SMITH : EYES OF PULMONATE GASTEROPODS. 253 



accessory retina is separated from the chief retina by a partition of 

 connective tissue, but I think it is not. 



The sensory cells of the accessory retina are in all essentials like those 

 of the chief retina, and their impregnation with methylen blue shows 

 that they are probably a functional part of the eye. Two such cells 

 (Plate 3, Figs. 28, 30) have already been mentioned. They are provided 

 with large but typical radiculae, and their neurites accompany to the 

 optic nerve those of the retinal cells of the chief retina in their vicinity. 

 The "neck" portion of the sensory cells of the accessory retina do not 

 suffer so much lateral compression as do the cells in the chief retina, 

 because apparently they are not arranged into ommatidial groups as in 

 the chief retina. Figure 25 shows a part of a very large sensory cell, 

 situated at the front of the accessory retina. Several pieces of neurite 

 are also shown. Figure 36 is a composite drawing made by superim- 

 posing the camera sketch from the section which contained the most of 

 the neurite over the one from the next section, which contained the 

 cell body. The two sketches fitted so exactly that there is no doubt as 

 to our having to do here with a sensory cell which shows a relatively 

 long stretch of neurite. 



The indifferent cells of the accessory retina represent the pigment 

 cells of the retina in one of their functions only ; they serve as a me- 

 chanical support to the sensory cells. They have throughout their 

 length the same prismatic form that the corneal cells do ; furthermore, 

 they resemble these in being filled with a very inconspicuous cyto- 

 plasm. Their nuclei, however, are a little larger than those of the 

 corneal cells. Their basal ends are not refractive and attenuated as 

 are the basal ends of the pigmented cells. 



The cells of the accessory retina do not have the radial arrangement 

 characteristic of the chief retina. In relation to the indiflei'ent cells 

 the sensory cells appear to be oriented in a most irregular manner (Plate 

 2, Fig. 13). Figure 16 shows their haphazard arrangement. It will be 

 seen, for instance, that the two oblique sections of rods in this figure are 

 surrounded by indifferent cells, and that at the same level several other 

 cells are cut through their nucleated part. 



There is great variation in this structure, but I have not found an 

 eye of Limax without the accessory retina. In most cases there is no 

 lens; occasionally there is no vitreous humor. TTheu the lens is present 

 it usually has about the same position as in Figure 13 (Ins 1 .). In one 

 case it was found in the most ventral position possible in the accessory 

 eye, in contact with the capsule. 



