100 ORGANISMS OF TISSUES 



which reproduce by budding. This phenomenon is called 

 metagenesis or alternation of generations since one (sexual) 

 generation (medusa) alternates with another (asexual) genera- 

 tion (hydroid). 



Other features of general interest mentioned under different 

 headings above may be summarized as (4) the bilamellate or 

 diblastic structure of Hydra representing a permanent gastrula; 

 (5) symbiosis or living together of an animal and pi ant for mutual 

 benefit; and (6) the combination of extra-cellular and intra- 

 cellular digestion, thus combining the digestive processes of 

 higher metazoa with those of the protozoa. 



