MORPHOLOGY OF ANIMALS 



simplest forms the symmetry is radial, but in more com- 

 plex forms it may be bilateral or biradial (Ctenophora, 

 Fig. 2, C). 



{b) Heteroxonia (= Bilateratd) are all other Metazoa 

 except the protaxonia. In these the chief axis of the gastru- 

 la does not directly become the chief axis of the adult. By 

 the greater growth of the gastrula on one side (ultimately 

 posterior, Fig. 3, B-D) the oral and aboral poles of the 

 gastrula become widely separated on that side and nearer 

 together on the opposite (anterior) side; thus the axis run- 

 ning from the apical pole of the gastrula to the blastopore 



Fig. 3. Diagrams of the origin of Heteraxonia from a typical gastrula. Chief axis of 

 gastrula indicated by arrow, apical (sensory) pole shaded, oral pole at blastopore. A, 

 typical gastrula, radially symmetrical ; B, bending of gastrular axis by greater growth of 

 gastrula on dorsal-posterior side and consequent establishment of bilateral symmetry; C, 

 stage corresponding to adult flat-worm (Turbellaria) with mouth in middle of ventral side, 

 without anus and with two nerve cords (shaded) running from brain aroimd oesophagus 

 and along ventral side of body ; D, stage corresponding to an annelid, the trunk segmented 

 into several somites, with mouth near anterior end of body, anus at posterior end, nervous 

 system consisting of brain, circumoesophageal ring and ventral chain of ganglia and with 

 a pair of nephridia in each somite. 



I 163 



