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BULLETIN OF THE BUREAU OE FISHERIES. 
between neighboring stations. At some localities Copepoda were most abundant 
at the bottom, as at station 38, where the quantity just above the bottom was 
about 30 times as great as at the surface (3,320 at the surface, 94,500 at the bottom, 
a depth of 6 m.). This gathering of Copepoda at the bottom was observed only 
at this station; at the nearest stations no indication was found of any increase of 
these forms in the lower strata. 
It is quite possible that in this case we are dealing with a so-called “swarm” 
of plankton organisms. Such swarms of Cladocera and Copepoda have been 
Fig. 15. — Distribution of Crustacea in Lake Keokuk. (The mean content of plankton and the mean numbers of Copepoda and 
Cladocera in cross sections of the lower part of Lake Keokuk, July 15-30, 1921. Stations 23-25 are located opposite Nauvoo, 
111. Heavy line, , represents the mean volume of plankton per cubic meter of water; plain line, , the number of 
Copepoda per cubic meter of water; dotted line, , the number of Cladocera per cubic meter of water. The figures on the 
lines are the averages computed from the data of three stations on the given cross section of the lake. The serial numbers 
of stations are given under the abscissae. Scale: One division of the abscissa — 1 mile; one division of the ordinata— 1 cm.* 
of plankton for heavy line; 100 Copepoda and Cladocera for plain and dotted lines.) 
described often in limnological literature, but nothing definite is known of the real 
cause of such gatherings. E. G. Moberg (1918), investigating the horizontal dis- 
tribution of plankton in Devils Lake, N. Dak., describes swarms of plankton ani- 
mals, which “are at times visible, even at considerable distances, to the naked 
eye.” He found also that the zooplankton in Devils Lake shows a great irregu- 
larity in horizontal distribution and suggested that this irregularity “is due to the 
habit of swarming among plankton animals, due perhaps to a social instinct, similar 
to that found in many other groups of the animal kingdom.” 
