52 TROPISMS 



and above on a positively heliotropic animal its progres- 

 sive motions are stopped, and in some cases a tendency 

 to turn a somersault backward may even arise. The case 

 is similar to that of galvanotropism when the current goes 

 through an animal lengthwise (see previous chapter). 

 We must conclude from the observations of Holmes and 

 Garrey, which will be discussed farther on, that if the 

 head of a positively heliotropic animal is turned to a 

 source of light its forward motions are facilitated and 

 the backward motions rendered difficult; while in the case 

 of a negatively heliotropic animal it is just the reverse. 

 If the animal now moves to the right or to the left the 

 illumination of the two eyes or of the two sides of the 

 body becomes different again, causing a forced movement, 

 whereby the plane of symmetry of the moving animal is 

 caused to go through the source of light again ; with the 

 head toward the source of light w T hen the animal is posi- 

 tively heliotropic or away from it when it is negatively 

 heliotropic. 



2. Dieect Peoof of the Muscle Tension Theory of 

 Heliotropism in Motile Animals 



The fact that light causes forced movements, like 

 those described in the case of brain lesions and of galvano- 

 tropism, has been proved by many observers, and espe- 

 cially clearly by Holmes and Garrey. Holmes worked 

 on the positively heliotropic water scorpion Ranatra 228 

 When this animal is illuminated from the right side, the 

 legs on the right side of the body are bent and those on 

 the left side extended (Fig. 17). This effect is identical 

 with the one observed in Palamonetes, when a galvanic 

 current goes sidewise through the animal. Hence Ranatra 



