BEHAVIOUR OF SHORE ANIMALS 233 



aquarium is turned round through 180° so that the Hght 

 enters its opposite end, the worm during the next day or 

 two bends its tube in such a way that the axis of symmetry 

 of its circle of gills is again in the direction of the rays 

 of light. A similar response to light is shown by the 

 hydroid Eudendrium. If, when the colony is brought 

 into the aquarium, the old polyps are cut off, the new 

 polyps which are regenerated will bend towards the light in 

 the same way as the tube-worm (Loeb, op. cit.). 



One must not fall into the error of assuming that all reac- 

 tions to light are of the nature of tropisms. There is a 

 forced character about a tropism which it is not difficult to 

 test by experimental means. 



One important type of response which cannot be inter- 

 preted in this way is that consequent on a brusque change 

 in the intensity of a stimulus : as when a tube-worm retracts 

 its tentacles under the influence of a sudden shadow. In 

 the case of non-sedentary forms this reaction usually takes 

 the form of an abrupt stoppage followed by a turning 

 movement of 180°, after which the animal continues its 

 progress in a direction directly opposite to the one previously 

 followed. This is what happens (in varying degree) when 

 a positively phototaxic animal reaches the limit of a shadow, 

 or when an animal climbing up a slope encounters an abrupt 

 change of gradient. The term applied to this type of 

 reaction is " differential sensitivity." It has been much 

 studied by Bohn (191 1), who regards it as of great importance. 

 When acting in combination with tropisms, it gives an aspect 

 of considerable complexity to the behaviour. For instance, 

 an adult brittle-star does not move directly towards the area 

 of shadow which it ultimately reaches, but the path it follows 

 after the initial impulse is spent is frequently a tortuous 

 one, and is governed by the attraction of the darkened area, 

 acting in conjunction with the oscillations caused by varia- 

 tions in the degree of illumination of the surfaces the animal 

 encounters. Actually, the movements of the animal are 

 governed by a combination of three factors : (i) the initial 

 impulse resulting from the state immediately preceding the 



