SMITH : EYES OF PULMONATE GASTEEOPODS. 277 



medium, but that there shall be a multitude of ultimate fibrils oriented 

 and exposed to the light like the needles on a young pine twig. That 

 the orientation of the fibrils is more easily maintained if they are im- 

 bedded immovably in a matrix belonging to the sensory cell is quite 

 evident. 



Summary. 



(1) Intra vitam staining shows (a) that the pigment cells of the retina 

 of Helix and Li max are indifferent, and (IS) that the pigment-free cells 

 are sensory. 



(2) The pigment cells are attached to the eye-capsule by fine root-like 

 proximal branches — the radicular. 



(3) Each sensory cell gives off proximally one neurite to the optic 

 nerve and (probably) one or more branched processes which attach the 

 cell to the capsule. 



(4) By the use of the polariscope it is shown (a) that the fibrillae 

 of the rods are not artifacts but normal structures of the living cell ; 

 (IS) that the fibrils of the rod are doubly refractive to light. 



(5) The neurofibrillae from the rod pass into the cell at first parallel 

 and distinct. In the vicinity of the nucleus they pass over into a net- 

 work, which gives off one or more fibrils to the neurite. The network 

 is not segregated into two regions, peripheral and perinuclear, as Apathy 

 found in ganglion cells of the leech, but is either uniformly distributed, 

 or massed into main paths through the cell. 



(6) The mantle of the rod contains the ultimate, recipient fibrillae of 

 the optic apparatus, imbedded in a matrix of delicate, passive material 

 produced by the sensory cell. 



(7) Histological conditions reveal appearances of pigment migration 

 in the pigment cell of the retina of Planorbis trivovis, but the con- 

 ditions under which the migrations occur have not been determined. 



Postscript. 



By an unaccountable oversight the preliminary paper by R. Backer, " Zur 

 Kenntnis der Gastropodenaugen " (Zool. Anz., Bd. 25, p. 548, 21. Juli, 1902), 

 entirely escaped attention until to-day, when it accidentally came to my 

 notice. The final paper by Backer has also been published (Arbeiten Zool. 

 Inst. Wien. Bd. 14), as I learn from the cards of the Concilium Bibliographicum 

 (Zurich). Unfortunately this volume is not accessible in Cambridge. 



The main conclusions reached by Backer in his preliminary account are as 

 follows: The non-pigmented cells of the retina are sensory; the pigment cell3 

 are not. Each of the former terminates at its base in a nerve fibre, distally in 



