PLANKTONIC STUDIES. 579 



this here because in pUinktology the iuterestiug and complex, vital 

 relations of pelagic organisms, their manner of life and economy, are 

 very often called biological instead of oecological problems.* 



PLANKTON AND BENTHOS. 



If nnder the term Halohios we embrace the totality of all organisms 

 living in the sea, then these, in (ecological relation, fall into two great 

 chief groups, benthos and planlcton. I give the term benthos \ (in opposi- 

 tion to pJanldon) to all the non-swimming organisms of the sea, and to 

 all animals and plants wliich remain upon the sea bottom either fixed 

 (sessile) or cai)able of freely changing their place by creeping or run- 

 ning (vagrant). The great oecological differences in the entire mode of 

 life, and consequently in form, which exist between the benthonic and 

 planktonic organisms, justify this intelligible distinction, though here 

 as elsewhere a sharp limit is not to be drawn. The benthos can itself 

 be divided into littoral and abyssal. The littoral-benthos embraces the 

 sessile and vagrant marine animals of the coast, as well as all the 

 plants fixed to the sea-bottom. The abyssal-benthos^ on the other 

 hand, comprises all the fixed or creeping (but not the swimming) ani- 

 mals of the deep sea. Although as a whole the morphological char- 

 acter of the benthos, corresponding to the ijhysiological peculiarities 

 of the mode of life, is very different from that of the planlcton, still 

 these two chief gToui)S of the halobios stand in manifold and intimate 

 correlation to one another. In ])art these relations are only phylo- 

 genetic, but also in part at the present day of an ontogenetic nature, as, 

 for example, the alternation of generations of the benthonic polyps and 

 the planktonic medusiie. The adaptation of marine organisms to the 

 mode of life and the organization conditioned thereby may in both 

 chief groups be primary or secondary. These and other relations, as, 

 well as the general characteristics of the pehigic fauna and flora, have 

 already been thoroughly considered by Fnchs (112) audMoseley(7). 



PLANKTON AND NEKTON. 



The terra inanhton may be used in a wider and in a narrower sense; 

 either we understand it as embracing all organisms swimming in the 

 sea, those floating passively and those actively swimming; or we may 

 exclude these latter. Hensen comprehends under plankton " every- 

 thing which is in the water, whether near the surface or far down, 

 whether dead or living." The distinction is, whether the animals are 

 driven involuntarily with the water or whether they display a certain 

 degree of independence of this imiietus. Fishes in the form of eggs 



*The terms biologj- aiKlVcology are not interchangeable, because the hitter only 

 forms a part of pliysioUtgy. Comp. my " Geuerolle Morphoh)gie," 18(50), Bd. i, p. 8, 

 21; B(l. II, p. 286; also my " Ueber Et\vickclungs;;inig nnd Aiifi^abc dcr Zoologie/' 

 Jena. Zeitsch. fur Med. u. Nat., Bd. v, 1870. 



t/^eVOo?, the bottom of the ocean; hence the organisms living there. 



