ILM 



GENERAL PRIXCIPLES OF ZOOLOGY 



In llic case of liiradial symim-try (tv^. oi'l there is the main axis, as in llie hist, 

 and two other unequal axes at riL;ht angles to this, the inequality consisting in 

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

 of tliese is called the sai^itlal axis, the other the irans\-ersc. Planes ]iassing 



Fig. 89. — Ilaliomnia i-riiiac:us. a radiolarian, a. oxlcrnal, /, inlcrnal, latticed s|ihcrical 

 skeleton; ck, central capsule; wk, exixa-capsular soft parts; 11, nucleus. 



Fig. go. — Vouno; Clirysmwa (after Clans'). 1, perradii; II, iiUerradii; (;/, gaslral 

 lilaments; sk, sensor\- pedicels. 



through the main axis and cither 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 [lassing throuL;h the 



