of any cone in the eye of this animal would be con- 

 ducted directly to the rhabdome under the given cone. Light entering 

 a cone obliquely to its axis would fall upon one of its pigmented sides, 

 where, if not absorbed, the light would suffer reflection. As the sides of 

 the cone are not parallel but approach proximally, the light would not 

 undergo simple internal reflection as in a cylinder, but would be so 

 turned at each reflection that it would eventually be discharged from 

 the end of the cone at which it entei*ed. Thus oblique light would not 

 reach the underlying rhabdome at all. This action, by which the axial 

 light of the cone is conducted to the rhabdome and the oblique light is 

 discharged, has been called by Exuer ('91, p. 59) the catoptric action of 



