188 LESLIE B. AREY AND W. J. CROZIER 



rior end of the foot during active creeping. The lateral wave 

 produces an appreciable sideways shifting of the whole animal. 

 Parker ('14) also noted that ''by swinging the anterior portion 

 of the foot to one side and the posterior portion to the other ; the 

 animal can rotate its body with the middle of its foot as a pivot." 

 We have observed that in the case of these turning movements, 

 which are frequently employed by chiton, the anterior end of the 

 animal is the one primarily and principally concerned; the ante- 

 rior end of the foot is, as a whole, pushed over to one side and 

 diagonal retrograde waves bring the rest of the foot into the new 

 position. The posterior end of the foot is pushed, as a whole, 

 toward the side opposite the anterior one, but relatively not so 

 far. During the turning maneuver the shell of the animal and 

 the girdle are usually left behind, but after one or two pedal 

 waves have passed, the foot (now straight) is held stationary, 

 while the whole body of the chiton is swung slowly into the new 

 position. We have spoken of diagonal waves upon the foot 

 during turning; these waves are diagonal so far as the anterior 

 end of the foot is concerned, but they usually become almost 

 perfectly transverse before they reach the middle of the animal's 

 length. 



From the foregoing account it will be seen that there is, in 

 comparison with most gastropods, a considerable degree of flexi- 

 bility as to the use of the foot as a whole and as to the nature of 

 the muscular coordinations producing pedal waves upon its sur- 

 face, although this flexibility does not by any means involve 

 such complexity of movements as appears in the foot of the 

 gastropod Cyprea (Olmsted, '17 a). In the main Chiton tuber- 

 culatus progresses anteriorly by means of retrograde pedal waves; 

 these waves in their characteristic form run almost entirely 

 lengthwise on the foot from the anterior end backward and are 

 not free to course in all directions across the foot as they are in 

 the pedal disc of sea anemones (Parker, '17 b). Undoubtedly, 

 this difference in the character of the pedal waves in the two 

 cases is determined by the nature of the nervous arrangements 

 within the pedal organ, and in fact the disposition of the nervous 

 system of chiton allows us to analyze the relation experimentally. 



