1. The form and frequency of the waves passing down the bodies of small freeliving nematodes (Panagrellus, Rhabditis and Turbatrix) depend on the nature of the external medium.

2. Observations of animals moving in such media as syrup, agar gels, and dense suspensions of particles suggest that the relationship between the speed of progression of the animal to the speed of propagation of the waves along the body, depends on the relative resistance exerted by the medium to displacement of the body in directions normal to and tangential to its own surface.

3. When swimming in water the body of a nematode yaws periodically in a transverse plane and the axis of progression does not coincide with that of the waves; the displacement of an element of the body relative to the medium depends on its position on the body. The envelope of one complete wave exhibits two characteristic nodes.

4. A suspended particle originally situated near the anterior end of a swimming animal moves tangentially backwards along the surface of the body, but the speed at which it does so is not constant and is always much less than that at which the waves travel relative to the ground. The average speed of a particle close to the surface of the body is about one quarter of that of the waves.

5. When plotted relative to the ground the path of displacement of a particle exhibits characteristic loops. When viewed from above and in the direction of the animal's progression, all particles on the left side of the body traverse their loops in a clockwise direction; all those on the right side move anticlockwise.

6. There is a highly characteristic pattern of circulation round the body of the animal. Water in the vicinity of a wave crest moves in the opposite direction to that of the propagation of the waves, but in the same direction as the waves when situated in a wave trough. The circulation extends for a considerable distance from the surface of the body.

7. The flow round the body of a swimming nematode is essentially the same as that in the neighbourhood of an undulating sheet of rubber. Its analysis presents an interesting but difficult hydrodynamical problem.

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