ay 
2. Some Recent Advances in our Knowledge of 
the Freshwater Fauna of India, 
(A lecture delivered before the Asiatic Society of Bengal 
on the evening of March 27th, 1912.) 
By N. ANNANDALE, D.Sc., F.A.S.B. 
Until a few years ago it was no exaggeration to say that, 
except as regards the fish and shells, we knew far more about 
the animals which live in the Bay of Bengal at a vertical depth 
of two miles than we did of those of the Calcutta tanks. Since 
I became Superintendent of the Indian Museum a little more 
and could not be comprehensive, they will, I hope, prove a 
useful starting-point for what still remains to be done. To- 
night I propose first to give a rapid summary of what we know 
of the freshwater fauna of India from a systematic point of 
view, noticing some recent discoveries in each of the more im- 
portant groups, and then to discuss in rather greater detail 
certain problems to which I have myself devoted special atten- 
tion. ese problems are (1) the biological relations between 
different kinds of freshwater animals; (2) seasonal changes in 
the life-cycle of the lower freshwater invertebrates, and (3) the 
direct effect of environment on plastic organisms such as fresh- 
water sponges. It is not yet possible to deal in a satisfactory 
manner with the geographical distribution of the Indian fresh- 
water fauna as a whole. 
I. 
The only Indian mammal that is a permanent denizen of 
fresh water is the Gangetic Porpoise (Platanista gangetica), a 
highly specialized form that occurs also in the Indus. As it 
never visits the sea, the fact that it inhabits two rivers which 
now flow in opposite directions is a proof that these rivers were 
united at a not very remote period. 
It is natural that fish should have attracted more attention 
in the past than any other aquatic group of animals, and a 
hundred years ago Hamilton (or, as he at one time called him- 
