390 S. J. HOLMES 



to face the light, the animal begins to swim through the water 

 for a short distance, then settles down, reorients itselt and 

 makes another excursion. Pecten frequently closes its shell 

 upon the approach oi a large moving object, and the experi- 

 ments of the author have convinced him that this reaction is 

 not a shadow reflex such as is performed by many other mol- 

 lusks, but a reaction based upon the movement of the object 

 in its field of vision. 



The numerous tentacles that fringe the margin of the mantle 

 are organs that are very sensitive to touch and chemical stimu- 

 lation. If the tentacles come in contact with a staifish the 

 mollusk commonly swims away. The juices of the starfish 

 diffusing in the water evoke the same lively response. If Pecten, 

 which commonly lies upon its more convex right side, is turned 

 over it is able to right itself by opening and closing the valves 

 of the shell. Bauer concludes from several experiments that 

 both the eyes and the statocyst are concerned in the righting 

 reaction. 



Bonn (3) has discovered that in actinians pressure of the 

 water in which they are contained tends to cause them to ex- 

 pand. Certain copepods and the larvae of the lobster may be 

 caused to react negatively to light by only a slight increase of the 

 pressure of the water. 



Bohn (4) believes that there are two kinds of sensitivity, one 

 to light, another to shade, corresponding to antagonistic chem- 

 ical reactions. Causes that accelerate oxidations tend to make 

 animals positively phototactic; causes that inhibit oxidations 

 produce attraction to shade. Periods of sensitivity to light 

 tend to alternate with periods of sensitivity to shade. Young 

 lobsters are rendered temporarily positive to light by acids and 

 more permanently positive by alkalis. 



Planarians which had been illuminated on one side and which 

 had been turning away from the source of stimulus were found 

 by Boring (5) to keep turning toward the opposite side after 

 being transferred to a situation in which the light was the same 

 on both sides of the body. Apparently the more stimulated 

 side had lost some of its sensitivity. After turning away from 

 the light for a while the planarian begins to make sudden turns 

 toward the light which for a time increase in frequency, but 

 finally disappear. Boring considers these turns to be "a form 



