1. Experimental determinations of the hydrostatic pressure in the pseudocoel of living Ascaris lumbricoides were made by a direct method, using a glass helix pressure gauge and by an indirect method using an indentation gauge, both of which are described.

2. The mean value of this pressure was 70 mm. Hg (95 cm. of water), and showed wide and often rhythmical variations from 16 mm. to as high as 225 mm. Hg. Observations on the behaviour of artificially distended worms and of the tension developed by the muscles confirm these results.

3. The mechanical structure of the cuticle, with its inextensible spiral fibrils, forming a basketwork at an angle of 75° to the longitudinal axis, provides for an anisometric expansion and contraction under the action of the longitudinal muscles which is closely in accordance with the observed changes in volume and length.

4. A discussion of the significance of this mechanism and of the high internal pressure suggests that the great similarity of form among nematodes is determined to a considerable extent by mechanical factors.

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