194
THE ENTOMOSTRACA OF EPPING FOREST.
divisions varies somewhat according to different authorities, but
the following arrangement is perhaps the most usual :—
Although open to criticism, it is occasionally convenient to
speak of the three sub-divisions of the Branchiopoda as if they
were equal in rank to the three remaining orders, and this
will be done in the present paper. Of the six divisions then
enumerated above, the Phyllopoda, Cladocera, and Branchiura
are almost exclusively inhabitants of fresh-water, the only
exceptions being that some Phyllopods live in brackish water
and salt lakes, and that a very few Cladocera and Branchiura
are marine. The Ostracoda and the Copepoda, on the other
hand, are mainly marine animals, although having numerous
fresh-water representatives. The Cirripedia are exclusively
marine.
With the exception of the Phyllopoda, all the fresh-water
Entomostraca may be classed as microscopic animals, i.e., animals
which require magnification to show even the main characters
of their organisation. But the members of the different groups
vary very much in respect to size. Thus the Phyllopoda range in
length from 11/5" to 1/3", the Cladocera from nearly 1/2" (Leptodera
hyalina) to 1/110" (Alonella nana, said to be the smallest known
Arthropod), the Ostracoda from 1/6" (Cypris barbata) to 1/55" (Cyclo-
cypris lavis) and the free-living Copepoda from 1/5" (Diaptomus
superbus) to 1/70" (some of the smaller Harpacticids). The fresh-
water parasitic Copepods may, however, be much larger than J",
and there exist both larger and smaller marine forms of Ostracoda
and Copepoda than those mentioned above.
Number of Known Species.
A point which does not seem to be very generally realised
in relation to the Entomostraca is the great variety of different
forms included in the group. The prevalent idea appears