144 THE PHYSIOLOGY OF EARTHWORMS 



Peristalsis, Co-ordination, Movement and Locomotion 



Co-ordination of the body wall musculature. When an earthworm 

 crawls along the surface of the ground, or through the channel that 

 comprises the burrow in which it lives, it does so by the co- 

 ordinated contractions of the longitudinal and circular muscle 

 bands that go to make up the bulk of the body wall. From a position 

 of rest the contraction of the circular muscles may have one of four 

 effects: the contracting end may elongate, the opposite end may 

 elongate or it may thicken, or both ends may elongate. Which 

 event takes place depends not on the contraction of the circular 

 muscles of the contracting end but upon the state of contraction of 

 the longitudinal and circular muscles in other parts of the body 

 (Chapman, 1958). These actions are seen in Fig. 45. 



Much of the muscular reactions of the body wall depends upon 

 the presence within the segments of coelomic fluid. Each segment is 

 in effect a self-contained sac of fluid, for although the septa between 

 the segments are incomplete ventrally where the nerve cord passes 

 through, it is possible to seal off this hole by a sphincter muscle 

 around the nerve cord. Injection of dyes into individual segments 

 shows that no transmission of fluid from segment to segment occurs 

 when peristaltic waves pass along the body. If the animal is 

 anaesthetized and muscle tone lost as a result then coelomic fluid 

 may pass between segments. The average hydrostatic pressure 

 amounts to about 16 cm w^ater in the anterior segments, and 

 approximately 8 cm water in the rear segments, although greater 

 values may be reached during the rapid wriggling movements 

 sometimes shown by worms. The pressure exerted by the earth- 

 worm prostomium upon the ground is greatly increased when the 

 animal is in the enclosed space of its burrow (Newell, 1950). Roots 

 and Phillips (1959) have shown photographs of the method by 

 which the prostomium gains the necessary purchase for burrowing. 

 In a period of quiet locomotory activity an earthworm moves along 

 by first contracting the circular muscles of the anterior end, 

 causing the anterior segments to extend. The contracting region 

 passes posteriorly, and as the pressure lessens the anterior longi- 

 tudinal muscles contract, drawing the posterior end forward. The 

 posterior region is unable to slip backwards for the chaetae of the 

 body wall are aligned to point backwards. The first wave of 

 contraction is followed by others in a rhythmic succession and the 



