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We are familiar with many cases in different animals, where we 

 have regions of the body sensitive to light, though in these regions 

 we can demonstrate no visual organs. The mantle edge of some 

 Lamellibranchs and the prostomium of Earthworms are instances in 

 point. In many of the Protozoa and Protophyta the phenomena of 

 heliotropism indicate a sensitiveness to light, apparently the whole 

 protoplasm being reponsive to such stimulation unaided by pigment. 

 In still other cases, apparently we find the presence of pigment suf- 

 ficient to make ordinary cells sensitive to light rays; e. g. the eyes 

 of some Medusae, and possibly the stigmata of some Protozoa and 

 Protophyta. I do not know of any descriptions ^) of well developed 

 optic organs in connection with which no pigment is found. In many 

 species of Salpa, however, we have just this condition. 



In S. pinnata, chain form, for example, we have four pairs of 

 well developed eyes, consisting of clusters of rod cells in connection 

 with which no pigment is found. I give in Figure 6 a draw- 

 ing of a sagittal section of the ganglion of the chain S. pinnata, 

 showing the large, dorsal eye and one of each pair of smaller eyes. 

 In the chain forms of other species we have more or less similar 

 conditions, except that the rod cells usually are more irregular in 

 shape. It is noteworthy that in S. costata we have a small eye in 

 the ganglion, composed of irregular rod cells, and that in this 

 case we have pigment present (Fig. 7e'). This eye, then, is 

 functional, though its rod cells are so irregular. What shall we say 

 of the small eyes of S. pinnata, whose rod cells are so perfectly deve- 

 loped? Does the absence of pigment preclude their functioning as 

 optic organs? 



In the solitary form of some species we have in connection with 

 the larger, horseshoe-shaped, pigmented eye, masses of rod cells which 

 are far removed from any pigment (see Fig. 8 a;). Are these cells 

 functional? If so they function unaided by any visual pigment. 



In the eyes of Vertebrates the functions of the pigment epithelium 

 and visual purple are not understood. The phenomena of their trans- 

 formation under the influence of light do not correspond with the 

 visual perceptions produced. It has been suggested that there is some 

 other invisible substance in the retina, whose transformation under 

 light stimulation is the immediate cause of visual perception. The 

 existence of three such invisible substances in the retina has been 

 postulated to explain color vision. It is probable that the human eye 



1) Except my descriptions of the smaller eyes of Salpa. 



