1. The type of locomotory activity which can be displayed by an intact leech depends upon the presence or absence of particular patterns of exteroceptive stimulation reaching the nerve cord via the suckers or ventral surface of the body. If such peripheral patterns be present, the animal can walk but cannot swim, if such patterns be absent the animal can swim but cannot walk.

2. The frequency of the ambulatory rhythm is normally determined by the rhythmical adhesion of the suckers. Adhesion can only occur when a sucker is in the protruded position.

3. The protrusion of the suckers is dependent upon the isotonic contraction of the body muscles. The posterior sucker is not protruded during isometric contraction of the longitudinal muscles. It invariably occurs when these muscles contract isotonically. Fixation of the posterior sucker induces active relaxation of the longitudinal muscles. Protrusion of the anterior sucker is dependent upon contraction of the circular muscles.

4. If the isotonic contraction of the longitudinal muscles is impeded by an external resistance the ambulatory rhythm is reduced in frequency; in extreme cases, the rhythm ceases and is replaced by a very powerful and prolonged contraction of the longitudinal muscles. These phenomena are attributed to the activity of stretch receptors whose presence is inferred from the electrical activity set up in the sensory and motor fibres of the segmental nerves. It is suggested that the normal ambulatory rhythm is best regarded as a closed chain of peripheral reflexes.

5. The transmission of the waves of longitudinal and circular contraction along the body, during ambulation, is shown to be independent of the integrity of considerable regions of the somatic musculature.

6. Decapitated leeches only exhibit an ambulatory rhythm in response to strong stimulation. The loss of muscular tone characteristic of an animal deprived of the suboesophageal ganglion is associated with a marked reduction in the arhythmical electrical discharges detectable in the segmental nerves of an isolated nervous system subsequent to removal of the suboesophageal ganglion.

7. Both intact and decapitated preparations swim readily when released from all tactile stimuli. The frequency of the swimming rhythm is affected by changes in the viscosity of the medium and by tension applied longitudinally to the body. The rhythm can be accelerated or suppressed by appropriate exteroceptive stimulation.

8. The whole nerve cord exhibits a marked electrical rhythm so long as it is in organic connexion with a limited region of the body displaying mechanical swimming movements; the frequencies of the mechanical and electrical rhythms are identical.

9. No electrical rhythm, comparable to that which might be expected to be characteristic of ambulation or swimming, has been detected in the isolated central nervous system.

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