REACTIONS TO LIGHT 261 



the reactions are associated with objects which have some 

 vital relation to their existence as food or a source of dan- 

 ger. In the higher animals, man in particular, pleasure 

 and other emotions enter in as factors in the response, and 

 the objects represented by the condition of light which 

 causes the response may consequently have additional signi- 

 fications. I do not however wish to be understood as advo- 

 cating the exclusion of such factors in the reactions of lower 

 organisms, for, while they have not been demonstrated in 

 these organisms, they may exist for all that is known to the 

 contrary. 



In the organisms in Group a all the tissue appears to be 

 equally sensitive to light. In many of those in Group b, 

 Euglena, Stentor and Planaria, for example, some parts of 

 the body are undoubtedly more sensitive than others, and 

 it may be that the sensitive tissue is confined to a small 

 area definitely located, as for instance in Euglena near the 

 eye-spot. This tissue serves to distinguish changes of inten- 

 sity, but owing to its positional relation to non-sensitive 

 tissue (see Fig. 11) which intercepts the light from one side, 

 and the movement of the organism, especially the rotation 

 on the long axis, it serves also to locate the direction from 

 which the strongest light comes. Thus we have in these 

 organisms structures which may be termed direction eyes. 

 In Planaria and Arenicola larvae the highly sensitive tissue 

 is nearly surrounded by opaque tissue which admits light 

 only from one side (see Fig. 26), and consequently serves to 

 locate more accurately the direction from which the light 

 comes. In these organisms the lateral head movements 

 are also of importance in locating the direction of the light. 

 While the photosensitive tissue may be confined to limited 

 regions in some of these forms, we are certain that it is not 

 in others. Planaria and earthworms e.g. are known to 

 respond after the more highly sensitive anterior end has 

 been removed. Histological investigations in the latter 

 seem to indicate that the photosensitive elements are fairly 

 well distributed over the entire body surface. 



