Harper, Behavior of Corethra. 439 



The Method of Locomotion.— Ml ALL speaks of the movements 

 as having the quickness of a deflection of the magnetic needle. 

 The unit of movement in locomotion consists (i) of a side sweep 

 of the posterior half and a similar but less pronounced stroke of 

 the head in the same direction. The head may also move up 

 or down. The side sweep of the tail may be carried till the end 

 touches near the middle of the body or it may be of much less 

 extent. (2) After the contraction the body straightens quickly, 

 but not bending to the opposite side. The drifting motion that 

 results is nearly always accompanied by rotation, with the head in 

 such cases always turned away in the direction opposite to that oi 

 the contraction (Fig. i). A parallel position may be maintained, 

 but there is never rotation of the head in the direction of the con- 

 traction. A consideration of the factors shows that the lateral 

 sweep of the head and tail to the same side would have opposite 



Fig. I. Diagram of locomotion. A, pigmented air sacs. 



effects upon the tendency to rotate. The return movement of the 

 tail appears most effective in producing the rotation. It seems to 

 be due to elasticity alone. The extent of the movement varies 

 from a slight twitching of the two ends up to a movement with 

 rotation to 180°. Locomotion is thus seen to be in the nature of a 

 recoil or reaction to the single muscular contraction. The lashing 

 of the body is like that of cilia, driving the body in the opposite 

 direction. 



MiALL speaks of the movements as due to the sculling motion 

 of the tail. That description does not include the action of the 

 anterior end. The body tapers posteriorly. The muscles are 

 mainly segmental, binding together adjacent segments. The side 

 stroke of the more powerful anterior end though less extensive is 

 effective in locomotion. It appears more effective in the contrac- 

 tion, the posterior end in the return movement. 



