VISUAL TISSUES 



231 



Charybdea we shall not describe, its type being represented later by 

 other forms. 



Another medusa, however, has an intermediate type which should 

 be considered here. This form, Aurelia aurita, has one part of its body- 

 wall, consisting of all three layers (the mesoglcea weakly represented), 

 developed into an eye that is a simple inverted type. That is to say, the 

 light passes through the cells and strikes their visual cell-organ from 

 a proximal position. This eye is pictured in Figure 203, and we see that 

 a portion of the animal's ectoderm has been proximally produced in 

 two places to form two knobs, one of which is larger than the other. 

 These knobs are composed of long sensory cells of ectodermal origin, 

 which rest on the inner surface of a pocket-like layer of cells which have 

 been invaginated distally (or evaginated if they are considered with 

 reference to their distal surface) to form these pockets. 



The cells which line the pockets have developed pigment in their 

 proximal ends (these ends are di- 

 rected distally with reference to the 

 animal's exterior), and form the pro- 

 tective and absorptive layer of the 

 eye. There is no lens connected with 

 this eye, although some medusae have 

 a well-developed one. The mesoglcea 

 seems to be crowded out entirely in 

 the structure, and the figure shows 

 but one cell belonging to this layer 

 and lying at some distance from the 

 point at which the eyes are formed. 



. The eye of a planarian worm fur- 

 nishes an example of a simple eye of 

 slightly greater complexity than of 

 the starfish and Solen and with more 

 highly specialized visual cells than in 

 the medusa eyes which have just 

 been described. We shall study an 

 eye from Planaria torva and another FlG - 204. Axial section of a single eye of 



e A ,. , , , . /T -,. Planaria torva. B, a similar section of 



from Amandia polyopthalmia (Fig. 



204), and at the same time compare 

 them with the larger eye found in 

 another planarian worm, Planaria 

 gonocephala. 



Among the many eyes distributed 

 over the dorsal surface of Planaria torva are some composed of a 

 single visual cell. As in most planarians, this is sunk beneath the 



a side eye of Amandia polyopthalmia; 

 vis.c., visual cell nucleus; pg.c., pigment 

 cell ; vis.r., sensory cell-organ of light per- 

 ception, corresponding to visual rod; nv. 

 /., nerve fiber of visual cell extending to 

 central ganglion. (After R. HESSE in 

 Zeits.f. wiss. Zool.) 



