periodically, wholly or in part, leave the open water for a littoral 

 or benthal existence. They are periodic planktonts. Some organ- 

 isms, such as many of the rhizopods and diatoms and Hydra, appear 

 in the plankton under certain conditions of temperature and food. 

 They temporarily adopt the limnetic mode of life as a result either 

 of a change in their specific gravity due to internal changes, such as 

 an increase of the gaseous or fatty contents of their protoplasm, or 

 to changes in the buoyancy of the water due to changes in 

 temperature or in substances in solution in the water, or because 

 of the abundance of food in the open- water. They become 

 under these conditions actively adventitious planktonts. Still other 

 organisms are released from their usual contact with or attach- 

 ment to the substratum, or from their association with debris 

 or vegetation of shore or bottom, by movements or disturbances in 

 the water, and are swept into the open water only to return again 

 to their customary habitat when conditions favor. Practically all 

 of the smaller organisms inhabiting the shore and bottom and the 

 debris and vegetation found thereon are liable thus to enter the 

 open water, and to be found in forced and temporary association 

 with the eulimnetic fauna and flora. They are passively adventi- 

 tious planktonts. 



Another class of organisms which occur in the plankton are those 

 which either as internal or external parasites find in plankton organ- 

 isms either a host or a substratum for attachment. These are in a 

 certain sense passive planktonts, and they may be distinguished from 

 other passive planktonts as attached or parasitic planktonts. Sharp 

 lines between these various classes of organisms found in open water 

 can not be drawn upon distinctions based upon their degree of 

 dependence upon the bottom and shore. An equally vague line 

 separates the organisms of the plankton from those more active 

 forms which by virtue of their powers of locomotion are to a con- 

 siderable degree independent of waves and current, and are able 

 freely to maintain their position in their preferred habitat. Among 

 the organisms commonly included in the plankton, the flagellates, 

 rotifers, and Entomostraca exhibit some degree of activity, such as 

 is seen in their limited vertical migrations, while larger organisms, 

 such as Leptodora hyalina and the larvae of Corethra, are capable of 

 movement sufficient to give them considerable independence in the 

 matter of their position in the water. We thus find degrees of inde- 



