PLANKTONIC STUDIES, 583 



pteropods, and licterupods, very many Crustacea, etc. ; (/>) Chimopekujic, 

 whicli appear at tlie surface only in winter and in summer are bidden 

 in the depths — radioiaria, medusa', siphouophores, ctenophores, a part 

 of the pteropods and heteropods, many Crustacea, etc.; (c) Allopehujic, 

 which perform irregular vertical wanderings, sometimes appearing at 

 the surface, sometimes in the depths, independently of the changes of 

 temperature, which condition the change of abode of the nyctipelagic 

 and chimopelagic animals; the final cause of these wanderings ought 

 to be found in different oecological conditions, as of reproduction, of 

 outogeny, of food supply, etc. 



Finally one may call spanipelagic those animals which always live 

 in the ocean dejiths (zonary orbathybic), and come to the surface only 

 exceptionally and rarely. This does not apply to a few dee])-sea ani- 

 mals which once every year ascend to the surface, but only for a short 

 time, for a few weeks or perhaps I'or a single day, c. g., Athoryhia and 

 PhysopJiora among the siphonophores, Charybdea und Per ijjhylla among 

 the medusie. The final cause of this remarkable spanipelagic mode 

 of life must lie chiefly in the conditions of reproduction and ontogeuy. 

 These animals must be much more numerous than present appearances 

 show. 



HOLOPLANKTONIC AND MEEOPLANKTONIC OEGANISMS. 



Numerous organisms pass their whole life and whole cycle of devel- 

 opment hovering in the ocean, while with others this is not the case. 

 Tliese rather pass a ]>art of their life in the benthos, either vagrant or 

 sessile. The first groiip we call holoplanldonic^ and the second mero- 

 pJanMonic. To the holoplanktonic organisms, which have no relation 

 whatever to the benthos, belong* the greater part of the diatoms and 

 oscillaria, all murracytes and peridinea; further all radiolaria, many 

 globigerina, the hypogenetic medusie (witliout alternation of genera- 

 tions), all siphonophores and ctenophores, all chsetognatha^, pteropods, 

 the copelata, pyrosoma, and thalidia„etc. Among these we find '^purely 

 pelagic, zonary, or bathybic*' forms. 



The meroplanlctonic organisms, on the othei- hand, which are found 

 swimming in the sea only for a part of their life, passing the other 

 part vagrant or sessile in the benthos (either littoral or abyssal), are 

 represented by the following groups: A x:)art of the diatoms and oscil- 

 laria, the planktonic/?/con7,9, the metagenetic medusre {Cras2)edotawit]i 

 hydroid nurse, Acraspeda with scyphistoma nurse), some turbellariaus 

 and annelids, etc; further, the "pelagic larva;" of hydroids and corals, 

 many helminths and echinoderms, acephala and gasteropods, etc. 



