102 ORGANISMS OF TISSUES 



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 two different types of organ- 

 isms for mutual benefit; and (6) the combination of extra-cellu- 

 lar and intracellular digestion, thus combining the digestive 

 processes of higher metazoa with those of the protozoa. 



