MUSCLE, NERVE AND LOCOMOTION 77 



are explained by supposing that in response to peripheral stimula- 

 tion the sub-oesophageal ganglion excites the ventral nerve cord 

 to co-ordinate crawling activity. There are four situations in which 

 it might fail to do so: (i) when there is no ventral peripheral 

 stimulation, (ii) when inhibited by the brain, (iii) when the 

 ganglion has been removed and (iv) when the nerve cord has been 

 cut. Taken together these account for all the experimental and 

 behavioural observations quoted above. 



According to Schluter (1933) removal of the anal ganglion 

 inhibits swimming so that leeches dropped into water fall passively 

 to the bottom. Removal of the brain of such leeches restores the 

 powers of swimming. It is thus possible that the division of labour 

 between the various ganglionic masses is as follows: the sub- 

 oesophageal ganglion is mainly responsible for initiating and 

 maintaining crawling movements, the anal ganglion for swimming 

 movements and the supra-oesophageal ganglion for inhibiting 

 locomotion under certain circumstances. 



It is interesting and instructive to compare the locomotion 

 of leeches with that of earthworms. In the latter, the basic 

 mechanism is that a group of segments becomes elongated, obtains 

 a point of attachment anteriorly by protrusion of chaetae and then 

 shortens, drawing the posterior segments forward. Chaetae are 

 then protruded on the posterior segments before the next phase 

 of elongation begins. Normally, several waves of activity are 

 present in the body at one time, but the author has observed that 

 in certain circumstances one wave of activity may occupy almost 

 the whole body at least in certain species of earthworm. From this 

 condition it is only a short step to the arrangement found in leeches. 

 The anterior and posterior suckers replace the chaetae as means of 

 attachment and in crawling the whole body is involved in one wave 

 of activity. 



It is no longer necessary for the body to be divided internally 

 into a number of distinct hydraulic units capable of independent 

 activity and this is probably the functional reason for the loss of 

 septa and the obliteration of the spacious coelom in leeches. 

 Earthworms are capable of co-ordinated movement after nerve 

 cord transection and even after the body has been completely 

 severed and joined again by stitches. Apparently a peripheral 

 mechanism for the transmission of locomotor reflexes is present in 



