smith: eyes of pulmonate gasteropods. 255 



"When this part of the cell is free from pigment, it appears to contain a 

 vacuole. More distally these cells have a cylindrical form, which they 

 preserve up to the place where they are lost to view in the dense pig- 

 ment zone. The cells whose nuclei lie near the capsule are very long 

 and slim distal to their nuclei, and do not seem to have the enlargements 

 just described for the other kind. If the pigment extends quite to the 

 nucleus, this part of the cell appears as a thin, black, granular line. 

 The possible width of the pigment zone is thus relatively greater than 

 in Limax ; but as the pigment does not ordinarily fill the cells to the 

 nuclei, the zone appears relatively narrower than in Limax. 



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

 ment 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 by a few experiments whether dif- 

 fering light conditions would produce corresponding changes in the posi- 

 tion 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-haematoxyliu, followed by orange-G. Similar prepara- 

 tions were made from specimens which had been kept in darkness for an 

 hour or more before killing. Comparisons were then made between the 

 two kinds of preparations in order to learn whether the position of the 

 pigment was different in the two cases. More cases in the "light" eyes 

 had the pigment reaching quite to the nuclei than in the " dark " eyes ; 

 but there was such a lack of uniformity in different animals, and even in 

 the same retina, that the evidence was not at all conclusive. Repeated 

 experiments did not lead to more definite results. I am satisfied that 

 the pigment does travel up and down in the pigment cells under the 

 influence of some stimulus, but just what is the exciting factor I have 

 not determined. I have not been able to get any evidence of pigment 

 migration in the eyes of either Helix or Limax. 



Neurofibrillae in the Eye of Limax. 

 In the following account of the fibrillae I shall first present the evi- 

 dence concerning them and leave the theoretical considerations, as far 

 as possible, for subsequent discussion. In the figures, beginning with 

 Figure 37 (Plate 4), darkness of tone or sharpness does not mean that 

 some fibrils in the sections appear darker than others, but that the fibrils 



