242 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
more complete trial, I dissolved various neutral 
salts and other substances, such as sugar, ete., in 
fresh water till it was of the density of sea-water ; 
but in all cases, on immersing Sarsiz in such solu- 
tions, death was as rapid as that which followed 
their immersion in fresh water. 
The Fresh-water Medusa. 
On June 10, 1880, it was noticed that the 
fresh water in the large tank of the lily-house of 
the Royal Botanical Society, Regent’s Park, was 
swarming with a small and active species of Medusa, 
previously unknown to science—it being, indeed, at 
that time unknown to science that any species of 
Medusa inhabited fresh water, although it was well 
known that some of the other Hydrozoa do so. 
Examination showed that the new species belonged 
to the order Trachomedusz, and the Petaside of 
Haeckel’s classification—its nearest known relative, 
according to Professor Ray Lankester, being the genus 
Aglauropsis, which occurs on the coast of Brazil. 
The Medusa was called Limnocodium (Aiur, a 
pond, and xédwy, a bell) sorbii by Professors All 
man and Lankester. I am indebted to the kindness 
of Professor Allman for permission to reproduce his 
drawing of the animal. (Fig. 31.) It is remark- 
able that, although this Medusa has reappeared 
every June in the same tank, no one has yet suc- 
ceeded in tracing its life-history. Nor is it known 
from what source the tank first’ became impregnated 
with this organism. No doubt the germs must 
