184 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



ing which the number of chromosomes is reduced to half the body- 

 number. Spermaries and ovaries can be seen in the Hving Hydra as 

 little lumps on the ectoderm. The spermaries are always found near 



the free end of the body, the 

 ovaries, when present, being 

 nearer the base. The egg 

 is fertilized while still at- 

 tached to the parent and 

 develops into an embryo 

 surrounded by a protective 

 chitinous case, in which 

 stage it sinks to the bottom 

 of the pond for a resting 

 period before emerging as 

 an adult. 



Asexual development 

 also takes place. A small 

 bulging area, formed by 

 the interstitial cells, ap- 

 pears on the side of the 

 body, which more or less 

 rapidly grows into a short 

 column surrounded by 

 tentacles, depending on the 

 food supply available for 

 the parent Hydra. When 



* 



fully developed the bud 

 may separate from the 

 parent and lead a separate 

 existence. A Hydra fre- 

 quently produces more than 

 one bud on a single animal. 



young 

 hixd 



sperrr? 

 — cells 

 forming 



ec.todJ2.rm 

 endocferm- 



jonriing" 



Longitudinal section through the body of a 

 Hydra, showing both sexual and asexual repro- 

 ductive structures. 



Regeneration 



Although regeneration takes place in other groups of animals it is 

 best seen in the phylum, Coelenterata. The primitiveness of Hydra 

 is shown by the fact that it can regenerate or replace lost parts by 

 growth of the body cells. It may be cut lengthwise or crosswise, or 

 even into small pieces, and the fragments will, under favorable con- 

 ditions, give rise to complete individuals. 



