EFFECTS OF LIGHT AND DARKNESS ON THE EYE 521 



from their direct ancestral stock, thus making the inference that 

 these forms arose from a much lower level on the phylogentiq 

 tree than is usually suggested — lower even than Hubrecht's 

 nemertean ancestry for the vertebrates; or (c), we must have 

 in these three groups three cases of parallel evolution. We have 

 suggested these alternatives by way of indicating the fields of 

 general interest into which this study has led us, without com- 

 mitting ourselves to any of them. The analogy however, be- 

 tween these eyes and those of vertebrates is so striking that we 

 have become interested in trying to find an analogy of function 

 between the visual elements of the vertebrate and Prorhynchus. 



The pigment cells of vertebrates are influenced by light and 

 its absence. "In general it may be said that light induces an 

 expansion and the absence of light a contraction, of pigment 

 cells;" Arey ('16 b, p. 360). 



The accessory cell or pigment cell associated with the visual 

 cell of Prorhynchus is closely applied to the rhabdome or recep- 

 tor of the retinula (fig. 1). In eyes that have been subjected to 

 alternation of day and night conditions, when the specimens 

 are free to seek and find optimum conditions of illumination, 

 this cell in each eye is found to have a uniformly stratified cyto- 

 plasm. Little change is to be observed in these cytoplasmic 

 strata when the eyes have been illuminated continuously for 

 forty-eight hours or longer. Figures 10, 11, 12 show the closely 

 crowded strata of cytoplasm in the pigment cells that have been 

 continuously illuminated for two nights and two days or longer. 

 It is to be seen here that, if anything, these eyes show a con- 

 traction of the pigment cells. We have met with one apparent 

 exception to this type of reaction on the part of an illuminated 

 pigment cell. This exception is shown in figure 9. Here the 

 cytoplasmic strata are less crowded and lie less regularly about 

 the contour of the rhabdome. Examination shows that this is 

 the right eye of the animal and that the left eye has a typical 

 Hght adapted pigment cell. It is suggested by this that the 

 right eye might have been shaded in some manner. When we 

 first undertook to expose animals to the continued illumination, 

 it was found that the specimens would frequently disgorge some 



