RETINAL PIGMENT OF PLANORBIS 369 



n. opt.). The optic nerve is covered by a sheath of connective 

 tissue continuous with that of the optic sac. The work of 

 Babuchin ('65), Henchman ('97), and particularly that of 

 Smith ('06) has proved that the sensoiy cells of pubnonate 

 gasteropods are of fibrillar composition, the terminal brush- 

 like neurofibrils of the rod mantle (fig. CJhrl.") being continuous 

 with the fibers of the rod axis {fhrl.') of the cell body, and of the 

 neurite. The length of the rod is 6 )u or 7 m- 



The lens (fig. B, Ins.) is a large pear-shaped body occupying 

 almost the whole interior of the optic sac. It is a non-cellular 

 secreted mass, the outer portion of which forms a denser cap- 

 sule or rind. 



The vitreous humor (fig. B, hu. vit.) fills whatever spaces inter- 

 vene between the central zone of the retina and the lens, as well 

 as the spaces between the indi\-idual rods. 



EXPERIMENTAL PART 



a. Effect of light and darkness 



The initial experimentation consisted in determining whether 

 or not the retinal pigment of Planorbis undergoes positional 

 changes, whereby a characteristic distribution is assumed in 

 light and in darkness. 



The citation of Smith's statement ('06, p. 255) relative to the 

 tests performed by hun on this animal will serve to show both 

 his methods and his results : 



In spite of the fact that the rods, lying, as they do, distal to the 

 pigment zone, cannot be protected by pigment migration, as can the 

 rods in cephalopods, the variable position of the proximal region of 

 the pigment suggests the possibility of pigment migration in the eye 

 of Planorbis. I therefore attempted to determine b}- a few experiments 

 whether differing light conditions would produce corresponding changes 

 in the position of the pigment. Several specimens of Planorbis were 

 placed for an hour or more in water in a white, porcelain dish which 

 was set in a sunny window. Their heads were then cut off with scissors 

 and fixed in Perenyi's fluid. Sections were made in the ordinary way 

 and stained in Heidenhain's iron-haematoxylin, followed by orange-G. 

 Similar preparations were made from specimens which had been kept 

 in darkness for an hour or more before killing. Comparisons were 



