124 



GENERAL PRINCIPLES OF ZOOLOGY 



In the case of biradial symmetry (fig. 91) there is the main axis, as in the last, 

 and two other unequal axes at right angles to this, the inequality consisting in 

 that organs occur in the line of the one that are not found in the other. One 

 of these is called the sagittal axis, the other the transverse. Planes passing 







FIG. 89. Haliomma erinaceus, a radiolarian. a, external, i, internal, latticed spherical 

 skeleton; ck, central capsule; wk, extra-capsular soft parts; n, nucleus. 



FIG. 90. Young Chrysaora (after Claus). I, perradii; II, interradii; gf, gastral 



filaments; sk, sensory pedicels. 



through the main axis and either of the others will divide the animal symmet- 

 rically. Corals, sea anemones and ctenophores belong here. 



Bilateral symmetry has the same three axes, the two ends of the main or 

 longitudinal axis being dissimilar, as well as those of the sagittal axis. These 

 axes define three planes which have received names. That passing through the 



