4l8 MR. GURNEY ON THE CRUSTACEA OF EAST NORFOLK RIVERS. 
salinity was perceptible to rough analysis at high and low 
water. As a matter of fact, I believe the water had been fresh 
all through the summer. Yet, in spite of that one finds, in 
addition to the fresh-water species, several species which are 
generally found in brackish or salt water. On the other hand, 
when (Nos. 2 and 3) the water was salt no fresh- water species 
were to be found. The fauna in the neighbourhood of Acle 
is a good deal richer, in ordinary conditions, than it is below. 
In the patches of P otamogeton pectinatum growing in certain 
places along the shores between Muckfleet and Acle there 
is an abundance of Crustacea. 
si THE MUCKFLEET. 
I 
The Muckfleet Dike, as its name implies, was once a broad 
shallow dike leading freely from Ormesby, Rollesby and 
Filby Broads into the river about a mile below Acle Bridge. 
It is now a very narrow dike shut off from the river by sluice 
gates, its function being to conduct water from the river to the 
Broads whence it is drawn off for the supply of Yarmouth by 
the Ormesby Waterworks. 
The only part of it with which we are concerned is the 
part nearest the river, where it widens out into a large pool 
with a considerable amount of vegetation consisting of 
Myriophyllum, Ranunculus aquatilis, P otamogeton pectinatum 
and a little Ulva. The depth of water varies considerably 
according as the supply from the river is shut off by the sluice 
gates or not. The composition of the water in the Muckfleet 
is thus largely a matter of human regulation, but, at its river 
end, it is subject to great fluctuations of salinity, similar to 
those in the river. During high tides in the river the water 
becomes, by slow infiltration, highly saline, while at other 
times, when opportunity offers, fresh water is drawn in, and 
also, no doubt, there is a considerable dilution caused by 
reflux of water from the Broads. The fauna varies with 
these changes. It does not appear that floating forms are 
drawn in from the river, so that marine Calanoids are never 
found ; but fresh- water Plankton species often occur, and 
