FORM OF THE BODY 



133 



The radial symmetry is to be referred to the radial action of the 

 environment, which is the same in all directions at right angles 

 to the principal axis. This type of symmetry is characteristic 

 of plants, and inasmuch as these radial animals approach plants 

 in their life habit, they are affected by their environment like 

 plants and consequently approach plants in their structure. 

 A b 



ab. 0. 



Fig. 63. — Diagram of radial symmetry as represented by a medusa. 

 view; B, lateral view; o-ab.o., principal axis. 



A, Oral 



317. Universal Symmetry. — In case external forces are the 

 same in all directions the corresponding response form would be 

 a sphere. This might be called universal symmetry. Such a 

 condition is actually approached only by a few protozoa which 

 float in the w^ater, unattached, and are continually turning over 

 and over. 



318. Asymmetry. — Varying degrees of asymmetry arc found 

 among fixed or sluggish types. This is more often true of 

 colonial forms which grow by a process of budding and become 

 asymmetrical by unequal growth. In these cases, however, the 

 individual may be perfectly radial. 



319. A study of some exceptional cases will serve to "prove 

 the rule." The shell of gasteropod molluscs is asymmetrical. 

 However, when the animal is completely withdrawn into the 

 shell and is then completely asymmetrical, it is also to all in- 



