ANATOMY OF LIMNOCNIDA TANGANYICA. 287 
and consequently its waters are saline. This Medusa (if 
Medusa it be) may be descended from fresh-water ancestors. 
The first question that naturally occurs to the student of 
evolution is, How did the fresh-water Meduse get into fresh 
water? It is clear that they must either have originated 
there, or they must have immigrated from the sea. The latter 
of these alternatives is the only one worthy of consideration, 
since we are not acquainted with any fresh-water Celenterate 
from which the known lacustrine species could be derived ; 
whereas the sea must always be regarded as the true home 
and birthplace of Medusz. If this proposition be accepted, 
there is no reason why all fresh-water Celenterates known at 
present cannot be regarded as having descended from marine 
ancestors. 
In the first place, the mere fact of the difference of salinity 
between fresh and salt water does not seem to be an insuperable 
difficulty in the transition from a marine life to a lacustrine 
one. The marine forms Aurelia aurita and Crambessa 
Tagi, already mentioned, often frequent brackish water. The 
only essential condition is, that the change from salt to fresh 
water must be very gradual, as any sudden substitution of one 
for the other causes almost instantaneous death to a soft 
gelatinous creature like a Medusa, owing to the very violent 
osmotic action which occurs in animals with a soft skin. 
Experiments demonstrating this point were made long ago by 
Beudant, and are described by Semper in his ‘ Natural Con- 
ditions as they affect Animal Life.’ 
The circumstances under which the postulated slow change 
of environment might occur are of several kinds. All marine 
creatures found in fresh water have probably either wandered 
up rivers from the sea, or have been living in bays or lagoons 
which have become cut off from the sea by the upheaval of 
land or by the silting up of the connecting channel, or in some 
other way familiar to the geologist. In any of the latter 
cases there may perhaps have been an intermediate epoch 
when the body of water, completely separated from the sea at 
low water, was still in connection with the sea at high tide. 
voL. 86, PART 2,—NEW SER. U 
