FORM OF THE BODY 135 



of fishes. But they soon turn on one side and in the adult 

 continue in this attitude, with one side toward the earth. In 

 this case the principal axis is maintained in the same relative 

 position, but the dorso-ventral and right-left axes are trans- 

 posed in space. In response, the form of the animal also under- 



FIG. 65. A sea-urchin (Clypeaster) in which a secondary bilateral symmetry 

 is impressed on a radial organism. Oral view, slightly reduced. 



goes a change, so that the dorsal and ventral surfaces become 

 symmetrical and the right and left sides unsymmetrical. The 

 plane of symmetry is thus revolved 90 on the principal axis. 

 It would be more correct, however, to say that while the animal 



