172 Wisconsi7i Academy of Sciences, Arts, and Letters. 



growing upon the bottom, Avhile the larger part of the lake is so 

 deep that only the floating vegetation can exist. It is easily 

 seen, then, that a shallow lake wdll be "'^plankton-rich'' as com- 

 pared with a deep lake. Fishermen recognize this fact, and ex- 

 pect the shallow lakes to be better for their sport. 



It is evident, then, that the living limnetic vegetation must 

 be at or near the surface, where it can have an abundance of 

 light. Animal life, h&w^ever, is not limited in this way. It was 

 long ago shown that in the sea there was an abundant surface 

 fauna and an abyssal fauna, but in regard to the condition of 

 the intermediate region there has been some dispute. Agassiz 

 has claimed that there is a region intermediate betw^een the top 

 and bottom, which is entirely devoid of life. This has been dis- 

 puted by some authors, and late explorations seem to indicate 

 that no region between the surface and bottom is entirely free 

 from animals. A similar condition exists in the lakes. By far 

 the most abundant fauna is at and near the surface, but animals 

 are found in greater or less numbers at all depths. The larger 

 part of the plankton is found within thirty or forty feet of the 

 surface ; but the same kinds of animals that form the fauna of 

 the upper waters may be found at all depths, altliough in small 

 numbers. Limnocalanus is an example of an animal which be- 

 longs to the intermediate regions. It, too, may be*found in small 

 numbers at anv depth from the surface dov\Ti, but it seldom 

 occurs in any considerable numbers outside the intermediate 

 region. 



Limnocalanus and Daplmia pulicaria are perhaps the only 

 animals in fresh v/ater which belong distinctly to the zonary 

 plankton, although Cyclops hrevispinosus is much more abun- 

 dant between five and twenty meters than it is near the surface. 



Collectors of plankton m.aterial have known that they could 

 ordinarily make much more abundant collections at night than 

 in the daytime. This has led to a belief that there is a verti- 

 cal migration of the plankton, tow^ards the surface at night, and 

 away from it in the daytime. It was supposed that the whole 

 body of the plankton moved up and down. This idea has been 

 proved to be false. What movement there is is within quite nar- 

 row limits near the surface, and all members of the limnetic 



