PLANKTONIC STUDIES. 583 
pteropods, and heteropods, very many crustacea, ete.; (b) Chimopelagic, 
which appear at the surface only in winter and in summer are hidden 
in the depths—radiolaria, medus, siphonophores, etenophores, a part 
of the pteropods and heteropods, many crustacea, ete. ; (c) Allopelagie, 
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 ecological conditions, as of reproduction, of 
ontogeny, of food supply, ete. 
Finally one may call spanipelagic those animals which always live 
in the ocean depths (zonary or bathybic), 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., Athorybia and 
Physophora among the siphonophores, Charybdea and Periphylla among 
the medusxe. 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 MEROPLANKTONIC ORGANISMS. 
Numerous 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- 
planktonic. To the holoplanktonie 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 meduse (without alternation of genera- 
tions), all siphonophores and ctenophores, all cheetognathie, pteropods, 
the copelata, pyrosoma, and thalidia, ete. Among these we find “purely 
pelagic, zonary, or bathybic” forms. 
The meroplanktonic organisms, on the other 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 part of the diatems and oscil- 
laria, the planktonic fucoids, the metagenetic meduse (Craspedota with 
hydroid nurse, Acraspeda with seyphistoma nurse), some turbellarians 
and annelids, etc; further, the “pelagic larve” of hydroids and corals, 
many helminths and echinoderms, acephala and gasteropods, ete. 
