998 PRINCIPLES OF STRATIGRAPHY 



to the plankton of the larger fresh water seas by a number of 

 writers, notably O. Zacharias, who uses the term Jicicoplankton for 

 the plankton of shallow ponds and potamoplankton for that of 

 rivers. 



E. Lirnnonekton (Heleonekton, Potanwnekton). 



The nekton of fresh water, like that of the sea, is typically 

 represented by the fish, though other classes of animals, such as 

 the aquatic mammal, are also represented. The merostomes of the 

 Palaeozoic also appear to belong here. An interesting example of 

 a meronektonic life is evidenced by the tadpole, while an epinektonic 

 condition exists in the larval fresh water mussel (Unio) which 

 attaches itself to the gills of fishes. 



F. Limnobenthos {Helcohenthos, Potamohenthos). 



Both sedentary and vagrant benthos occur in fresh water; the 

 former is chiefly represented by plants, but a number of inverte- 

 brates also belong here. Such are the sedentary infusoria, rotifers, 

 and bryozoa, and the fresh water sponges and hydrozoa, though 

 the principal members of the latter class are only temporarily at- 

 tached and may move about at will. The vagrant benthos, on the 

 other hand, is w^ell represented by molluscs, worms, and Crustacea, 

 though the crawfish also leads at times a benthonic life. 



G. Atmoplankton. H. Atmonekton. I. Atmobenthos. 



Among the air-breathers permanent planktonic and nektonic 

 types are unknown, though many unicellular plants, especially bac- 

 teria, live in the atmosphere for a considerable period of time, and 

 must during that period be classed as atmoholoplankton. The 

 meroplankton, however, is well represented, chiefly by the spores and 

 seeds of plants, which are wafted about by the winds, and so be- 

 come widely dispersed. Terrestrial nekton is represented by in- 

 sects, birds, flying reptiles, and bats. None of these lead a per- 

 manently nektonic life in the air, for all return, more or less fre- 

 quently, to the substratum. Nevertheless, during their period of 

 flight, which often is very long, they must be considered as nekton 

 of the air. The benthos of the earth's surface is pretty sharply 

 divided into sedentary benthos, or plants, and vagrant benthos, or 



