No. 413.] SPIRAL SWIMMING OF ORGANISMS. 375 
swimming movement seem to have this tendency, to strike so 
as to turn the animal toward the dorsal side, strongly ingrained. 
Many of these animals cannot rise from the bottom so long as 
the dorsal side is down. In such a case the dorsal side of the 
head repeatedly strikes the bottom until, by revolving on 
the long axis, the dorsal side is turned toward the free water ; 
the animal then swerves off the bottom in that direction. Some 
Fic. 8. — Diagram showing course followed by Ploesoma when swimming without revolving 
on the long axis. The animal continually swerves toward the dorsal side, hence follows 
a circular path. 
of the rotifers, if they attempt to swim freely through the water 
without revolving on the long axis, turn backward somersaults, 
over and over, describing thus small circles. I have seen 
Ploesoma thus describe circles (Fig. 8) for considerable periods. 
But as soon as the animal begins at the same time to revolve 
on the long axis, without otherwise changing its movement, 
the effect is striking. The purposeless circular movement 
