268 bulletin: museum of comparative zoology. 



accomplished to any desired degree, by the method already explained. 

 A less prolonged action of the gas leaves the pigment very faintly brown, 

 so that the pigment cells are readily distinguishable from the sensory 

 cells. Possibly the pigment in the eyes of other molluscs will yield to 

 the same treatment. 



Not only does methylen blue as an intra-vitam stain give a more 

 accurate picture of the form of the unpigmented cells of the retina than 

 it is possible to acquire by any other means, but, accepting the prevalent 

 view that it selects the nervous cells only, the stain gives conclusive 

 information as to the functions of the two kinds of cells in the retinas 

 of Helix and Limax. The unpigmented cells are undoubtedly sensory, 

 just as Henchman maintained. The failure of the pigment cells to stain 

 helps to corroborate Hesse's view that they probably have no sensory 

 connections. The staining of the sensory cells is so easy by this method 

 that the pulmouate eye offers favorable material for the demonstration 

 of a simple, though clearly unique, type of visual cell. As occasion 

 offers, the same method should be applied to a study of the eyes of 

 those gasteropods in which the sensory cells are apparently pigmented. 

 This method of staining also removes any doubt that might exist as to 

 the functioning of the accessory retina in the eye of Limax. 



It is too hazardous to deny that in any pultnonate the indifferent 

 pigment cells can be affected by light. The possibility of pigment 

 migration under the influence of light is not to be overlooked. 

 Parker ('99) has shown that in the rhabdome of an amphipod (Gam- 

 marus ornatus) the position of the pigment is not the same after the 

 animal has been in the dark for a time as it is in an animal which has 

 remained for some time in the light. In the former case some of the 

 pigment travels in a proximal direction, leaving a part of the rhabdome 

 quite unsurrounded by black pigment. When the same animal passes 

 into the light again, the pigment travels distally to surround the rhab- 

 dome. Parker concludes that the pigment in the latter position protects 

 the rhabdome from over-stimulation by light internally reflected from 

 the white pigment. In dim light the withdrawal of pigment permits the 

 internal reflecting apparatus to become effective. I have been able 

 to show, in a research recently published (Smith, :05), that the photo- 

 tropic response of Gammarus annulatus varies under the two condi- 

 tions. Although the pigment in these amphipods lies in the sensory 

 cells, — on account of which the comparison to the gasteropod eye (Plan- 

 orbis) is not quite exact, — it is probable that pigment movement is 

 a direct response to light-stimulation in Planorbis as well as in the 



