138 . PHYSIOLOGY 



that a central spacj is formed with one opening to the 

 outside. 



The jellyfish is closely related to the hydroid (a salt- 

 water form very similar to the fresh-water hydra) but 



Fie. 77.— Jellyfish. (From Hertwig, after Haeckel.) 



the resemblance has been obscured, for the depressions 

 and elevations on the surface of the hydroid have been 

 intensified in the jellyfish, and through the separa- 

 tion of the two layers in the wall by a structureless jelly 



