254 bulletin: museum of comparative zoology. 



Before closing this general account of the structure of the eye, I 

 desire to refer to an abnormality which was twice found in the eye of 

 Limax. In the first case the preparation was made by the vom Rath 

 method ; in the second, the stain was methylen blue. In each there 

 was, near the optic nerve, a secondary outfulding of the optic capsule in 

 such a way that a little chamber was cut off from the main capsule. 

 Connecting the two there was a narrow passage. In the chamber were 

 found the nucleated portions of several sensory cells, their distal parts 

 protruding through the opening into the pigment. Thus the rods of 

 these cells occupied their normal places in the internal or central zone. 

 In one of the cases there was an irregular mass of pigment in the cham- 

 ber. No effort was made to trace the nerve-fibres, nor to learn how 

 common this abnormality is. 



Some Observations on the Eye of Planorbis. 



The eyes of Planorbis differ somewhat from those of Helix and Limax. 

 The shape of the eye has been illustrated by Willem ('92 a ). The lens 

 has the general form of a cone, the base being turned outward. The 

 retina conforms to the shape of the lens. The sensory cells, as seen in 

 ordinary sections (compare Plate 2, Fig. 20, which is a serai-diagram- 

 matic sketch of a part of the retina of Planorbis), appear to resemble 

 those of Limax in every way, except that the rods are shorter, approach- 

 ing those of Helix in form. The rods are less easily made out than in 

 Limax or Helix, for their fibrillae are more delicate. 



The pigment cells vary much more in form than the sensory cells. 

 They may be roughly divided into two groups. To one group belong 

 pigment cells whose nuclei lie in a proximal position next to the cap- 

 sule ; to the other, those whose nuclei lie either in the vicinity of the 

 nuclei of the sensory cells or even distal to them. Both are attenuated 

 proximad of their nuclei, and the latter have the form of long, clear, 

 highly refractive rods. The position of the pigment varies greatly. In 

 general the pigment zone usually appears proportionately much narrower 

 than in Helix or Limax. In the pigment cells Avhose nuclei lie near the 

 capsule, the pigment may either extend quite down to the nuclei or be 

 confined to a relatively short stretch of the cell, or its proximal limit 

 may reach some point intermediate between these extremes. The same 

 statement applies to the pigment cells whose nuclei lie more distally. 

 In the latter there is usually an enlargement of the cell immediately 

 distal to the nucleus, and this enlargement may be filled with pigment. 



