The Mechanism of Locomotion in the Leech {hirudo Medic

CONSIDERABLE uncertainty still exists concerning the nature of the mechanism which maintains the normal locomotory rhythm of terrestrial animals. Two types of theory have been advanced: (i) That the rhythm is determined by the existence of a closed chain of peripheral reflexes, each phase of a complete locomotory cycle setting up automatically the particular pattern of peripheral stimulation requisite for the elicitation of the succeeding phase in the cycle, (ii) That the rhythm is, fundamentally, a property of the central nervous system which is, however, susceptible to modification or extinction by peripheral stimulation. Similar theories have been applied to the locomotory rhythm of aquatic animals but in such cases the balance of opinion is in favour of the view that the rhythm is of central and not peripheral origin. The common leech (Hirudo medicinalis Ray) is of interest for such investigations for two reasons: (i) It exhibits two clearly denned and distinct types of locomotory rhythm, (ii) During normal terrestrial locomotion, the two phases of each step are very clearly related to the two very simple and clearly defined patterns of peripheral stimulation emerging from the suckers. The leech possesses three main groups of muscles (circulars, longitudinals and dorso-ventrals); during terrestrial movements the dorso-ventrals remain inactive, whilst the circulars and longitudinals exhibit a characteristic rhythm of activity; during aquatic locomotion, the dorso-ventral muscles are held in a state of sustained contraction, the circular muscles are relaxed, whilst waves of contraction pass alternately down the ventral and dorsal bands of longitudinal muscles. The present paper deals with the neuro-muscular mechanism of both types of progression.