ii6 C. M. Child. 



it extends at present and with the change in position the tension 

 on the tissues at this point has decreased until now it has become 

 pressure and these parts are "too long" for the position they must 

 occupy under the altered conditions, A comparison of Figure i8, 

 the form of the piece during locomotion, and Figure 19, the form 

 during rest, when the parts are not subjected to longitudinal ten- 

 sion, renders it still more evident that the tension due to move- 

 ment is the cause, not the effect of the change in form. When the 

 piece is at rest the angle between the original axis and the axis of 

 the new body is always greater than during locomotion and the 

 folds at X disappear. In other words the change in direction of 

 the new body does not precede but follows, and does not even keep 

 pace with the change in direction of locomotion. These facts 

 leave no room for doubt that the tension due to locomotion is the 

 efficient factor. 



In a previous section the fact was noted that locomotion in Lep- 

 toplana is chiefly ciliary when the animal moves quietly, but that 

 when strongly stimulated the movements are to a large extent 

 muscular. The same is true of these pieces. When stimulated only 

 slightly they progress at a uniform rate, largely by means of the 

 cilia, but under stronger stimulation the margins of the head are 

 used in the manner described and the body is drawn forward by 

 strong muscular contractions usually alternating on the two sides. 

 A marked difference in direction between the two kinds of loco- 

 motion was observed in this piece and indeed in many other similar 

 pieces. The direction of locomotion by muscular contraction was 

 sometimes after strong stimulation in a curve to the left while that 

 of ciliary locomotion was always toward the right. During loco- 

 motion the muscular activity of the right side of the head — the 

 new tissue — appears to be greater than that of the old tissue on 

 the left. In ordinary locomotion the muscular play of the mar- 

 gins of this part is much more conspicuous than on the left. Ap- 

 parently the new parts are in a more active condition functionally 

 than the old, and doubtless under strong stimulation are capable 

 of more work. When the piece turns to the left after strong 

 stimulation the difference in muscular activity between the two 

 sides Is much more marked, that of the right side being clearly 

 much greater. 



