594 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 
and Phwodaria) move in the three lower zones (vertical distribution, 
4, § 232-239). The dependence of their appearance upon the various 
conditions of life has been investigated by Brandt (24, p. 102). 
D.—CaiLENTERATES OF THE PLANKTON. 
The ancestral group of the celenterates has important significance 
and manifold interest for the natural history of the plankton; still 
thisappliesin very varied degrees to the different principal groups of this 
numerous circle (comp. 30, p. 522). The, great class of the sponges, 
which belongs exclusively to the benthos, has never acquired a pelagic 
habit of life. The phylum of the platodes also needs no further reference 
here. We know, to be sure, a small number of pelagic turbellarians 
and trematodes. Arnold Lang, in his monograph on the sea-planarians 
or polyclads (1884, p. 629), mentions as “purely pelagic” or oceanic 
8 species and 4 genera (Planocera, Stylochus, Leptoplana, Planaria). 
Parasitic trematodes are occasionally found as “pelagic parasites” in 
meduse, siphonophores, and ctenophores; but these trematodes and 
turbellarians are usually found only individually; they never appear 
in such quantities as are characteristic of the majority of the plankton 
animals. Much more important for us is the third type of the ccelen- 
terates, the diversified chief group of the nettle animals or Cnidaria 
(30, p. 524). 
Cnidaria.—With reference to the mode of life and the form condi- 
tioned thereby, one may divide the whole group of Cnidaria into two 
great principal divisions, polyps and acalephs, which since the time of 
Cuvier have lain at the foundation of the older systems. The polyps 
(in the sense of the older zodlogists) embrace all nettle animals, which 
are fixed to the bottom of the sea, hydropolyps as well as secyphopolyps 
(Anthozoa). They belong exclusively to the benthos. Only a few forms 
have acquired the pelagic mode of life (Minyade, Arachanactis, larvee 
of Actinie, Cerinthide, and some other corals). The second principal 
division of the nettle animals, the Acalepha, embraces, in the sense of 
their first investigator Eschscholtz (1829), the three classes of meduse, 
siphonophores, and ctenophores; all swimming marine animals, which, 
from their richness in forms, their general distribution in the ocean, and 
their abundant occurrence, possess much importance for plankton 
study. Since the above-mentioned pelagic polyps (Minyade, etc.) on 
the whole are rare, and never appear in great quantities, we need make 
no further reference to them here. Much more important are the Aca- 
lephs, which ofter a fund of interesting problems for plankton study. 
Commonly, all these animals are roughly termed “pelagic,” but a 
new consideration shows us that they are so in a very different sense, 
and that the distinction which we have made above in reference to their 
chorological terminology here finds its complete justification. We will 
first consider the meduse, then the siphonophores and ctenophores. 
