42 



ZOOLOGY 



SECT. 



Pcedogenesis, or the development of young by a sexual process from 

 individuals that have not attained the adult condition, is a 

 phenomenon which is to be observed in some groups of animals. 



7. SYMMETRY. 



The general disposition or symmetry of the parts in an animal 

 presents two main modifications the radial and the bilateral. 

 The gastrula (p. 24) is the simplest and most generalised form among 

 multicellular animals or Metazoa ; but no adult animal retains 



FIG. 28. Diagram of the axes of the body 

 Ali, primary axis ; ab, erf, 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.) 



this simple shape. In the gastrula we may imagine a central 

 primary axis (Fig. 28, AB) passing through the middle of the blas- 

 topore and of the archenteric cavity, and a series of secondary axes 

 (ab, cd) running at right angles to this to the outer surface. In a 

 symmetrical gastrula the secondary axes would be all equal. Many 

 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- 



