CHAPTER 7 



MUSCLE, NERVE AND 

 LOCOMOTION 



1. The Muscular System 



Locomotion in annelids in general is brought about by the 

 antagonistic action of two sets of muscles, the longitudinal muscles 

 whose fibres lie parallel to the longitudinal axis of the body and 

 the circular muscles whose fibres lie in a plane at right angles to 

 the longitudinal axis. The forces produced by one set of muscles 

 are used to stretch the others through the hydraulic action of the 

 fluid enclosed within the body wall. Thus an annelid may be com- 

 pared with a fluid filled cylinder which, when the circular muscles 

 contract, becomes long and thin and when the longitudinal 

 muscles contract becomes short and thick. In earthworms there 

 are internal septa which limit the movement of coelomic fluid and 

 help one part of the body to be longitudinally contracted while 

 another part is longitudinally extended, but in many polychaetes 

 the septa are reduced or absent so that several segments work as a 

 unit. In leeches the whole body works as a unit and all septa have 

 disappeared. Moreover the coelomic fluid has been replaced by 

 mesenchyme cells but these are sufficiently deformable to provide 

 the hydraulic mechansim described above. 



Between the outer circular muscles and the inner longitudinals 

 leeches have a double layer of oblique muscles whose fibres run 

 at approximately 45° to right and left of the longitudinal axis 

 (Fig. 33). This condition is unique among annelids. In most 

 species they run spirally round the body in complete right and left 

 geodesic helices. The mode of action of these muscles has not 

 been investigated experimentally but Clark and Cowey (1958) 

 have considered the geometry of a geodesic system of inextensible 

 fibres in some nemertean w^orms and turbellarians and this gives 



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