(1) Individuals of the common pond flat-worm, Polycelis nigra, were cut into three pieces transversely (heads, mid-pieces, tails), and the speed of their creeping (gliding) movement was tested.

(2) It was found that the heads were fastest, the mid-pieces intermediate, and the tails slowest.

(3) A gradual process of loss of speed with the continuation of captivity was very noticeable in all pieces.

(4) The rate of slowing down (deceleration) was calculated. In one collection this rate was highest in the tails, but in the other it was greatest in the heads.

(5) It appears that if pieces be allowed to regenerate without food they can attain a high speed, not only in excess of that previously shown by them when recently cut, but in excess of that of controls kept captive and uninjured for a shorter period.

(6) Various control experiments were made on whole worms, with reference to the effects on speed of captivity, size, injury to the anterior end (without decapitation), light-intensity, and anaesthesia.

(7) The hypothesis of metabolic gradients and physiological polarity in organisms, due to C. M. Child and his school, has been applied in some of its bearings to the foregoing results. The relative speeds of the three pieces are considered in relation to the evidence of metabolic gradients in respiratory activity, growth, etc., the rates of slowing down are compared in the two collections to the phenomena of direct and indirect susceptibility to poisons, while it is suggested that the recovery of a high speed in regenerated worms is connected with the presence of rejuvenated tissue.

It is hoped that the results may indicate the possible influence of physiological polarity upon the externally manifested, locomotory behaviour, as distinguished from the more fundamental vital activities. There is a tendency to attribute, without more ado, such behaviour of the animal as a whole to the activities of the central nervous system, an attitude which is open to some criticism in an animal of lowly nervous organisation and great regenerative powers.

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