No. 413.] SPIRAL SWIMMING OF ORGANISMS. 70 
Asymmetry appears sporadically in many different groups of 
the Rotifera ; possibly it may in every case be brought into 
relation with the spiral method of swimming. 
The Rotifera are a group of organisms excessively varied in 
form and movements, furnishing a most excellent opportunity 
for studies on the interdependence of 
structure and function. Some species 
(¢.g., certain species of Diaschiza) 
have the body so curved ventrally 
that the tendency to turn toward the 
dorsal side is more than compensated, 
and the animal tends instead to curve 
continualy toward the ventral side. 
a swimming 
: k E 5 FiG. 10. — V 
This tendency is of course likewise ^ zuchianis triquetra from behind 
after Eh t gt h the t 
hree 
corrected by the revolution on the 
axis of progression, the path taking 
here the form of a spiral with the ventral surface to the outside. 
Some few rotifers have become so modified that revolution on 
the long axis has become unnecessary for keeping a straight 
course. Thus Euchlanis triquetra (a view of which from the 
rear is shown in Fig. 10) has developed 
three great keels, one dorsal and two 
lateral, which tend to prevent swerving 
in any direction; so this animal fre- 
quently swims freely for long stretches 
without revolving, while the closely 
related Euchlanis oropha (having no 
keels) almost continually revolves as it 
passes through the water. 
| Revolution on the long axis, with the 
Fic. r.—One of the Gastrotri- resulting spiral path, is common also 
after Zelinka), visae be in many other animals. In the Gas- 
os cha (s on the trotricha (eg., Chaetonotus, Fig. 11) 
the locomotor organs (cilia are con- 
fined to a strip on the ventral side, (a) which necessarily 
results in giving the organism a tendency to turn toward the 
dorsal side. The revolution on the long axis is therefore of 
&reat importance for producing an effective forward movement. 
keels. 
