140 The Animal Mind 



quick withdrawal into the burrow (171, 327), but the re- 

 action time to light is much longer than that to mechanidal 

 stimulation. The part of the earthworm's body affected 

 by the light also influences the reaction. Darwin indeed re- 

 ported that the worms withdrew into their burrows only 

 when light fell on the head end (171), but decapitated 

 worms were found by Graber to respond to light like nor- 

 mal ones, only less strikingly (266), and Yung (833) ob- 

 tained evidence that sensitiveness to light is distributed 

 over the body. According to Hesse the anterior end of 

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

 region least (316). Not only the region, but the amount 

 of body surface affected, 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 occur- 

 rence of responses where the middle third and the posterior 

 third alone were stimulated is represented by the figures 

 2.4 and i respectively (552). 



In many of the marine worms well-developed eyes exist, 

 although not such as are capable of giving clear images. 

 Their function seems to be chiefly that of receiving stimuli 

 from shadows. Many tube-dwelling worms will with- 

 draw into their tubes if a shadow is cast upon them (285, 

 321, 650). 



Turning to the molluscs, we find that the siphons of the 

 Acephala, which are projected from the shell to take in 

 currents of water containing nourishment, are withdrawn 

 in response to sudden darkening in some cases, to sudden 

 illumination in others, and in still other instances to either 

 (195, 520, 650). The danger of arguing the existence of 

 sensory discrimination from structure alone is well shown 

 in the case of snails, for although many of them have eyes 



