66 



LEECHES 



contract maximally for a given body volume. When the body is 

 long and thin the spiral fibres are clearly reinforcing the action of 

 the longitudinal muscles and causing the body to shorten (Fig. 35a). 

 When the leech is short and thick the spiral fibres reinforce the 

 action of the circular muscles and cause elongation (Fig. 35b). 



(a) 



Fig. 35. Diagram illustrating how spiral muscles may reinforce 



the action of (a) the longitudinal muscles or (b) the circular 



muscles. For explanation, see text. 



There must be some intermediate position w^hen the spiral muscles 

 cause neither lengthening nor shortening, and this is when they 

 make an angle of 54° 44' w^ith the longitudinal axis. At this point 

 they serve only to increase internal hydrostatic pressure, imparting 

 rigidity to the body of the leech. This enables the leech to sit 

 upright on its posterior sucker, an activity vv^hich is characteristic 

 of certain leeches and important in their behaviour. 



Leeches also have a v^ell developed set of dorso-ventral muscles. 

 These, on contraction, make the body flat and ribbon-like, thus 

 increasing the efficiency of the dorso-ventral undulations used in 

 swimming. 



The longitudinal, circular and dorso-ventral muscle cells of 

 leeches are elongated unstriated cells with an inner axial sarco- 

 plasm and an outer rind of contractile myoplasm. In the longitu- 

 dinal and circular muscles they are usually simple spindle-shaped 



