8 
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.” hese distinctions are based) upon the nature ot tne 
environing body of water, and the terms are convenient, though 
the separation of these types everywhere 1n 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 alge, 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 
