270 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



isolation " of the rod is here impossible. This fact may help to explain 

 the small capacity for " vision " which gasteropods show. Although 

 Willem ('92") thinks he has demonstrated that Helix can see — in the 

 ordinary acceptance of the word — an object at a distance of a mili- 

 metre, it is evident to one who observes the actions of these animals 

 that vision in the gasteropods is quite subordinated to reflexes of a 

 directive nature, gasteropods being extremely sensitive to differences in 

 light-intensity. The lack of isolation of the rods by pigment explains, 

 in part at least, the lack in this class of molluscs of sharp vision such as 

 the cephalopods possess. 



Aside from preventing internal reflections within the rod zone, it is 

 possible that the pigment is directly protective to that part of the visual 

 cell which is surrounded hy it. We do not know that the middle part 

 of the sensory cell is not sensitive to light. Neither do we know how 

 or where light-vibrations are transformed into nervous impulse. If 

 the transformation takes place in the middle zone, the pigment may 

 serve some purpose there. 



The " optic ganglion " which Henchman ('97, p. 428) mentions as " a 

 funnel-shaped enlargement of the optic nerve containing oval nuclei," is 

 shown by the methylen-blue method to be a misinterpretation, for only 

 connective-tissue nuclei occur there normally. It may be that nucleated 

 portions of a few sensory cells had been pushed out into the base of the 

 optic nerve, as they are in the abnormal protrusions near the optic nerve, 

 already described, and that this condition led to the misinterpretation ; 

 or it may be that connective-tissue nuclei were mistaken for the nuclei 

 of the ganglion cells. Thus the old notion of an optic ganglion in the 

 gastropod eye, first introduced by Leydig ('57, p. 253) and adhered to 

 by Simi'oth ('76), Carriere ('85, p. 16), Hilger ('84), and Henchman ('97) 

 must be abandoned. 



In some minor details my observations have not agreed with those of 

 Henchman and Hesse on the accessory retina. Some of the discrepan- 

 cies may, however, find their explanation in the variability of this struc- 

 ture. Hesse figures the accessory retina as made up of cells arranged 

 exactly as in the chief retina. The sensory cells are given a radial 

 position between indifferent cells, which show a more granular cyto- 

 plasm than the corneal cells. 



Never, as far as I have observed, is there a regular arrangement of the 

 sensory cells. The typical, irregular arrangement may be seen in Figures 

 13 and 16 (Plate 2). The indifferent cells of the accessory retina re- 

 semble the corneal cells in every way, except that their nuclei are very 



