272 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



animals are doubly refractive. Howard based Lis argument for the 

 probable existence of fibrils in the rod of vertebrates partly on the re- 

 actions which he observed in the rhabdome of crustaceans, which, as is 

 known (Parker, '95), contains the peripheral endings of the optic appa- 

 ratus. The rod of Limax furnishes an example which is both more 

 accessible and more to the point, for in the mantle of the rod in Limax 

 the fibrils can actually be seen in the polariscope. Just what is the sig- 

 nificance of the double refraction of impulse-conducting fibrils is not yet 

 clear. 



The radial mantle-fibrils in the rod of Limax, which Babuchin discov- 

 ered, and the longitudinal fibrils in the axis of the rod of Pteroceras, which 

 Hensen pointed out, were first described in their proper relations to each 

 other by Henchman. Their function as neurofibrils, which might have 

 been suspected from the interpretations which Patten placed upon fibrils 

 which he observed in the eyes of various molluscs, remained as yet un- 

 announced. From the studies of Hesse we first get a conception of the 

 physiological meaning of fibrils in the rod of the gasteropods. He traced 

 the transition from the single fibrillar brush in the rod of Patella to the 

 broad, fibrillar mantle of Helix, and finally to the large, cylindrical rod of 

 Limax. Such a rod as the last differs in degree, but not in kind, from 

 the single brush of Patella, and Hesse unequivocally ascribed to these 

 end fibrils in the rod of the gasteropods the function of light-reception. 



Henchman described both the body of the cell and the axis of the rod 

 as "longitudinally fibrous," and Hesse reported similar indications from 

 the eye of Pleurobranchus. He was able to trace a single fibril through 

 the slim, pigmented rod-cell of Patella into the neurite. In other gas- 

 teropods he could only infer the presence of fibrils in the cell body. His 

 figures of the sensory cells of Helix show a very faint longitudinal stria- 

 tion, but he does not refer to the fact in his text. As far as Hesse's 

 observation of the single fibril in the rod-cell of Patella is concerned (or 

 the single fibril in the rod and rod-cell of the cephalopod eye, and even 

 the parallel fibrils in the case of Pleurobranchus), there is no reason to 

 suppose that it is erroneous : for whatever the condition in the pulmo- 

 nates, it is evident that the fibrils need not be similarly disposed in all 

 groups of molluscs. 



There is one thing, however, in ordinary sections of the retinas of 

 Helix and Limax which gives a false impression of parallel neurofibrils 

 in the cell body. It has been shown that, in cross sections through the 

 pigmented zone, the sensory cell exhibits a polygonal outline. Hence 

 in radial sections of the retina one sometimes gets a side view of this 



