MUSCLE, NERVE AND LOCOMOTION 73 



sub-epidermal nerve rings corresponding in number and position 

 with the inter-muscular rings, and it has been suggested (Miller, 

 1945) that there is also a network of nerve fibres just below the 

 epidermis comparable with that described for the earthworm by 

 Hess (1925). On the other hand Wilson (1960) studied the spread 

 of excitation over the body wall of Hirudo after stimulation of an 

 isolated segmental nerve and concluded that no nerve net was 

 present. Iwata (1940a, b) reached a similar conclusion regarding 

 a Japanese leech. The sense organs of the epidermis and the 

 proprioceptor organs in the body wall send neurones into the 

 peripheral network of fibres and from here they pass down the 

 segmental nerves to the ventral nerve cord. 



3. Locomotion 



Having discussed the structure and function of the nervous 

 and muscular systems we may now pass to the co-ordinated 

 activity of these systems as seen in locomotion. Broadly speaking, 

 leeches move in two ways: by swimming, which involves dorso- 

 ventral undulations of the body and by creeping, which involves 

 moving the anterior sucker forward and drawing the posterior one 

 up behind. Gray et al. (1938) resolved these activities into a 

 system of reflex responses. Stated rather simply, the sequence is 

 as follows : if Hirudo is freely suspended in water so that neither 

 the suckers nor the ventral body surface are able to make contact 

 with a solid object it will swim, the dorso-ventral muscles being 

 contracted and the circular muscles relaxed while dorso-ventral 

 undulations pass back along the body as a result of differential 

 contraction of the longitudinal muscles. If the anterior sucker is 

 now brought into contact with a solid object it attaches firmly and 

 the swimming stops ; the dorso-ventral and circular muscles relax 

 and the longitudinals contract, causing the body to become short 

 and thick. If now the posterior sticker is placed in contact with a 

 solid object it becomes attached and a wave of contraction passes 

 back over the circular muscles while the longitudinals are inhibited, 

 causing the body to become long and thin. Repetition of the last 

 two reflex actions results in crawling. Gray et al. therefore con- 

 cluded that if ventral peripheral stimulation is absent the leech 

 swims, if it is present it crawls ; and that the rhythm of crawling is 



