PLANKTONIC STUDIES. 583 



pteropods, and lieteropods, very many Crustacea, etc. : (/>) Ch imopelagic, 

 which appear at the surface only in wiuter and in summer are hidden 

 in the depths — radiolaria, medusa', siphonophores, cteuophures, a part 

 of the pteropods and heteropods, many Crustacea, etc.; (o) 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 dilferent ecological conditions, as of reproduction, of 

 ontogeny, of food supply, etc. 



Finally one may call spanipelagic those animals which always live 

 in the ocean de^jths (zonary orbathybic), and come to the surface only 

 exceptionally and rarely. This does not apply to a few deep-sea ani- 

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

 time, for a few weeks or perhaps for a single day, e. g., Athoryhia and 

 Physopliora among the siphonophores, Gharyhdea und Feriphylla among 

 the medusie. The final cause of this remarkable spanipelagic mode 

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

 These animals must be much more numerous than present appearances 

 show. 



HOLOPLANKTONIC AND MEKOPLANKTONIC ORGANISMS. 



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

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

 These rather pass a part of their life in the benthos, either vagrant or 

 sessile. The first group we call holoplanktonic, and the second mero- 

 planldonic. To the holoplanktonic organisms, which have no relation 

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

 oscillaria, all murracytes and i^eridinea; further all radiolaria, many 

 globigerina, the hypogenetic meduste (without alternation of genera- 

 tions), all siphonophores and cteuophores, all cluetognatha^, pteropods, 

 the copelata, pyrosoma, and thalidia, etc. Among these M"c find ^'purely 

 l^elagic, zonary, or bathybic" forms. 



The meroplan'kionie organisms, on the other hand, which are found 

 swimming in the sea only for a i^art of their life, passing the other 

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

 represented by the following groups: A part of the diatoms and oscil- 

 laria, the planktonic/»coyV7s, the metagenetic medusa^. {C r a spedota with 

 hydroid nurse, Acraspedn with scyphistoma nurse), some turbellarians 

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

 many 'helminths and echinoderms, acephala and gasteropods, etc. 



