e.n.a. 



e.fi^ 



"^ 



<P 



LUMBRICUS TERRESTRIS 169 



relaxation of the other. If a worm which is lying quiescent, in a 

 moderately contracted state, starts moving, the following things 

 happen in order. The circular muscles of the first segment con- 

 tract, making it longer and nar- 

 rower ; then the second segment 

 behaves similarly, and the third 

 and fourth, and so on, so that the 

 anterior part of the body is 

 elongated. When this contraction 

 wave has reached about a third 

 of the way along the body, the 

 longitudinal muscles of the first 

 segment begin to contract ; they 

 are followed by those of the 

 following segments, and the second ^^ 

 type of waves has begun. As 

 each wave dies out it is succeeded 

 by another. These waves by 

 themselves would not lead to 

 locomotion at all, but merely to 

 successive extensions and with- 

 drawals of the two ends of the 

 body, the mid-point or centre of 

 gravity remaining stationary ; 

 something like this may some- 

 times be seen when a worm tries 

 to move on a wet glass plate. To 

 translate the waves into pro- 

 gression there must be some 

 greater resistance to movement 

 backwards than there is to move- 

 ment forwards. This differential 

 resistance is achieved in three 

 ways. It can often be seen that 

 the part of the body which 



is moving forward is raised off the ground ; this eliminates 

 surface friction. In ordinary slow crawling the head is lifted and 

 thrust forward ; the contraction wave dies out about half-way 

 along the body, and when the anterior end shortens the tail part 

 is raised slightly and dragged forwards. Secondly, the chaetae play 

 some, though probably a minor, part in movement. When the 



c.n.i 



Fig. 119. — A diagram showing the 

 mode of ending of the sensory 

 nerve fibres in the epidermis of the 

 earthworm and the relation of this 

 type to that which is found in 

 some other animals. 



See also Fig. 418. 



A, The arrangement found in the earthworm : 

 B, that of the worm Nereis ; C, that of a 

 fish ; D, that of a frog or man. 



c.n.s., Ending of the neuron in the central nervous 

 system ; ep. , ending in the epidermis. 



