Ji8 



AN INTRODUCTION TO ZOOLOGY 



unfavorable conditions and extreme age result in a decrease 

 (116). 



Frequently specimens of Hydra are found which possess buds 

 in various stages of development. Several buds are often found 

 on a single animal, and these in tui n may bear buds before detach- 



g 



..b 



l.d. 



FIG. 54. Diagram of a longitudinal section of Hydra; b., bud ; b.d., basal 

 disk; bl., blastula ; ec., ectoderm; en., entoderm ; g., gastrula ; gv.c., 

 gastrovascular cavity ; hy., hypostome ; m., mouth ; m.e., mature egg ; 

 mes., mesoglea; m.t., mature testis ; n., nematocysts ; p.b., polar bod- 

 ies ; /., tentacle ; y.e., young egg ; y.t., young testis. All of the struc* 

 tures shown do not occur on a single animal at one time. 



