RETINAL PIGMENT OF PLANORBIS 367 



venient to follow the suggestion of Smith ('06) and distinguish 

 three concentric zones, which, although of an arbitrary nature, 

 are on the whole rather clearly defined. The peripheral zone 

 (fig. C, rtn. ex.) is in contact with the optic capsule and is quite 

 free from pigment; it contams the cell nuclei. The pigment of 

 the non-sensory cells is aggregated in a middle or intermediate 

 zone (fig. C, rtn. m.); peripherally its limits are ill defined be- 

 cause of irregular processes, which extend down into the cells, 

 whereas centrally a sharp line of demarcation separates the 

 pigment from a third or central zone {rln. i). The position of 

 the boundary conmion to the peripheral and intermediate zones 

 is somewhat dependent on the degree of pigment migration 

 under various envii'onmental conditions, which will be dis- 

 cussed in the main part of this paper. The internal or central 

 zone (fig. C, rtn. i.) is of constant width and comprises the por- 

 tion of the retina between the pigmented zone and the lens. 

 It contains the rods (bac.) — the photoreceptive elements. 



Two kinds of cells form the retinal epithelium, the non-sen- 

 sory- pigmented cells (fig. C\ nl. cl. pig.) and the unpigmented 

 sensory cells {nl. cl. sm.). The pigmented cells, which are the 

 more numerous of the two, are grouped about the sensor>' cells 

 thereby isolating the latter from each other. These pigmented 

 cells are of two kinds. One set has its nuclei situated close 

 to the optic capsule, while the nuclei of the other set are near 

 the center of the cells and slightly distal to those of the sen- 

 sory elements. Both types of pigment cells are exceedingly 

 slender, although those with more distally placed nuclei gener- 

 ally have an enlargement just distal to the nucleus; according 

 to Smith ('06, p. 255) : "When this part of the cell is free from 

 pigment it appears to contain a \'acuole." 



The distribution of pigment is variable under different cir- 

 cumstances; the limit of proximal migration in either tj-pe of 

 cell, however, is conditioned by the position of the nucleus 

 (fig. C), for pigment is never found proximal to it. The length 

 of the pigment cells, and therefore the thickness of the retina 

 as a whole, varies at different levels in the eye. The thickest 

 portion of the retina is on the sides at a distance of about 60 m 



