ABSTRACT
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.
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.
- (a)
The excretory (water-vascular) system plays no obvious part in this mechanism.
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.
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.
The granular cells were not affected.
These, however, did not immediately recover their resistance to normal io per cent, sea water:
- (a)
7 hours after return to too per cent, sea water two worms were placed in normal 10 per cent. They were normal after 2 hours, but after 12 hours were again distended.
- (b)
A similar experiment after 27 hours in too per cent, sea water. Both normal after 17 hours in normal 10 per cent.
Experiments were performed to determine whether the feeding of the animal is upset by this vacuolation of the gut cells. Starved worms kept 12 hours previously in 10 per cent, sea water were supplied with pieces of earthworm. They were not as strongly attracted by the food as were controls in normal sea water. They were fixed 2 hours after feeding, and sections showed that, though a little food had been taken into the gut lumen, none had been ingested by the gut cells. The controls showed the gut cells crowded with food vacuoles.
Ellis (1933) has shown with Nereis diversicolur that calcium deficiency in 17 per cent, sea water, which in presence of calcium the animal can normally withstand, causes increased accumulation of water in the body. This is the result if calcium is removed after it has adjusted itself to dilute sea water as shown by the fall in weight. It seems then that in this animal also adjustment to hypotonic water is effected by a decrease in permeability of the body surface, tor which the presence of calcium is necessary.
