STRUTUiE AND I'll YsrOLOfJY OF ANIMALS 



41 



Alternates with a stage in which there occurs a true sexual mode 

 si reproduction. Such a phenomenon is termed alternation of 

 gensjratiqns or metagenesis. The term heterogamy is applied to 

 OftBOfl in which two different sexual generations — usually a true 

 sexual and a parthenogenetic — alternate with one another. 

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



Fir,. 27.— Fresh-water polype (Hydra), two specimens, the one expanded, the other contracted, 

 .showing multiplication by budding. M. 1 Id* Od.'S buds in various stages of growth. (From 

 Parker's Biology.) 



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. 23) is the simplest and most generalised form among 

 multicellular animals or Metazoa ; but no adult animal retains 

 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 



