THE ENTOMOSTRACA OF EPPING FOREST. 195
to be that the number of species of Entomostraca, especially those
living in fresh-water, is very limited. Compared with some other
animal groups, the Lepidoptera or Diptera for example, this is
quite true, but still the total number of recognised species is by no
means a small one. Vosseler (106)2 says that considerably more
than 2,000 species have been described. Many of these will no
doubt ultimately be found to be mere varieties, but considering the
vast areas of the earth's surface that have not yet been properly
searched for these animals, the number mentioned is probably
smaller than that of the species actually living at the present
time. The fresh-water forms hitherto recorded may be
estimated at 600, distributed as follows:—Phyllopoda 100,
Cladocera 200, Branchiura 20, Ostracoda 120, and Copepoda 160.
Of these we have in the British Isles, as far as yet known, only
about 190, namely, Phyllopoda 2, Cladocera 75, Branchiura 1,
Ostracoda 58, and Copepoda 54.
Distribution and Habitats.
The distribution of the Entomostraca is, like that of so many
of the lower groups of animals, world-wide, and this is true not only
of the group as a whole and its main divisions, but of the
majority of the genera and not a few species. Confining our
attention to the fresh-water forms, and from this point onwards
reference will only be made to these, we find, for example, that
all our British species, with only one or two doubtful exceptions,
extend to all parts of the European Continent; further, that
most of them have been found in North America and many also
in Asia, while some even occur in South America, South Africa,
Australia and New Zealand. Notwithstanding the fact of wide
distribution, however, and the consequent similarity between
the total faunas of different countries over large areas,
as for instance over the whole of Europe, it is also
true that the faunas of certain districts of limited extent
within each country possess characteristic features. This
is no doubt mainly due to differences in the types of the
lakes, ponds, and other waters of the different parts of a
country. The lakes and tarns of a wild mountainous region,
to take an extreme example, differ in almost every detail from
the lowland ponds and meres, and many of their Entomostracan
inhabitants also differ from those found in the latter. But this
2 The figures in brackets refer to the numbers of the papers in the Bibliography appended
(See Part IV.)