MOLLUSKS, ARTHROPODS AND VERTEBRATES 225 



angles with the rest of the body, after which it returns more 

 slowly with the appendages partially folded. When it 

 swims rapidly the posterior end strikes backward at the 

 same time that the anterior end straightens, and moves 

 forward when the anterior end strikes backward. Thus the 

 creature alternately bends and straightens in rapid succes- 

 sion and forces itself through the water. Ordinarily it 

 swims on either the right or the left side. When it changes 

 its direction of motion it always turns toward the dorsal 

 surface. If the ray direction is changed it turns directly 

 toward the source of light if the dorsal surface is illumi- 

 nated, but if the ventral surface faces the light it first 

 rotates on its long axis through 180° and then turns. 



Orientation is not brought about by unequal action of 

 symmetrically located sensory and locomotor organs. W^hen 

 the organism turns, either the entire anterior end alone or 

 both ends must strike differently than when it swims 

 straight ahead. The appendages on opposite sides of the 

 body act the same. The organism as a whole must be 

 involved in the process of orientation. This cannot be due 

 to a " compulsory automatic turning of the head " toward 

 the source of light, in accord with Loeb's explanation of 

 orientation. In some way or another these creatures do 

 keep the anterior end directed toward the general source 

 of illumination. Just how this is done I am unable to say. 

 It may be that the organism receives a localized stimulation 

 when it turns so that the anterior end is no longer directed 

 toward the light, and orients by means of a differential 

 response to such a stimulation. There certainly is no evi- 

 dence indicating that light acts constantly as a directive 

 stimulation, nor is there any indicating that " the head 

 is . . . bent until its symmetrical photosensitive points are 

 . . . struck at the same angle by the rays of light." The 

 process of orientation is by no means as simple as these 

 explanatory quotations from Loeb demand. 



h. Discussion. — Orientation in Corethra larvae, which, 

 according to the work of Harper (1907) bend sharply alter- 



