42 



ZOOLOGY 



SECT, 



animals are in the adult condition similar in their symmetry to the 

 gastrula, except that there are special developments along a series 

 of regularly arranged radiating secondary axes ; these radial 

 developments may be in the form of tentacles or radially arranged 

 processes (Fig. 29), or may assume the character of a radial arrange- 

 ment of internal parts. Such an animal is said to be radially 

 symmetrical. The body of a radially symmetrical animal is capable 



— c 



d- 



Fig. 28. — Diagram of the axes of the body. 

 AB, primary axis ; ab, ed, secondary 

 axes. The lower figure is a transverse 

 section of the upper one showing its two 

 secondary axes. (From Gegenbaur.) 



Fig. 29. — Radial symmetry. Letters as 

 in Fig. 28. The processes at A are 

 the tentacles ; the lower figure repre- 

 sents the upper or oral surface. (From 

 Gegenbaur.) 



of being divided into a series of equal radial parts or antimcres, 

 each of which is symmetrically disposed with regard to one of the 

 secondary or radial axes. 



In animals which are not permanently fixed, locomotion usually 

 takes place in the direction of the primary axis of the body, and 

 one side, habitually directed downwards, becomes modified differ- 

 ently from the other which is habitually directed upwards : lower 

 or ventral surface becomes distinguishable from an upper or dorsal. 

 Thus the radial symmetry is now disturbed ; the secondary axes 

 have become unequal ; the dorso-ventral or vertical secondary axes 



