484 
NOTES AND MEMORANDA. 
water was salt. Here are the most favorable conditions 
possible for turning marine animals into fresh water animals; 
in fact, the change of mode of life presents no difficulty. 
Below no doubt the water is always salt, but the fish find a 
fluid less and less salt as they rise to the surface. We 
caught the mullets in the almost fresh water with a net, and 
with them the mussels and crabs. I even saw an abundance 
of Medusae, and a species of Rhizophora sw'imming in the 
creek above the sand-flats, where there was scarcely any 
salt at all in the water, yet evidently in most perfect 
health.” 
Professor Agassiz writes, It strikes me as if the conse- 
quences resulting from the finding of this freshwater Medusa 
had been somewhat overdrawn. In the first place, w^e have 
two genuine freshwater Hydroids, Hydra and Cordylophora, 
and in the second place, as far as my experience goes, it is 
not conclusive of so fatal an action of fresh water on Medusae 
as Romanes would lead us to believe in. We have quite an 
estuary leading out back of Boston Harbour, extending on 
tlie one side to form what we call the back bay, and beyond 
this up the Charles River as far as W atertown, where there is 
a dam, about seven miles from the inner extremity of the 
harbour proper. Here the Charles River falls into this 
estuary as a freshwater stream sufficiently large at times to 
affect the saltness of the estuary below it at low tide, so 
that at Cambridge, half way from XVatertown to Boston, the 
w^ater is salt only at the highest tides, quite brackish during 
the first half of the ebb, and comparatively fresh during the 
last part of the tide. At W. Boston Bridge, about one mile 
from the head of the harbour, the water at the last part of 
the tide is fresh enough and tastes but little salt. At this 
bridge there is an abundance of Hydroids which thrive 
remarkably well on the drainage of the district, and grow to an 
unusually large size. The species found there which has no 
free Medusa is Laomedea gigantea. While of the Hydroids 
which have free Medusae we find Eucope diaphana, Eucope 
pyriformis , Ohelia commissuralis . All these species are, 
therefore, twice during twenty-four hours exposed to salt 
water and to nearly fresh water, and thrive remarkably well 
under the treatment, as must of course their free Medusae, 
which I have caught both at high tide and low water — in 
salt and in nearly fresh water. 
“ Other of our Medusae also find their way into this 
estuary, and I have found in fresh water at loiv tide active 
Sarsiae, Tiaropsis, and also Aureliae, which seemed unaf- 
fected by the large quantity of fresh water in which they 
