ABSTRACT
It is an interesting fact that the water content of the edible snail, Helix pomatia, varies with the physiological condition of the animal. Duval (1930) showed that the osmotic pressure of the blood varied from Δ = − 0·37° to ‒ 0·43° in hibernating animals, and from — 0-30° to — 0-40° in active summer animals. Cardot and Troussier (1929) found that the blood volume was considerably greater in active than in hibernating animals, and Brand (1931) showed the same to be true of the water content of the whole body. In the present paper, the observations of Duval (1930) on the osmotic pressure of the blood are repeated, using the very accurate thermo-electric method described by A. V. Hill (1930 a).
The principle of the method is as follows. Two pieces of filter-paper, soaked respectively in the unknown fluid and in a standard NaCl solution, are placed on the two faces of a sensitive differential thermopile. If they have different vapour pressures, the two fluids will evaporate at different rates, so that there will be a temperature difference between the faces of the thermopile. This can be measured electrically, and, by previous calibration, the vapour pressure of the unknown fluid can be determined. Only very small amounts of fluid are required.
In the present research, the thermopiles employed were those described by Hill (1931, p. 44) as “type a of the micro-form.” No particular modification was made in the technique employed by him.
The snails were obtained in the hibernating condition from France. In Table I, the data for the hibernating animals are compared with those for three groups of active animals from the same hibernating stock batch. They were easily activated by immersing them in warm water, and were kept active on cabbage leaves, at about 20° C., for 1, 2 or 3 weeks after emergence from hibernation.
To make the measurements, the blood from a single individual was collected in a small stoppered bottle by puncturing the heart. The filter-paper was left to soak in the blood for at least half an hour to attain equilibrium (Hill, 1930 b). The blood was not centrifuged, since it was assumed that the heat production, if any, of the blood cells was negligible. The vapour pressure of the blood was measured against that of 0·70 per cent, (by weight) NaCl, and the calibration was made each day with this NaCl solution and distilled water.
The blood from every individual was tested with three different thermopiles. The measurement with each thermopile was repeated, after interchanging the two solutions between the two faces of the instrument, and the mean of these two values is tabulated in Table I. In order to show the accuracy of the method, the results obtained with the three different thermopiles L1, L2; and L3 are given separately. It will be seen that the three values for each individual snail coincide well with one another.
It is clear in Table III, where the results are summarised, that the blood of active snails has a very much lower osmotic pressure than that of hibernating ones, and this confirms the results of the authors quoted above. It may be pointed out that there is considerable variation between different individuals, especially in the hibernating animals, but taking the standard deviation into account, the mean osmolar difference between active and hibernating animals is evidently significant. It may be noticed that the two ranges overlap; the lowest hibernating value is 0-51 per cent., and the highest active value is 0·58 percent. It is also clear that the mean osmotic pressure does not vary in active animals once they have emerged from the hibernating state.
Strictly speaking, it may not be right to compare the figures here given with those of other investigators, because of the lack of experimental control for humidity and temperature. The good agreement with the results of Duval (1930) is nevertheless striking (Table III). Duval’s animals were collected in the field, not kept, as mine were, in the laboratory.
The results from snails from another hibernating stock batch are given in Table II, as additional evidence for the tendency of the hibernating snail to have, on the average, a blood of higher osmotic pressure. In Exps. 5, 6, 7 and 8, each pair (active and hibernating, the former having been active for about 3 weeks, the latter’s epiphragm having been removed) was kept without food or water in a small (1 litre) enclosed glass jar in a refrigerator for 1 week previous to the determination. This was done in order to diminish the difference of environmental conditions, but still the osmolar difference appeared.
It is, however, not appropriate to conclude from this that active snails possess, as a matter of physiological necessity, the more diluted blood even under the same humidity and temperature. It is likely that such simple treatment alone was not sufficient as an experimental control to equalise the osmotic difference in question.
SUMMARY
The vapour pressure of active and hibernating Helix pomatia has been measured by A. V. Hill’s thermo-electric method.
Hibernating blood is isotonic, on the average, with 0-69 per cent. NaCl, and active with 0-50 per cent. NaCl.
ACKNOWLEDGEMENTS
These determinations were made at the request of Mr G. P. Wells, of the Zoological Department of University College, London. My best thanks are due to Prof. A. V. Hill, F.R.S., in whose laboratory the work was done, for facilities for the determinations, to Mr G. P. Wells and Mr N. H. Howes for their kind help, especially in the supply of snails, and to Mr J. L. Parkinson and Miss M. Hetherington for assistance in the technique.