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 its economy except as competitors. 



The zooplankton includes representatives of a considerable 

 range of groups, though both in species and individuals the Proto- 

 zoa, Rotifer a, and Entomostraca predominate among the animals. 

 Representatives of other groups are in the main adventitious. 



Among the Protozoa, the Rhizopoda 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 Heliozoa are 

 few both in species and individuals. The Mastigophora (which in 

 our discussions include all green and brown flagellates often clas- 

 sified with the Chlorophyce and Ph&ophycece] vie with the Chloro- 

 phycecs and Bacillariacecz for the first place as converters of the 

 inorganic (and perhaps also the dissolved organic) matter into food 

 for the zooplankton. 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 Ciliata 

 furnish a few constant members of the plankton, and numerous 

 adventitious and parasitic species. During the low water of 

 autumn, when bacterial contamination is 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 Chlorophyceoz and Mastigophora, which enter largely into their 

 food. This abundance of the Rotifer a 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. 



