THE SENSORY RESPONSES OF CHITON 189 



During ordinary locomotion one or two waves appear upon the 

 foot of Chiton ; usually two waves are present when the animal is 

 ■engaged in turning, but even in the absence of pivoting move- 

 ments, one wave may appear at the anterior end before its prede- 

 cessor has reached the posterior extremity of the foot. These 

 waves are 5 to 7 mm. in anteroposterior extent, and involve the 

 full breadth of the foot. They require fifteen to thirty seconds, 

 usually about twenty-five seconds, to pass from one end of the 

 foot to the other. The speed of progression of the pedal waves 

 is less at lower temperature; it is identical in either sex, pro- 

 vided the chitons are of the same size. At 27°C, the speed 

 of propagation of the wave is usually about 12-15 cm. per min- 

 ute, being therefore faster than the rate of movement of the 

 pedal wave in actinians (Parker, '17 b, 1 to 3 cm. per minute). 

 The pedal wave is a region, occupying about one-tenth to two- 

 tenths the area of the foot, which is temporarily lifted from the 

 substratum (Olmsted, '17 a) and locally moved forward by mus- 

 cular contraction. In backward locomotion, which may readily 

 be induced by partial illumination of the shell, the retrograde 

 character of the pedal wave is retained. This is especially evi- 

 dent in Ischnochiton purpurascens, which creeps freely back- 

 ward if stimulated by horizontal light striking the anterior end 

 of the shell. 



In chiton there can sometimes be seen a distinct longitudinal 

 depression running the full length of the foot in the midline, as 

 if the foot were about to be folded together lengthwise. This is 

 more easily seen in Ischnochiton. No trace of this activity is 

 apparent in the pedal waves, however. Nevertheless, it can be 

 shown that the foot is controlled in a bilateral manner. If an 

 incision be made into the foot sufficiently deep to divide the con- 

 nectives which join the pedal nerve strands, the lateral halves 

 of the foot exhibit independent wave movements. If such an 

 incision is made at the posterior end, a normal pedal wave may 

 bifurcate when it reaches the anterior end of the incision, one 

 half of it becoming obliterated while the other half may continue. 

 'Stationary waves,' sometimes opposite, sometimes unilateral, 

 appear on a foot completely divided in this way; four or five such 

 waves may be present at once, 6 to 7 mm. apart. 



