MOLLUSKS, ARTHROPODS AND VERTEBRATES 23 1 



fore continue in this direction. Orientation in these forms 

 takes place in principle precisely as Jennings' description 

 demands. 



(4) The same factors are involved in the process of 

 orientation in the colonial forms, Volvox, Eudorina and 

 Pandorina, as are involved in the orientation of the ciliates 

 and flagellates. The orienting stimulus is due to a change 

 of intensity on some part of the zooids. This is due (i) to 

 the change in the surface of the zooids exposed as the 

 colonies rotate, and (2) to the transfer of the zooids from 

 the illuminated side of the colony to the shaded side 

 and vice versa. The change of intensity causes a definite 

 response in the zooids, a shock movement or avoiding reac- 

 tion, which consists in an effort to turn toward the side fac- 

 ing the anterior end of the colony. Owing to the way in 

 which the zooids are united there are no errors in the process 

 of orientation in the colonies as a whole. They never turn 

 in the wrong direction as many of the protozoa frequently do. 

 The reaction of Volvox in light is not like the reaction of 

 this form in a constant electric current, as Bancroft assumes. 



(5) Hydra viridis moves fairly accurately toward or from 

 a source of light. When attached it does however not take 

 a definite axial position with reference to the direction of 

 the rays. It may bend toward any point of the compass. 

 There is no definite relation between the direction of bend- 

 ing and the side illuminated. But the anterior end is 

 directed toward the source of light a greater part of the 

 time, and the animal usually travels only in this direction. 

 When the anterior end is shaded the animal tends to turn, 

 but when this end is illuminated it tends to travel. This 

 indicates that the tendency to become oriented and the 

 tendency to travel are not the result of the same factors. 

 The former may be due to the stimulation by light owing 

 to a change of intensity, the latter to the action of light 

 owing to its constant intensity. There is no evidence indi- 

 cating that the reaction in light is the same as the reaction 

 in a constant electric current. 



