THE NANNOPLANKTON OF BOMB-CRATER POOLS. 233
Only small quantities of water are required as a rule for the
purpose of centrifuging, about a quarter of a fluid ounce (7 c.c.)
being usually sufficient. Even smaller amounts, down to 1 or
11/2 c.c, will often yield quite good samples of nannoplankton,
but for centrifuging very small quantities of water specially small
tubes fixed on a special head of the centrifuge are required.
There seems no need on this occasion to go further into the
question of the technique of centrifuging water in order to obtain
nannoplankton organisms. Those who may be inclined to
follow up the subject will find ample details in a paper already
quoted (Lohmann 1911) and in two papers by the present author
(Scourfield 1911 and 1930).
From what has already been said as to the very small size of
nannoplankton organisms it is evident that they can only belong
to the unicellular types of plant and animal life, that is to say
to the Protophyta and Protozoa. But the surprising thing is
that within those great groups they do actually represent so
many different types. Thus, in freshwater, in addition to many
kinds of Bacteria and a few of Fungi, there are representatives
of the Volvocales, Chlorococcales, Conjugates, Xanthophyceae,
Chrysophyceae, Bacillariophyceae (Diatoms), Cryptophyceae,
Dinophyceae, Euglenineae and Mxyophyceae among the Algae1
and of the Rhizopoda, Heliozoa, Zooflagellata and Ciliata among
the Protozoa.
As regards the important role played by nannoplankton
organisms in the general economy of the sea and fresh waters it
may be said to depend upon three principal factors, namely
(1) their constant presence in waters of practically every kind,
(2) their rapid rate of reproduction under favourable conditions
and (3) their general character as "producers," rather than as
"consumers," for the great majority, being essentially plants,
can synthesise, from inorganic materials, such organic substances
as oils, starches, &c. Owing to factors (2) and (3) these minute
organisms form the basis of subsistence for many rather larger
forms and therefore contribute largely to the foundation of the
food-chain which leads from the lowest to the highest forms of
aquatic life.
Having now, it is hoped, made clear what kind of organisms
are included under the term nannoplankton, we may proceed to
the immediate subject of the present paper. First of all a few
words may be devoted to the bomb-craters and pools themselves,
to be followed by the detailed account of the forms of nanno-
plankton which have actually been found in them.
1 For classification of the Algae see Fritsch, 1935.