MR. a. J. ROMANES ON THE LOCOMOTOR SYSTEM OE MEDUSAE. 
745 
Thinking it a curious circumstance that the mere absence of the few mineral substances 
that occur in sea- water should exert so profound and deadly an influence on the nervo- 
muscular tissues of the Medusae, 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. Sarsice 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 polypite were strongly retracted, as in the case of ex- 
posure 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 Sarsice 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 Sarsice, 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 ten- 
tacles and polypite 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 &c., in fresh water till it was of the 
density of sea-water ; but in all cases, on immersing Sarsice in such solutions, death was 
as rapid as that which followed their immersion in fresh water. 
VII. GENERAL SUMMARY. 
The “ fundamental experiment ” of paralyzing nectocalyces by removing their margin 
has this year been repeated a vast number of times on various species of naked-eyed 
Medusae, and always with the same result. In the case of the covered-eyed Medusae, I 
have confirmed such of my previous observations as required confirmation, from the fact 
of their having been at variance with those of Dr. Eimer. I have continued to find that 
the lithocysts are the exclusive seats of spontaneity, so far as the “ primary movements ” 
are concerned. I have failed to detect the slightest evidence of spontaneity on the part 
of the contractile zones. I am still unable to say that the “ secondary contractions” 
appear to me either more “feeble,” “inefficient,” or “local” than the primary ones; 
neither can I modify my previous statements as to the time during which these 
