HALOBENTHOS LOIXOPLANKTOX 997 



deposited anywhere else, as in the case with })lanktonic and nektonic 

 organisms. The sedentary benthos is, to a large degree, dependent 

 for food on those organisms which are swept within its reach 

 by the currents, while the vagrant benthos is more actively en- 

 gaged in seeking out its food. Large numbers of sedentary ben- 

 thonic animals have assumed a radial structure, especially well 

 typified in corals and crinoids, and also shown in the corona of 

 the barnacle ; while others, such as brachiopods, have a bilateral 

 symmetry of high degree. Some of the lower vagrant benthonic 

 animals, e. g., the Echinoidea, are also built on the radial plan, but 

 the majority of the free benthonic animals are bilaterally symmetri- 

 cal. Among the vagrant benthos the struggle for existence is most 

 intense, and, as a result, the variety of adaptations and the wealth 

 of form and color are almost unlimited. Transitions from the 

 vagrant benthos to the nekton are numerous, and it sometimes 

 becomes difficult to decide if an animal belongs to the vagrant 

 benthonic or to the nektonic type. The gradation is just as com- 

 plete as between nekton and plankton. In general, a radial form 

 may be said to be characteristic of the sedentary benthos, while a 

 bilaterally symmetrical form is as characteristic of the vagrant 

 benthos. Examples of change of form with change of habit occur 

 in many classes. Both plants and animals, themselves belonging to 

 the sedentary benthos, may lead, secondarily, a vagrant life by 

 being attached to vagrant benthonic forms. These may be regarded 

 as vagrant epibenthos. Among plants, algae are the most familiar 

 examples, while among animals hydrozoa (Hydractinia on shells 

 inhabited by hermit crabs), actinians, and bryozoans furnish the 

 most characteristic examples. 



D. Limnoplankton. 



Fresh water plankton is met with in all fresh water lakes, 

 ponds, and streams. It not infrequently occurs in surprising 

 amounts. Thus studies of the Illinois River and its tributaries 

 have shown that it averages 2.7 parts per million of the water in 

 the stream, and that the total average moving down stream past 

 any given point aggregates 75,000 tons per annum, or about 8.5 tons 

 per hour (6). The holoplankton here is largely composed of 

 minute plants, while the meroplankton, consisting both of larval 

 plants and animals, constitutes a very large percentage of the 

 mass. Finally, the pseudoplankton makes up a not inconsiderable 

 portion of this mass. The term limnoplankton has been restricted 



