METHODS AND APPARATUS 



CHAPTER I 



APPARATUS, METHODS AND PURPOSES OF THE 

 INVESTIGATION 



Historical 



The plants and animals that inhabit the open waters of ponds, lakes, 

 streams, and the oceans constitute what is known collectively as 

 the plankton^ A multitude of organisms is included under this 

 term, ranging, in fresh water, from forms as simple as the bacteria to 

 forms as complex in structure as Crustacea and insect larvae. The 

 fresh-water planktonts vary in size from minute bacteria to organisms 

 that are 15 millimeters or more in length, but the vast majority of 

 them are microscopic in size. The plants float freely in the water and 

 are subject to the action of waves and currents. Most of the animals, 

 on the other hand, are more or less active swimmers, but they are not 

 powerful enough to be independent of the waves and currents so thaf 

 their distribution is also governed chiefly by the general movements 

 of the water. 



The insect larvae and some of the plankton Crustacea have been 

 known to scientists for two and a half centuries, but the smaller plank- 

 ton organisms had to await the development of the compound micro- 

 scope. Exception must be made of some of the smaller forms, more 

 especially some of the algae, since they became familiar to the layman 

 as well as to the scientist without the aid of the microscope because they 

 frequently appear in great abundance. In the quiet waters of lakes 

 these algae may become abundant enough at certain times of the year 

 to form a thick scum on the surface of the water ; this is the so-called 

 '* water bloom." 



In spite of the fact that biologists became familiar with many of 

 these forms at a comparatively early date, no attempt was made to 

 study the plankton from a quantitative standpoint until 1882 when 

 Hensen ^ undertook such investigations on the Baltic and North seas, 

 and on the Atlantic ocean. His studies extended from 1882 to 1886 

 and the publication of his results in the latter part of 1887 stimulated a 

 great interest in both marine and fresh-water investigations of this 

 character. As a result a voluminous literature upon this subject has 

 appeared in the last three decades. 



^Fiinf. Ber. Kom. zur wissen. Untersuch. d. deut. Meere, 1887, pp. 1-107. 



