president’s address. 1O5 
then it has migrated into rivers, and is now found in quite 
fresh water, having even invaded the water pipes of Hamburg, 
and, in company with the mollusc Dreisscna polymorpha, 
completely blocked them. This, with the exception of the 
common Hydra, is the only Hydroid living in fresh water, 
and it is of special interest to Norfolk, as it grows freely in 
the Hickling district. I must add, however, that the waters 
of Hickling are not absolutely fresh, though I need not go 
into the causes of that here, further than to say that their 
saltness is in no way due to tidal influence. 
The limnetic species found in the sea are less numerous ; 
while there are many fish which come up the rivers to spawn, 
there are none, except the Eel, which seek the sea for this 
purpose. 
Of hydrachnids and insects there are several species which 
have a marine habitat. 
The Frog has been found living and spawning at Greifswald, 
on the Baltic, where, however, the percentage of salt is very 
small (under 1 %). 
The power of resistance of lacustrine animals to salt, and 
of marine animals to fresh water, is very various, and is not 
always in proportion to the apparent delicacy of structure. 
For example, Semper has experimented with Frogs, and 
found that they die in two and a half hours in a solution of 
5 % of salt, but that they can live without apparent 
inconvenience in a 1 % solution. Beudant found that by 
very slowly increasing the quantity of salt in solution he 
was able to accustom species of Limnea, Physa, Planorbis, 
and Ancylus to a 4 % solution, while he was quite unsuccessful 
with Unio, Anodonta, and Cyelas ; on the other hand, in 
the reverse experiment, he found Mytilus able to live in quite 
fresh water (Mytilus is found in the Baltic, in waters of 
very low salinity ; but there it is dwarfed in size). 
Kennel describes a lagoon in Trinidad which is cut off 
