628 NEUROMUSCULAR MECHANISMS IN CHITON 



^ The strong suctional attachment of Chitons, following stimulation, 

 is effected by the girdle; the flexible girdle is pressed into contact 

 with the substratum, and a subsequent slight arching of the body 

 serves to produce a true "sucker" action quite independent of the 

 activity of the foot. The surface of the foot does, however, suck 

 locally (Parker, 1914), an area several millimeters square being 

 sufficient to afford a suctional field adequate for the support of the 

 whole body. An undisturbed Chaetopleura is supported in this way, 

 for the girdle is then lifted permitting the entrance of respiratory 

 water currents. 



A Chaetopleura detached from its substrate and placed on its back 

 undergoes, in the light, a pill-bug-like curHng up. In weak light or 

 in darkness, the animal after a short time opens out, twists longi- 

 tudinally, and may succeed in righting itself. The ventral surface is 

 quite sensitive to light, and to shading. Chaetopleura attached to a 

 glass slip may be caused to release the attachment of the foot by the 

 illumination of the ventral surface. Either increase or decrease of 

 light intensity causes a Chaetopleura lying on its back to curl up more 

 completely. By means of such changes of light intensity, produced 

 on the ventral surface through the animal's own movements while 

 beginning to "open up" after being curled, a more or less rhythmic 

 succession of curling contractions may be maintained. The pos- 

 terior end of a curled up Chaetopleura is commonly more sharply 

 bent than the anterior, which thus overlaps it (as noticed also by 

 Sampson, 1895). The girdle musculature is not strongly contracted 

 when the Chiton is curled up. 



The degree of longitudinal curvature of the body of a Chiton, and 

 the degree of elevation of the girdle are each controlled by the rela- 

 tive tensions of antagonistic muscle groups (for structure, cf. Sampson, 

 1895; Plate, 1901). The important muscles are: (1) a median dorsal 

 group, composed mainly of fibers running from the anterior edge of 

 each shell plate to the ventral surface of the anterior region of the 

 next anterior plate; (2) longitudinal muscles at either side of the 

 body (flexor antagonists of the extensor muscles (1) ); (3) the dorsal 

 (extensor) and ventral (flexor) muscles of the girdle, set perpendicular 

 to its margin. The contact irritability of the foot and the activities 

 of these muscle groups were studied especially in relation to the 

 effects of strychnine, nicotine, and curare. 



