9 
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 areina 
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 larve 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- 
