594 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



and Phccodaria) move in tlie three loAver zones (vertical distribution, 

 4, § 232-239). Tlie dependence of their appearance upon the various 

 conditions of life has been investigated by Brandt (24, p. 102). 



D. — ('(ELENTEKATES OF THE Pl.AXKTOX. 



The ancestral group of the coelenterates has important significance 

 and manifold interest for the natural history of the plankton j still 

 this applies in very varied degrees to the different principal groups of this 

 numerous circle (comp. 30, p. 522). The great class of the spojiges, 

 which belongs exclusively to the benthos, has never acquired a ])elagic 

 habit of life. The phylum of the 2)latodes 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-phiuarians 

 or polyclads (1884, j). G29), mentions as "purely pelagic" or oceanic 

 8 species and 4 genera [Flanocera, Stylochns, Leptoplana, Planaria). 

 Parasitic trematodes are occasionally found as "pelagic parasites" in 

 medusiie, 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 coelen- 

 terates, the diversitied 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 zoiilogists) eral)race all nettle animals, Avhich 

 are fixed to the bottom of the sea, hydropolyps as well as scyphopolyps 

 (Anthozoa). They belong exclusively to the benthos. Only a few forms 

 have acquired the pelagic mode of life {Minyadoe^ Arachanactis, iarvte 

 of Actinioe, Cerinthidce, and some otlier corals). The second principal 

 division of the nettle animals, the Acalcpha, embraces, in the sense of 

 their first investigator Esclischoltz (1829), the three classes of meduste, 

 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 {Minyadce, 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- 

 lejyJis, which offer 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 Avill 

 first consider the medusae, then the siphonophores and ctenophores. 



