78 



PRINCIPLES OF ANIMAL BIOLOGY 



of symmetry. An axis is a line around which something rotates, or 

 around which things are placed. The planes of symmetry may be 

 thought of as rotating on the axis of symmetry. Symmetry of this sort 

 is known as radial. In one of the major groups of animals (Fig. 65) the 



PLANE OF 

 -BILATERAL SYMMETRY 



CENTER OF- 

 UNIVERSAL SYMMETRY 



AXIS OF 

 RADIAL SYMMETRY 



-PLANE OF 

 31LATERAL SYMMETRY 



ASYMMETRY 

 Fig. 64. — Types of symmetry illustrated by familiar objects. 



bodies regularly possess radial symmetry. The arms or tentacles of the 

 animals of that group limit the number of planes that divide them 

 symmetrically; in practice the symmetry is called radial if there are two 

 or more such planes all having the axis line in common. 



Sometimes there are many planes of symmetry having, not a line, 

 but a point in common. Symmetry is then said to be universal, and 

 the common point is the center of symmetry. In a sphere, any plane 

 that passes through the center is a plane of symmetry. Not many 

 animals have a spherical form, but Eudorina (Fig. 52) approaches it. 



An object may possess symmetry of two types. A football, for 

 example, has radial symmetry around its long axis, but bilateral sym- 

 metry in relation to the plane halfway between its ends. Some cells 

 have approximately that form, as do also some protozoan colonies. 



