REACTIONS TO LIGHT IN THE EARTHWORM. 19 



skin behind the clitellum, making a conspicuous mark. The 

 dorsal pores are very prominent, exuding an abundant yellowish 

 mucus. The everted buccal cavity is used as a proboscis, and 

 is thrust out constantly in its feeling movements. The blood 

 vessels are prominent, shining distinctly through the skin. The 

 very numerous, minute, diffuse nephridia are a feature which, 

 along with the continuous circles of setae, have caused consider- 

 able discussion as to whether these conditions are primitive for 

 earthworms or secondarily derived. 



THE THEORY OF TROPISMS. 



The orientations to light and other stimuli, which are among 

 the most striking phenomena in the behavior of the lower ani- 

 mals, have received various explanations. After the first anthro- 

 pomorphic explanations of these movements, based upon likes and 

 dislikes, there came an apparent revolution of ideas bringing in 

 explanations of seemingly great simplicity. As the physiology 

 of plants, particularly of the higher plants, had made consider- 

 able progress towards a solid physico-chemical basis, there was 

 a transference of conceptions based upon plant physiology to 

 the realm of animal behavior and the orientations of the lower 

 animals were illuminated by analogies drawn from plants. For 

 example, we find the assertion of identity between heliotropic 

 phenomena in plants and animals. The mechanism of the 

 tropism was not a reflex according to this conception, but was 

 a unique form of movement to be added to the classification of 

 animal movements into reflex, instinctive and voluntary. 



The current theory of phototropic or -tactic phenomena as ap- 

 plied, for example, to the earthworm, was that when light strikes 

 one side of the animal so as to cause unequal stimulation of the 

 two sides, it changes the tone of the muscles on the side affected. 

 The muscles of one side are thus either relaxed or their tension 

 is increased according as the animal happens to be positively or 

 negatively phototropic. It is bent away from or toward the 

 source of stimulation by the direct action of the environment 

 upon the protoplasm. The tropism is accordingly regarded as a 

 peculiar kind of forced movement, dependent upon the chemical 

 nature of the protoplasm. 



