CNIDARIA 



45 



" Alternation of generations " in Hydrolds depends upon 

 the regular alternation of non-sexual generations (hydroid 

 polyps), increasing by lateral budding, with a sexually 

 developed generation (hydromedusa or sessile gonophore). 



The account of the development thus far affords some 



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suggestions of the manner in which the hydroid polyps and 

 the hydromedusfe may be referred back to one and the 



rates the cavity of the bell from the outside world and in which a two- 

 layer arrangement of the ectodermal cells can be recognized at an early 

 epoch (Weismann, No. 49, p. 260). 



