have a drifting habit and includes all forms of both macro- and 

 microscopic life which float free in the water or, if motile, are un- 

 able to swim against currents. Open- water plankters are called 

 euplankton (true plankton). Many algal species existing as such 

 have elongations of the cell, or bear long spines, whereas others may 

 gain buoyancy through the possession of mucilage. There is evidence 

 that pseudovacuoles in the cells of many blue-green plankters aid in 

 this connection. Forms which are unattached but are caught among 

 filamentous algae and other vegetation and reproduce in shallow 

 water are called tychoplankton. The minute phytoplankters which 

 pass through the meshes of a fine (No. 20) bolting-cloth collecting 

 net are here termed nannoplankton. A special term, periphijton, may 

 be applied to the organisms which form associations on the stems 

 and leaves of aquatic plants. Benthic algae, benthos, are the organ- 

 isms which live on the bottom especially in deep water, for ex- 

 ample, Chara, Nitella, Dichotomosiphon, and some species of 

 Cladophora. 



[3] 



