56 Studies in Animal Behavior 



the. light reactions of the larvae of the marine worm 

 Arenicola. These larvae are oblong in shape, with 

 two eye spots at the anterior end. Near either ex- 

 tremity there is a band of cilia by means of which 

 the larvae swim through the water. The larvae are 

 positive in their reactions to light, and will follow 

 a light around in various directions. Orientation 

 to light is brought about by bending the body to the 

 stimulated side. If the larva is between two sources 

 of light from which the rays intercept one another 

 as they fall on the animal at an angle of ninety de- 

 grees, the larva will take a course midway between 

 the two lights. If one light is turned off the larva 

 bends immediately to the other one. By arranging 

 a mirror so as to throw a small spot of light on dif- 

 ferent parts of the body, Mast has shown that 

 when light is thrown into one eye there is a strong 

 bend of the body toward the stimulated side. The 

 parts behind the eye spot show no definite reac- 

 tion. It is evident that orientation in this form is 

 due to different intensities of illumination on two 

 sides of the body. So far as can be ascertained ori- 

 entation takes place directly and automatically, with- 

 out any conscious decision on the part of the animal. 

 The movements of the larva appear little more 

 voluntary than the precise movements of certain Pro- 

 tozoa or the swarm spores of algse. Let us pass to 

 animals somewhat higher in the scale of life. 



Some years ago when on the Atlantic coast at 

 Woods Hole, Mass., I studied the behavior of vari- 



