48 THE SCIENCE OF LIFE. 



inifera. In other cases the budding is internal, and 

 the progeny may or may not remain attached to the 

 parent. 



15. Alternation of generations is a term given to ex- 

 press a mode of reproduction in which " the parent finds 

 no resemblance in his progeny until he comes down to 

 his great-grandson." The Jelly-fish, or Medusae, from 

 the huge masses cast up by the waves of the sea-shore, 

 to the tiny bell no bigger than a pea, are developed in 

 this manner. A ciliated germ, like some of the Infusoria 

 in form, swims about awhile, then becomes attached, 

 elongates, and develops into a polyp like the Hydra. 

 The polyp becomes wrinkled and subdivides until it 



looks like a pile 

 of saucers with 

 scalloped edges. 

 This breaks in- 

 to segments, 

 each of which 

 becomes a jel- 

 ly-fish, which 

 enlarges and 

 produces fresh 

 germs. Fig. 5, 



FIG. 5. Diagram illustrative of the Development of Hydrozoa. . 



(The specimen is one of the Lucernaridae.) 



i. Ciliated embryo or "planula." 2. Hydra tuba, showing production dlf- 

 a single individual. 3. Hydra tuba undergoing segmentation. 



4. The segmentation becoming more complete. 5. More ad- iCl'S ITOm ITietcl- 

 vanced stage, in which the tentacles are developed from the first , . i 



or basal segment. 6. Segmentation complete, giving rise to a HlOrpnOSlS, SUCn 

 free swimming Medusoid. as a butterfly 



undergoes in passing from the egg to the perfect insect, 

 or as most animals pass through in the embryonic state. 



