232 bulletin: museum of compakatiye zoology. 



fiddler crab gave like results. If blinded and deprived of otocysts, the 

 crabs rolled both forward and backward when walking or running; 

 this effect was still more apparent when the animals were placed in the 

 water. 



2. Removal of Sense Organs and its Effect on the Compensation 

 Movements of the Eyes. 



The following experiments, carried out on Gelasinus pugilator, confirm 

 the work done by Clark ('96). When a crab is tilted to the right or 

 left, forward or backward, the eyestalks tend to keep their original direc- 

 tions, thus seemingly moving through a certain angle. Such move- 

 ments, which have been observed also for the head and eyes of many 

 vertebi'ates and insects, are called compensation movements, and the 

 angle of movement, the angle of compensation. 



The angle of compensation in the fiddler crab was measured by means 

 of the apparatus desci'ibed by Clark ('96), a small table to which the 

 animals could be securely fastened and tilted about their chief horizontal 

 axis. A scale ruled to degrees enabled one to read accurately the angle 

 of compensation, and the angle through which the animal was turned. 

 The long eyestalks of the fiddler crab make it easy to determine the 

 angle of the eye movements. 



The angle through which the animals wei-e turned was in all cases 

 45° first to the right, then to the left, about the chief, or longitu- 

 dinal axis of the body. In each experiment fifty animals were used, the 

 average being taken as the angle of compensation. These animals were 

 most of them kept twenty days, and the angle then measured again, 

 thus guarding against abnormal conditions. 



a. Normal Animals. In normal crabs the eyestalks are so held as to 

 make an angle of about 22° with the vertical. The eye movements are 

 always correlated, and if the animal's body is tilted to the right (45°) 

 the right eye makes a compensating movement of 18° upward, the left 

 eye one of 25° upward ; rotated to the left, the conditions are just re- 

 versed, the right eye now moving through an angle of 25°. The move- 

 ment of the eye of the side toward which the animal is rotated is in 

 each case less by about 7° than that of the other eyestalk. This is due 

 to interference of the carapace with the eyestalk, preventing its passage 

 through a greater angle. 



b. Both Eyes blinded. Tilting either to right or left had the same 

 general effect as in normal animals, but the right eye described an arc 

 of only 13°, the left eye one of 20°, or vice versa. There is thus a 



