58 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



aquarium. A Cerianthus is laid on the wire net. After 

 a few minutes the foot of the animal begins to bend 

 downward and to work its way through one of the 

 meshes of the net. In the oral pole no change takes 

 place except that the tentacles lay themselves close to- 

 gether, so that they look like a brush whose handle 

 is formed by the body of the animal. The animal 

 forces its body farther and farther through the meshes 

 until it is at last able to keep itself in a vertical posi- 

 tion as represented in Fig. 14. This orientation can 

 be reached in half an hour. If we turn the wire net 

 over as soon as the animal has reached the position 

 represented in Fig. 14, so that the foot is up, it does 

 not pull itself out of the net again, but the foot near 

 the tip begins to bend downward vertically. The 

 bending then progresses from element to element of 

 the body, from the foot toward the head, until the tip 

 of the foot reaches the wire net, when it again pushes 

 itself through as far as possible. If the wire net be 

 turned over again, the process is repeated. Thus the 

 animal can be forced, simply with the aid of gravit- 

 ation, to weave itself in and out of the net. Fig. 15 

 shows a Cerianthus that has been forced to push itself 

 through three times in this manner. The drawing 

 was made from life. In these experiments we have 

 an example of a geotropic irritability, in other words, 

 of positive geotropism. As this kind of irritability is 

 very common in the roots of plants, it follows that for 

 the mechanism of these reactions no specific qualities 

 of the ganglion-cells are necessary. If a transverse 



