THE ENTOMOSTRACA OF EPPING FOREST. 197
are occasionally to be seen at the bottom, and still more often
among plants, they are not bottom forms in a proper sense of
the term, nor have they any means of clinging or attaching
themselves to the vegetation in the water.
The next section, that of the intermittent swimmers, is a
very large one, comprising the majority of all the orders except
the Ostracoda. In this group also it is possible to roughly
distinguish two sub-groups, the one including species which live
habitually at the bottom, and the other, species which prefer to
live in and about the miniature forests of water plants. These
divisions, although not very rigidly defined from one another,
many species living apparently indifferently at the bottom or
among weeds, may nevertheless be conveniently referred to
separately. The bottom forms have evidently adopted the
easiest possible method of avoiding the irksomeness of a continual
use of their swimming organs, since it does not require any
modification of structure to cease swimming and drop to the
bottom. As examples of these animals may be mentioned
Latona, most of the Lyncodaphnidae (Macrothrix, &c.), Campto-
cerens, Leydigia, AIona quadrangularis and affinis, Alonella rostrata,
Pleuroxus uncinatus, Harporhynchus and Monospilus among the
Cladocera, some few species of Harpacticidae among the
Copepoda, and Cypria ophthalmica and Pionocypris vidua among
the Ostracoda.
The members of the other sub-group, namely, those which
live among the water weeds, all possess some means of clinging
or attaching themselves to the same. For this purpose repre-
sentatives of the Copepoda and Ostracoda have not required any
special modification, as the structure of the second antennae and
mouth organs in the former Order, and of the second antennae
and feet in the latter, is such that it enables any of these animals
to cling to almost any solid object in the water. The principal
forms belonging here, are, among the Ostracoda, species of
Cyclocypris, Cypris, and Notodromas, and among the Copepoda,
Diaptomus castor, most species of Cyclops, and nearly all the
Harpacticidae. The case is different, however, with the
Cladocera ; and here we can see that a large amount of modifica-
tion of quite different organs has taken place for enabling these
creatures to support themselves without the effort of continuous
swimming. Thus Sida crystallina attaches itself to aquatic plants
by means of a complex gland at the back of the head and neck,