MUSCLE, NERVE AND LOCOMOTION 69 



found that the chronaxies were 30 msec for Hirudo and 20 msec 

 for earthworm, while conduction rates were about 35 cm/sec. 

 The response of leech muscle to electrical stimulation is 

 decreased by immersion in hypotonic solutions of sodium chloride 

 or sugar (Winterstein and Ozer, 1949), but the same treatment 

 increases the amount of tonic contraction, suggesting that the 

 mechanism of contraction after stimulation is quite distinct from 

 the mechanism for maintaining tonus. Moreover, while increased 

 activity under electrical stimulation is associated with increased 

 oxygen consumption, there is no such relation between tonus and 

 oxygen consumption (Ozer and Winterstein, 1949). The oxygen 

 consumption is maximal when the muscle is immersed in 0-1 N 

 sodium chloride and at other concentrations the oxygen consump- 

 tion is lower irrespective of whether the tonus is increased or 

 decreased. 



2. The Nervous System 



The gross morphology of the central nervous system of Hirudo 

 has been described and the differences in other genera are mainly 

 ones of relative proportions. In this section we shall consider the 

 micro-anatomy of the nervous system of a typical segment in order 

 to provide a background for the understanding of its physiology. 

 As in most annelids the motor nerve cells are concentrated in the 

 ventral nerve cord while the sensory nerve cells lie peripherally, 

 in or near the sense organs. In earthworms the nerve cells are 

 widely distributed in the ventral half of the ventral cord with 

 special concentrations in the segmental ganglia, but in leeches the 

 motor cell bodies are found almost entirely in the ganglia, enclosed 

 within fibrous capsules so that they are sharply separated from the 

 mass of fibres which form the bulk of the nerve cord. There are, 

 however, certain cells in the fibrous mass. They are spindle shaped 

 when viewed from the side and star shaped in cross section. Miller 

 (1945) regards them as nerve cells but most authors, notably 

 Scriban and Antrum (1934) and Ito (1936) consider that they are 

 neuroglia, i.e. supporting cells whose processes bind together the 

 nerve fibres. 



Each ganglion has six cell capsules, two antero-dorsal in position, 

 two postero-dorsal and two median ventrals. A typical transverse 



