No.413.] SPIRAL SWIMMING OF ORGANISMS 371 
effective; the animal thus moves forward as if on a straight 
line, — the actual path being a spiral with a straight axis. The 
principle is the same as that by which a rifle bullet is given a 
straight course by making it revolve in the axis of flight. 
In the Hypotricha a similar course is followed, save that the 
swerving is to the right; in Chilomonas (Fig. 4) it is toward 
the lower lip (a). Such a spiral path is known 
to be followed by most of the free-swimming 
Protista, — by swarm-spores, flagellates, and 
ciliates in general; by Volvox, Eudorina, 
Pandorina, Platydorina, etc. In some of these 
organisms the course followed becomes almost 
an actual straight line, owing to the fact that 
the body is symmetrical, so that there is no 
pronounced swerving toward one 
side. Such is the case, for ex- 
ample, in Volvox. Here 
the revolution on the 
T long axis probably 
serves merely to com- 
pensate for any acci- 
dental deviations that 
may occur through in- 
| jury, unequal develop- 
Fic. 2. Fic, 3. Fic. 4. ment, and the like. 
F ee rostrum, after Biitschli, showing the Such cases are compar- 
Pas Pome ae eta, 4 atively rate; however, 
oral side, ` most of these organ- 
Fic. dM Chil, x os 1 . * 
‘nek — ATTN He INOOA e. ioma- being. markedly 
ee ee ee 
z 
- 
a 
r3 
a Pr 
i gage z 
Lp pre Sa neun > 
AI re p T 
unsymmetrical. 
The mechanical cause of the revolution on the axis.of pro- 
Sression has often been discussed. In the Ciliata there seem 
to be three possible factors: (1) an oblique stroke of the cilia ; 
(2) the oblique position of the peristome; (3) the unsymmet- 
rical form of the body, which is often of such a shape as to 
favor rotation in a given direction. That the first factor is the 
Primary one is indicated by the fact that the direction of revo- 
lution may be reversed in many of these organisms, even when 
