120 Herbert Eugene Walter 



corresponding reduction in the photoreceptive elements, which, 

 according to the experiments of Loeb ('94) and of Parker and 

 Burnett ('00) seem to be in some degree at least distributed over 

 the entire body. If this is true, there would result less stimulation 

 from the light and consequently a slower rate. That Dendro- 

 coelum lacteum when reduced in size does not suffer a similar 

 reduction of its photoreceptive apparatus is probable. The work 

 of Lillie ('01), wherein he showed the inability of headless individ- 

 uals of Dendrocoelum lacteum either to regenerate or to respond 

 to light, suggests that the photoreceptive apparatus of this species 

 is not scattered over the entire body, but is rather concentrated 

 anteriorly, in all probability consisting of the eyes only. If this 

 is true, a reduction in the size of the body would not necessarily 

 cause a proportionate reduction in the photoreceptors, and, 

 indeed, the proportion of the light-receiving elements as com- 

 pared with the mass of the body might increase as the worm 

 became smaller. In this connection it is interesting to note that 

 Gissler ('82) pointed out that in the case of Bdelloura Candida 

 increasing size of the body is accompanied by a decrease in the 

 size of the eyes, and so far may this inverse ratio be carried that 

 the eyes sometimes disappear entirely in large individuals. If there 

 actually exists some such inverse ratio between the size of the 

 photoreceptors and the mass of the body in the case of Dendro- 

 coelum lacteum, it is easy to see why the smaller worms travel 

 faster than the larger ones. 



By another series of experiments it was found that the smaller 

 worms of all four species, with the possible exception of Dendro- 

 coelum lacteum, orient with less accuracy than the larger worms 

 under the same external conditions. In these experiments, as in 

 the previous ones already described dealing with orientation, 

 each worm was placed at the center of a circle 10 cm. in diameter 

 and headed successively toward, away from, and at right angles 

 in both directions, to the incident light. The average amount of 

 deviation at the circumference of the circle from the direction in 

 which the worms were started, reckoned in degrees, gives a crite- 

 rion of their accuracy in orientation. The averages of behavior 

 obtained are indicated in Table XXXII. 



