24 SELECTION IN CLADOCERA ON THE BASIS OF 



NATURE OF SWIMMING MOVEMENTS OF CLADOCERA 

 SPECIES USED IN SELECTION. 



Daphnia pulex is rather rotund in form and is only slightly heavier 

 than water. It swims by vigorous strokes with perceptible intervals 

 between, thus producing a very jerky movement. It is pelagic in 

 its habits, and ordinarily its swimming strokes are repeated just 

 frequently enough to maintain its level in the water. When loco- 

 motion occurs the animal orients itself differently, i. e., instead of its 

 antero-dorsal axis being approximately vertical, it changes to ap- 

 proach a horizontal position with dorsal side uppermost ; its swimming 

 movements are more rapidly repeated and the animal's course may be 

 fairly straight. The jerky character of movement is still quite in 

 evidence. This species frequently attaches to the surface film of 

 the water, but almost never to the sides of the container. A vigorous 

 individual rarely or never rests upon the bottom. 



D. longispina is very similar to D. pulex in its habits and move- 

 ments, but is more slender and somewhat less active; it is not quite 

 so heavy; its axis does not so nearly assume a vertical position in its 

 " place-maintain ing" movements; and in its locomotion its move- 

 ments are somewhat less jerky, due to its relatively smaller antennae 

 and less vigorous strokes. Like D. pulex, it is distinctly pelagic and 

 does not rest upon the bottom. It does not ordinarily attach to the 

 surface film and never to the sides of a container. 



Simocephalus exspinosus is a rotund, rather bulky species, much 

 heavier than the two species of Daphnia. It settles rather rapidly 

 through the water, unless kept up by continued swimming movements. 

 It lies upon and moves about over the bottom much of the time, fre- 

 quently holds fast to the side of the container, and often attaches to 

 and moves about at the surface film. It usually swims with ventral 

 side uppermost. Its swimming movements are less jerky than in 

 either species of Daphnia, due to its relatively smaller swimming 

 antennae and more rapid antennal movements, though the jerky 

 character is somewhat evident. In nature S. exspinosus spends most 

 of its time upon the bottom and upon submerged aquatic vegetation. 



BEHAVIOR OF YOUNG OF DIFFERENT SPECIES WHEN 

 RELEASED IN THE EXPERIMENTAL TANK. 



In the experimental tank the vast majority of individuals of D. 

 pulex upon their release swam a few strokes without orientation, 

 then oriented with reference to the light and swam toward the source 

 of light. This orientation was not perfect in many cases, so that the 

 animal sometimes swam somewhat diagonally. But even when the 

 direction of movement was somewhat diagonal, the course was 

 usually fairly straight. This diagonal course occasionally led to the 

 side of the tank. In such cases the animal usually followed along 



