638 MU. R. GURNEY ON FRESH- AND BRACKISH-WATER CRUSTACEA. 
for the other Broads, and particularly for the ditches connected 
with them. 
For the purposes of this investigation my wherry, the “ Cyclops,” 
has proved invaluable, since by her means the ground has been 
covered far more thoroughly and easily than would otherwise 
have been possible. A wherry is better adapted than any 
other boat for such work, since she not only provides ample room 
for carrying the necessary apparatus, but also allows a considerable 
amount of microscopic work to be done on board in comparative 
comfort. I have also made extensive collections in ponds and 
ditches in the district, and have been very much assisted in this 
direction by Mr. R. A. Todd, who has sent me numerous collec- 
tions from various parts of the county, and has in this way supplied 
me with many very valuable records. 
As is well known, the Rivers and Broads of Norfolk are the last 
relics of the great estuaries which at one time penetrated far into 
the county. At the present day such estuarine conditions are only 
preserved in Breydon Water, but even now the salt water reaches 
some way up the rivers, and there is a tidal rise and fall extending 
in the Yare, for instance, as far as Norwich. Accurate informa- 
tion as to the tidal rise and fall at different points, and as 
to the distance up the rivers at which salt water can normally 
he detected, is not forthcoming at present, though hearsay 
evidence can be collected in abundance. Unfortunately, the 
Norfolk waterman often uses the term tide indifferently for an 
influx of salt water, a periodic (tidal) banking up of fresh water, 
and for the irregular rise and fall of water due primarily to rain, so 
that his evidence is not reliable. For the Bure, at all events, it 
can be stated that the salt water does not normally reach higher 
than Acle Bridge; but that during periods of exceptionally high 
tides, sometimes due to a prevalence of strong Northerly winds 
banking up the water in the North Sea, the salt water may reach 
right up to Ant mouth, and from there up the Ant even to Irstead. 
On such an occasion I have even taken marine Calanoids and 
living Ctenophores at Thurne mouth. It follows that the inhabi- 
tants of the rivers and some of the Broads must be exposed to 
fluctations of salinity of considerable, and sometimes of very great 
magnitude. At every tide there must be a great destruction of 
the floating forms in the lower reaches of the rivers, and in fact 
