206 THE ENTOMOSTRACA OF EPPING FOREST.
In this case it is useful to replace the glass tube at the end of
the net by a small cylindrical tin weighted with lead. By means
of this it is sometimes possible to obtain species not to be taken
quite close inshore. If the net is to be used in places where there
is much vegetation, the mouth may be covered with a cone (point
upwards) of wire gauze of fairly open mesh as suggested by
Prof. Birge. This will prevent the net from being choked with
weeds and will also keep out the larger animals such as pond-
snails, insect larvae, &c.
For the proper investigation of a piece of water of fair size
it is visually necessary to make at least three separate collections
with the nets, namely, one from the middle, one from amongst
the vegetation near the margin, and one from the bottom. If the
collector also makes it a rule to take wet mosses from all
situations, but especially such as grow on tree trunks or posts,
just above the water level, he will probably not miss very many
of the Entomostraca in this district. In other localities, where
large and deep lakes occur, it is quite essential to a proper
understanding of the fauna to collect from a boat by means of
tow-net and dredge, but even in those districts much may be
done with the simple collecting appliances previously mentioned.
Of course, for quantitative work in any district, a specially
constructed net with carefully graduated line, or its recently
introduced equivalent, the Plankton-pump, is necessary, but
these need not trouble us here.
The golden rule for the examination of the Entomostraca is
that it should be made while the animals are alive. No doubt
it sometimes happens that this is impracticable, and in such
cases it is usually better to preserve a collection for future
examination than not to look at the animals at all. But this
method should be the exception, not the rule. Work done upon
living specimens is not only more pleasant and infinitely more
suggestive, it is really easier than with preserved material, for
the tissues are more transparent as a rule, and the animals can
be manipulated so as to show almost any detail of structure with
a minimum of trouble. With a living Copepod, for instance,
be it the smallest Cyclops or Canthocamptus, it is always possible
to make out in a very short time all the points necessary for
specific determination, without injuring the animal in any way.
With preserved examples, however, this is not so easily accom-
plished, and it is held by some workers that nothing short of
dissection then suffices for the purpose.