128 The Animal Mind 



the head end (91), but decapitated worms were found by 

 Graber to show the same light and color "preferences" as 

 normal ones, though in a less marked degree (150), and Yung 

 obtained evidence that sensitiveness to light is distributed 

 over the body (473). According to Hesse the anterior end 

 of the worm is most sensitive, the tail next, and the middle 

 region least (168). Not only the region, but the amount of 

 body surface affected, also makes a difference. When the 

 whole length of the worm was illuminated, the percentage of 

 reactions was to that obtained where the front third only was 

 involved as 26 to 10.2, while the relative occurrence of re- 

 sponses when the middle third and the posterior third alone 

 were stimulated is represented by the figures 2.4 and i re- 

 spectively (312). The effect of colored rays has been found 

 to be proportionate to their intensity ; that is, the green and 

 yellow regions of the spectrum are most effective (473). 



Although the ordinary response of the earthworm to light 

 is negative, it has been possible to determine experimentally 

 a positive phototropism in Allolobophora fatida for very 

 low intensities, and the emergence of worms from their 

 burrows at nightfall has been referred to this tendency to 

 seek very faint light (i). 



No parallel in our own experience can be found for the sen- 

 sation received by an eyeless animal from light. In many 

 of the marine worms, however, well- developed eyes exist, but 

 not such as are capable of giving clear images. The function 

 of the eyes of marine worms seems to be chiefly that of receiv- 

 ing stimuli from shadows. Many tube-dwelling worms will 

 withdraw into their tubes if a shadow is cast upon them, and 

 the term "skioptic" has been suggested for this class of reac- 

 tions (158, 173, 373). The leech Clepsine shows the same 

 kind of behavior ; the slightest shadow cast on the surface of 

 the water in a dish where these animals are resting quietly 



