232 THE ESSEX NATURALIST.
dwarf) to denote all those very minute organisms which can
pass easily through the meshes of the finest silk gauze nets.
Naturally some specimens of these small organisms will find
their way into the collections made with nets, but they will not
be in greater numbers than in the same quantity of unstrained
water and they will be altogether crowded out and perhaps
consumed by the larger organisms concentrated by the nets.
There is no rigid upper limit of size for nannoplankton
organisms but for globular and broadly ovoid forms a diameter
of not much over l/1000th of an inch, (say 25-30u) may be
taken as a convenient maximum. Fusiform and needle-shaped
forms may of course be considerably longer than this if their
diameter is proportionately smaller. There is no lower limit of
size except what is determined by visibility under the microscope.
Whether there are organisms smaller than this is doubtful, and
in any case the ultra-microscopic viruses, whether organisms or
not, do not, so far as is known, occur in water. Nannoplankton
organisms, even when they occur in considerable numbers and
are viewed with the aid of a pocket lens, appear only as the finest
dust in the water.
Although it is not possible to concentrate nannoplankton
organisms by means of a net there are various ways' by which
they may be obtained practically by themselves and in sufficient
quantity for examination under the microscope. When very
abundant a drop of the water just as dipped, without any attempt
at concentration, may be all that is required, but usually it is not
possible to see more than a chance specimen or two in this way.
One obvious method of concentration is to strain the water
through filter paper and then to take some of the deposit for
examination. Many nannoplankton organisms are, however,
so delicate that they are damaged or destroyed in the process of
transference from the filter paper to the glass slide. Another
method is to add some preservative such as formalin to the water
and, after allowing the dead organisms to settle to the bottom,
to pipette off some of the deposit. This method has the dis-
advantage, however, that many of the organisms may be dis-
torted or even disintegrated by the action of the preservative.
On the whole the best method of obtaining considerable
numbers of nannoplankton organisms in a living and undamaged
condition is by means of the centrifuge. This is, in fact, the
method introduced by Lohmann (1908) and it is very efficient
for the purpose. Its success depends of course upon the fact
that nearly all small aquatic organisms are slightly heavier than
water and are therefore forced to the bottom of the tubes used
in the centrifuge and so can be pipetted off for examination.