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Change of Osmotic Pressure

sea-water, fresh-water, animals, water and mixture

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Noctiluca can stand a considerable decrease in osmotic pressure with out having its luminous response affected, as indicated in table 1. We may dilute the sea-water with fresh-water in any amount down to a pro portion of half and half, and the animals still give a normal response— e., they flash on stimulation. The response is not quite so bright as in the control in a half-and-half concentration nor in a mixture of 4 sea-water to 6 fresh-water, but the animals continue giving a normal response for 7 days. In the latter concentration the animals also give a steady glow for the first few minutes, owing doubtless to the dying condition of certain of them. If the amount of fresh water is still further increased, the animals do not flash on stimulation, but give a steady glow, lasting for 8 minutes in a mixture of 3 sea-water to 7 fresh-water and of 2 sea-water to 8 fresh-water; and for 5 minutes in 1 sea-water to 9 fresh-water, and for about 3 minutes in pure fresh-water. When the osmotic pressure is increased by concentrating the sea-water to half its volume, a steady glow is given for more than 20 minutes.

Especially interesting results have been obtained in this series of experiments with regard to specific gravity. Normally, noctilucas are less dense than sea-water, so that when placed in an aquarium jar they soon rise to the top, where they form a layer somewhat pinkish in color. Unlike the floating siphonophores, there are no air-bubbles, and unlike certain pelagic eggs, there are no large oil-drops in the animals, so that their lower specific gravity must be due to the fact that their salt-content is less than that of sea-water. When placed in sea water concentrated to half its volume they shrink, and when the sea-water is diluted with fresh-water they swell. The plasma mem brane, therefore, shows the usual semipermeability to the balanced salts of sea-water—i. e., impermeability to the salts and permea bility to water. Out in the open bay, although most of the ani mals are at the surface on a calm night, many can be seen through a glass-bottom bucket well below the surface. Moreover, on windy days, they are not found at the surface, indicating that they have the power of increasing their specific gravity and sinking, owing to a tem porary increase in permeability which allows the salts of sea-water to pass in or water to pass out. When noctilucas die they immediately

sink, the permeability having been increased, causing an increase in specific gravity.

That these animals can also decrease their specific gravity is shown by the following facts. In all dilutions of sea-water with fresh-water down to 6 sea-water to 4 fresh-water, they rise immediately to the top, becoming and remaining somewhat swollen for more than 10 days. These dilutions are still of greater density than the animals. In a mixture of 6 sea-water to 4 fresh-water they remain distributed through the water for about 15 minutes, and then rise to the top; this mixture is of approximately the same density as the noctilucas. In a mixture of half sea-water and half fresh-water and also in a mixture of 4 sea water to 6 fresh-water, the animals sink at first, their salt-content being now greater than that of the surrounding medium, but during the next hour they gradually swell and rise to the top. The process is wholly independent of movement of the tentacle. If water were merely absorbed by the animals until their concentration was the same as that of the surrounding medium, the animals would remain suspended through the liquid; but since they eventually float, they must keep on absorbing pure water until their salt-content and hence their specific gravity is again less than that of the new medium (5 sea-water to 5 fresh-water or 4 sea-water to 6 fresh-water), thus reestablishing their normal relation to their surrounding medium. This must involve the absorption of water against the osmotic pressure of the salts of sea-water, a condition contrary to physical laws. This regulatory mechanism is characteristic of living cells only and is not destroyed under any experimental conditions. Anesthetics, acids and alkalies, KCN, and pure salts of sea-water do not affect the regulation, except when they cause irreversible changes and death of the cells. It is quite probable that this peculiar type of osmo-regulation is not characteristic of noc tilucas alone, but may be possessed by other marine plankton forms without gas-chambers which float and sink as occasion demands.

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