PLANKTONIC STUDIES. 591 



Dionly only of a siiiiiie .species, iSaygassHm hacciferitm; but tliis lias the 

 greatest iiiiportauce, since, as is known, it alone forms the lioating 

 sargasso banks, which, cover such extensive portions of the ocean. Be- 

 sides this very important species, other fucoids are found floating- in 

 the ocean, especially species of Fucus {F. vesicnIosii.<i, F. nodoi^us, and 

 others). Still they never appear in such masses as the familiar "berry 

 weed." The floating sargasso banks are well known to have their char- 

 acteristic animal life, which Wyville Thompson accurately described 

 and fittingly termed nomadic (14, ii, pp. 9, 339). 



This remarkable sargasso fauna bears the same character in both the 

 Atlantic and the Pacific oceans and consists partly of benthonic ani- 

 mals, which live sessile or creeping on the sargasso weed, partly of plank- 

 tonic organisms which swim among the weeds; the latter are more 

 neritic than oceanic. Hensen has lately described this fauna as re- 

 markably poor, and could only find 10 species of animals in it (9, p. 

 216). The Challenger found more than five times as many species in 

 this same Atlantic sargasso, namely, 55 (6, p. 136). It is obvious that 

 the remarkable negative results of Hensen on this as on other plank- 

 tonic questions can have no value against the positive results of other 

 investigators. 



C— Protozoa of the Plankton. 



The two great chief groui)s of unicellular animals, Rhizopoda and 

 Infusoria, occur m the ocean in very difterent proportions, in the 

 reverse condition to that in fresh water. 



The Infusoria {Flagellata and Giliata), which chiefly form the pro- 

 tozoic fauna in the latter, are indeed represented in tlie sea by a great 

 number of species, but the most belong to the littoral benthos, and 

 only a few swimming species occur in such quantities that they are of 

 importance in the plankton, the N'oetilucidw among the Flagellata, the 

 Tintinnoida' among the Ciliata. Much greater is the wealth of the 

 ocean in Ehizopoda, calcareous-shelled Thalamophora and siliceous- 

 shelled JRaf?^o/<lHa. The accumulated masses of these shells form the 

 most important sediment of the ocean, while their unicellular soft l)odies 

 constitute the chief food supply for many planktonic animals. 



Infusoria. — As is known, the Infusoria do not play so great a role 

 in the life of the ocean as in that of the fresh water. It is true that a 

 great number of Flagellata and Ciliata occur in the neritic or littoral 

 fauna, but neither on account of the number of individuals nor the 

 richness of forms are they elsewhere of importance, and only a few 

 small grouj)s extend out into the open sea. It seems as if these tender 

 and for the most part uncovered Protozoa are not suited for the contest 

 which the wild " struggle for existence " offers here. The armored 

 rliizopods take their i)lace. Still two small and very peculiar groups of 

 Infusoria are found in very great numbers in the plankton, and some- 

 times in such quantities as to form the chief bulk; the Noctiluca among 



