W. J. CROZIER 633 



into curare solution tend on the whole to remain curled up for a 

 longer period (even in the dark) than is the rule in sea water, but, 

 even after 5 hours in curare, recovery in sea water is quite prompt. 

 The isolated foot of a Chiton, if exposed to curare, does not go into 

 spasms of contraction. Curare has a quite different effect on the 

 foot and on the whole animal in the case of gastropods, usually 

 inducing convulsive contractions. Even after 7 hours in curare the 

 tendency to curl up is not materially enhanced; the foot is usually 

 crinkled somewhat, but not decidedly, and the girdle is not strongly 

 contracted. It is of interest to note that curare has practically 

 no action on neuromuscular coordination in flatworms.^ 



In comparison with the definite curvatures of body and girdle 

 characteristically induced by nicotine and in early stages of strych- 

 ninization, it is found that in quinine solutions the curvature of the 

 mantle is quite irregular, while the body may be curled to varying 

 degrees. 



SUMMARY. 



1. The degree of curvature of the body and of the girdle of a Chi- 

 ton is determined by the activity of antagonistic muscle groups. At 

 a certain early stage in the strychninization of a Chiton the reciprocal 

 inhibition involved in the natural use of these muscle groups is re- 

 versed, such that extensor muscles, rather than, as normally, flexor 

 muscles, contract as the result of stimulation. This condition 

 involves a reversal, under strychnine, of the normally positive stereo- 

 tropism of the foot, and of the usual response of the mollusk to an 

 increased illumination of its ventral surface. Strychnine reversal of 

 this character is not a matter of the relative strength of the opposed 

 muscle groups, for the flexor muscles are the more powerful and are 

 the ones always shortened in tetanic contraction. 



2. Nicotine, in contrast to strychnine, primarily induces contrac- 

 tion of flexor muscles. Its effects, moreover, are in a degree selec- 



^ Vies (1907) thought the antero-posterior direction of the pedal waves in 

 Acanthochites suggestive of a worm-like neuromuscular organization, more primi- 

 tive than that in the foot of gastropods, where the waves are often direct; but the 

 retrograde waves producing locomotion in polyclads (Crozier, 1918) seem to be 

 rather more complex than in Chitons (Crozier, 1918-19). 



