74 



LEECHES 



determined by a recurrent pattern of stimulation via the ventral 

 suckers. It should perhaps be added at this point that crawling is 

 not produced entirely by elongating and shortening the body. 

 In most species, when the posterior sucker is drawn forward the 

 body is flexed in a dorso-ventral plane, enabling the posterior 

 sucker to be placed immediately behind the anterior one (Fig. 40). 



.^ 



^ 



Piscicola Erpobdella 



Fig. 40. Successive stages in the creeping of Piscicola and 

 Erpobdella. After Herter, 1929, modified. 



Chapman (1958) has pointed out that contraction of the dorso- 

 ventral muscles in the middle region of the body assists this 

 flexure. The movement is more marked in some species than in 

 others and in its extreme form is reminiscent of the movement of 

 a looper caterpillar. 



