Osmotic Regulation in Gunda Ulvae

Abstract

1. The triclad turbellarian Gunda ulvae in its normal habitat is daily subjected both to sea water and to pure stream water. Under experimental conditions it can live permanently in any concentration of sea water down to 5 per cent. It is thus able to maintain a relatively constant internal against a rapidly changing external environment. 2. The manner in which this is done is suggested by the results of the above experiments, from which it is concluded that the following is the course of events when the animal is transferred from pure to dilute sea water (e.g. 10 per cent.): (a) An initial inflow of water through the ectoderm into the parenchyma which causes swelling and a temporary lowering of activity. (b) This water is taken up by the gut cells in the form of intracellular vacuoles, a process which entails expenditure of energy and can be inhibited by cyanide. The parenchyma is thus restored to its original condition and full activity is recovered. (c) After the initial inflow of water the animal begins to set up a resistance, which appears to be effected by a decrease of permeability to water of the ectodermal membrane. (d This resistance is maintained and the gut cells remain vacuolated so long as the animal is kept in dilute sea water. 3. The excretory (water-vascular) system plays no obvious part in this mechanism. 4. Distinction must be drawn between the ultimate impermeability of the ectoderm considered as a membrane and the osmotic resistance of the individual cells of this layer and of the other tissues. The permeability of the ectodermal membrane can be reversibly increased by calcium deficiency, but the osmotic resistance of its individual cells cannot be broken down by these means. This can be brought about by decreasing the osmotic pressure of the water to that of 2 per cent, sea water. Under these conditions the presence of calcium may retard but does not prevent the final swelling and disintegration of the cells, a process which is irreversible. 5. It is suggested that both vacuole formation by the gut cells and the osmotic resistance set up by the individual cells of the other tissues are active processes entailing expenditure of energy, whereas the resistance of the ectodermal membrane to inflow of water into the parenchyma is a passive impermeability.