plankton has been designated in turn " limnoplankton " by Haeckel 

 ('90), a word which in a restricted sense is retained for the plankton 

 of lakes, while that of rivers has been distinguished by Zacharias 

 ('98a) as "potamoplankton," and that of ponds ('98) as "heleo- 

 plankton." These distinctions are based upon the nature of the 

 environing body of water, and the terms are convenient, though 

 the separation of these types everywhere in nature is difficult, if not 

 impossible. Owing to the smaller size of fresh-water basins as 

 compared with those of marine character, the shore and bottom be- 

 come more important as factors in the environment of the plankton. 

 Within the fresh-water environment we also find degrees of impor- 

 tance of the shore and bottom which in ascending scale dominate 

 in the lake, river, pond, and marsh. Although each of these repre- 

 sents distinct conceptions, in nature we find them imperceptibly 

 intergrading, and neither these conceptions, geographical nomen- 

 clature, nor local parlance give us any final criterion which will 

 enable us to use the terms with the precision which a scientific 

 terminology would demand. The distinctions between these forms 

 of fresh-water plankton must lie in the plankton itself, if anywhere. 

 As I shall attempt to show later, these distinctions, though appar- 

 ent, in some cases at least, are nevertheless of minor importance, 

 and depend very largely upon the relative predominance of the 

 adventitious littoral fauna and flora rather than upon distinctive 

 assemblages of eulimnetic species. The striking similarity of this 

 eulimnetic plankton in all these types of environment and in widely 

 separated continents is a biological phenomenon of far more sig- 

 nificance than these minor differences. These distinctions between 

 the different types of fresh-water plankton are thus more a matter 

 of terminology than of biological import. 



Among the organisms found in open water there are varying 

 degrees of dependence upon the shore and bottom. Some, as 

 Cyclops and many of the lower algas, have life cycles in which no 

 encysted or quiescent resting stage has been found, and actively or 

 passively their whole existence is passed in the open water. They 

 are at all times components of the plankton; that is, are continuous 

 planktonts. Others, as Dinobryon, many of the Rotifera and Cladoc- 

 era, and, in fact, the greater part of the eulimnetic organisms, have 

 an encysted stage which as a winter egg or a cyst descends to the 

 bottom and remains there for a season. Such organisms only 



