[34] "Mem. American Acad. Arts and Sciences," 1850, p. 229.
[35] The covered-eyed Medusæ survive a longer immersion than the naked-eyed--Aurelia aurita, for instance, requiring from a quarter to half an hour's exposure before being placed beyond recovery. Moreover, the cessation of spontaneity on the first immersion is not so sudden as it is in the case of the naked-eyed Medusæ--the pulsations continuing for about five minutes, during which time they become weaker and weaker in so gradual a manner that it is hard to tell exactly when they first cease.
Thinking it a curious circumstance that the mere absence of the few mineral substances which occur in sea-water should exert so profound and deadly an influence on the neuro-muscular tissues of the Medusæ, I was led to try some further experiments to ascertain whether it is, as Agassiz affirms, to the mere difference in density between the fresh and the sea water, or to the absence of the particular mineral substances in question, that the deleterious influence of fresh water is to be ascribed. Although my experiments lead to no very instructive conclusion, they are, I think, worth stating.
I first tried dissolving chloride of sodium in fresh water till the latter was of the same density as sea-water. Sarsia dropped into such a solution continued to live for a great number of hours; but they were conspicuously enfeebled, keeping for the most part at the bottom of the vessel, and having the vigour of their swimming motions greatly impaired. The tentacles and manubrium were strongly retracted, as in the case of exposure to fresh water, and the tissues also became slightly opalescent. Thinking that perhaps a fairer test would be only to add as much chloride of sodium to the fresh water as occurs in sea-water, I did so; but the results were much the same. On now adding sulphate of magnesium, however, to the amount normally present in sea-water, the Sarsia became more active. I next tried the effects of chloride of sodium dissolved in fresh water to the point of saturation, or nearly so. The Sarsia, of course, floated to the surface, and they immediately began to show symptoms of torpidity. The latter became rapidly more and more pronounced, till spontaneity was quite suspended. The animals, however, were not dead, nor did they die for many hours, their irritability continuing unimpaired, although their spontaneity had so completely ceased. The tentacles and manubrium were exceedingly relaxed, which is an interesting fact, as being the converse of that which occurs in water containing too small a proportion of salt. Lastly, to give the density hypothesis a still more complete trial, I dissolved various neutral salts and other substances, such as sugar, etc., in fresh water till it was of the density of sea-water; but in all cases, on immersing Sarsia in such solutions, death was as rapid as that which followed their immersion in fresh water.