520 WM. A. KEPNER AND A. M. FOSHEE 



of the watch glass. No effort was made to ehminate the heat 

 factor. In our prehminary experiments it was found necessary 

 to isolate the ones that were to be subjected to hght; for they 

 showed a great tendency to hide beneath one another, if they were 

 not separated. It was also found that they were killed by ex- 

 posure to the open northern sky. The specimens were, therefore, 

 placed ten feet away from a window in the laboratory. The 

 second lot of animals of each collection was placed in a vessel of 

 spring water and carried into a dark room. After forty-eight 

 hours each series was fixed for two to five minutes in chrome- 

 aceto-formalin. A given lot of fixing fluid was di\aded and one- 

 half of it was used on the animals of the hght series and the other 

 portion of the fixing fluid was used for fixing the animals that 

 had been kept in the dark. Of course the fixing of each series 

 was done in the light or the dark as determined by the condition 

 under which the members of the series had been kept. After 

 the fixing all other steps were carried on in the light. The 

 specimens from the fixing fluid were washed quickly in four 

 changes of tap water and quickly carried through the alcohols, 

 xylol and paraffin; so that the animals were in a paraffin block 

 within one half hour after they had been fixed. The sections 

 were stained in iron hematoxyhn in a few cases, but Mallory's 

 connective tissue stain was for the most part employed. 



The relative position of the \dsual element and accessory 

 cell in material so handled can readily be made out when immer- 

 sion lenses are employed. It can thus be seen that, as Hesse 

 has observed, the rhabdomes of the visual cells were not directed 

 towards the light as it entered the eye, but away from the light 

 and towards the cells of the pigment cup, the hght passing 

 through a great part of the \dsual cell before encountering the 

 visual rod or rhabdome. It is interesting to note that this 

 inverted position of the retinal elements is encountered in 

 vertebrates, some molluscs and turbellaria. In these three 

 groups we must, therefore, have eyes that represent either (a), 

 a suppression of this type of eye through many intervening 

 phyla to appear atavistically in these three groups; or (b), a 

 retention of this type in certain mollusca and all vertebrates 



