59 i KKi'OKT OF COMMISSIONER OF FISH AND FISIIERIKS. 



and riuvodaria) move in the three loMer zones (vertical distribution, 

 4, § 232-231)). Tlie dependence of their appearance upon the various 

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



D. — C'fELKNTEUATKS OK THK PlaXKTON. 



The ancestral grouj) of the calcntcrnfcs has important significance 

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

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

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

 which belongs exclusively to the benthos, has never ac(iuired a pelagic 

 habit of life. The jthylum of the platodts also needs no further reference 

 here. We know, to be sure, a small number of pelagic turbellarians 

 and trcmatodes. Arnold Lang, in liis monograph on the sea-planarians 

 or polyclads (1884, p. G29), mentions as "i)urely pelagic" or oceanic 

 8 species and 4 genera {Flanocera, Stylochus, Leptoplana, Flanaria). 

 Parasitic trematodes are occasionally' found as "pelagic parasites" in 

 medusa^, siplionophores, and ctenophores; but these trematodes and 

 turbellarians are usually found only individually; tliey never appear 

 in sucli 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 diversified chief group of the nettle animals or Onidaria 

 (30, p. 524). 



Gnidaria. — With reference to the mode of life and the form condi- 

 tioned thereby, one may divide the whole group of Gnidaria 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 zotilogists) embrace all nettle animals, which 

 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 mo<le of life {Minyadce, Arachanactis, larvte 

 of Actiniw, Gerinthidoej and some other corals). The second principal 

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

 their first investigator Eschscholtz (1820), 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. Sin(te the above-mentioned pelagic polyi)s {Minyada;, 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^ca- 

 lejyhs, 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 will 

 first consider the medusie, then the siphonopiiores and ctenophores. 



