The effect of variations in osmotic pressure upon a free-living soil nematode

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Abstract
1. The osmotic pressure of the medium in which the nematodes were cultured has been shown to be equivalent to that of about 30 mM. NaCl.2. The effects of immersion in a saline solution isotonic with the above, in glass-distilled water, and in hypertonic saline solutions, have been described. Particular attention was paid to changes in the length of the body, in the appearance and behaviour of the alimentary canal, and in the movement of the body.3. The results obtained have been discussed, and the following tentative conclusions reached:(a) The main aqueous exchanges occur through the cuticle and protoplasm covering the periphery of the body, and are controlled by the protoplasm rather than by the cuticle. The cuticle thus does not act as aja impermeable outer covering which insulates the animal from its environment.(b) The species normally lives in an environment which is hypotonic to the body fluids, and maintains its internal osmotic pressure by means of an active method of osmotic regulation. The alimentary canal is probably partially, but not wholly, responsible for this regulation.(c) Swelling occurs in distilled water because the removal of water in the initial stages is not rapid enough to cope with the extra inflow. Later the excess water is removed.(d) In concentrated solutions, water is removed from the body, and the internal osmotic pressure may also be increased to some extent by the penetration of osmotically active substances to the body, through the cuticle and hypodermis of the body wall. Penetration through the hypodermis is slower than through the cuticle.(e) During recovery in concentrated solutions, there is a slow removal of osmotically active material from the body. The hypertrophy of the lateral and post-anal organs, which sometimes occurs under these conditions, may be connected with this removal.