SMITH : EYES OF PULMOXATE GASTEROPODS. 



237 



the retinas of the species in which he has been able to see this fibril best 

 are the least suitable for study, because their sensory cells (as in Patella) 

 are not only exceedingly slender but also pigmented. In the large, un- 

 pigmented, sensory cells of Helix and Limax (which are more favorable 

 for study) he has been able to trace the fibrillae only the shortest 

 distance into the cells (see Fig. C). The clearest case which he pre- 

 sents (:00, Fig. 6) is in the eye of an opistho branch, PleurobranchuSj 

 in which he has shown parallel fibrils passing directly through the large 

 sensory cells. Hesse has very convincingly reinterpreted the earlier 

 work of Grenadier ('86) and asserts that the rod of the cephalopod eye 

 contains a single fibril which ends in a little knob distally. He shows 

 that the fibril traverses the length of the cell, being continuous with the 

 neurite. In another article Hesse (:02 a ) describes the rod of Limax 

 more in detail than did Henchman, only the title of whose paper he had 

 seen. In the body of the sensory cells of 

 Helix he (Hesse, :02 b , Fig. 15) represents a 

 striation more or less longitudinal, although 

 he does not call attention to the fact in his text. 



Thus, knowledge of the fibrillae in the 

 body of the sensory cells of gasteropods, as 

 in other groups, is almost entirely lacking. 

 Furthermore, it has not been established 

 with absolute certainty which of the cells of 

 the retina in pulmonate gasteropods are sen- 

 sory, for the reason that, hitherto, no one 

 has succeeded in this field with either methy- 

 len-blue staining or silver impregnation. 

 In addition, there are certain histological 

 questions of minor importance, the settlement of which would make 

 our understanding of the gasteropod eye more clear. 



The present research embraces some of these secondary questions, but 

 is concerned especially with the neurofibrillae of the retina of the pulmo- 

 nate gasteropods. 



I am greatly indebted to Prof. E. L. Mark for supervision and helpful 

 suggestions. 



Material and Methods. 



My studies of neurofibrils have been carried out almost entirely on 

 Limax. maximus L. For experiments with methylen blue, however, 

 Helix pomatia L. was also used. The aquatic species Planorbis tri- 

 volvis Say gave some evidence of pigment-migration. 



Fig. C. Helix pomatia, after 

 Hesse. «/*., Neurite of sensory 

 cell ; pzk., nucleus of indifferent 

 cell ; p., radicula; st., fibrillae. 



