14 
are often taken with the plankton by virtue of their semi-limnetic 
habit, but do not in the living state enter the food cycle of the 
plankton nor affect 1ts economy except as competitors. 
The zooplankton includes representatives of a considerable 
range of groups, though both in species and individuals the Proto- 
zoa, Rotifera, and Entomostraca predominate among the animals. 
Representatives of other groups are in the main adventitious. 
Among the Protozoa, the Riizopoda are constantly represented 
by many individuals and a considerable number of species, many 
of which may be adventitious, but most of which are wont to adopt 
the limnetic habit during the warmer months. The Helozoa are 
few both in species and individuals. The Mastigophora (which in 
our discussions include all green and brown flagellates often clas- 
sified with the Chlorophycee and Pheophycee) vie with the Chloro- 
phycee and Bacillariacee for the first place as converters of the 
inorganic (and perhaps also the dissolved organic) matter into food 
for the zodplankton. They are exceedingly numerous in our plank- 
ton both in species and individuals, and form quantitatively a con- 
siderable part of the plankton during the summer months. The 
usual method of plankton collection—by silk bolting-cloth—per- 
mits a large proportion of these organisms to escape. The Culzata 
furnish a few constant members of the plankton, and numerous 
adventitious and parasitic species. During the low water of 
autumn, when bacterial contamination 1s at its height, these organ- 
isms form a large part of the plankton. The small size of some of 
the ciliates, combined with their motility and flexibility, renders 
the loss by their escape through the silk net considerable. The 
Suctoria furnish but few species and individuals—mainly adventi- 
tious or attached to other planktonts. 
The Rotifera constitute, both in species and individuals, the 
most important single group of analytic organisms, that is those 
of distinctly animal metabolism, occurring in our plankton. This 
may in part be due to our shallow warm waters and to the abundance 
of Chlorophycee and Mastigophora, which enter largely into their 
food. This abundance of the Rottfera may prove to be character- 
istic of the plankton of rivers ( potamoplankton) as contrasted 
with that of lakes (limnoplankton). While many rotifers are 
eulimnetic, the plankton also contains numerous adventitious 
species. 
