COELENTEKATA 31 



but not extending so far to the south as the brown Hydra. 

 They are found attached to weeds, in ponds and lakes, and in 

 quiet streams and rivers. 



Hydra has a cylindrical body, and the free end is surrounded 

 by a number of tentacles. The structure of the body is simple. 

 It consists of a double sac made up of many cells closely 

 adpressed. The outer layer is called the ectoderm, and the 



FIG. 13. Hydra, after Trembley. On the left, attached to a twig, Hydra is 

 seen in the expanded state and_.budding, in the retracted state, and in 

 the act of attaching the tentacles to the surface film. By releasing the 

 base and applying it to the surface of the water it becomes suspended 

 from the surface as shown. The succession of figures to the right illustrate 

 in the background rapid movement by looping, and in the front still more 

 rapid progression by somersaulting. 



inner layer the endoderm. The two layers are continuous 

 at the mouth opening, which is situated at the free end of the 

 body. The mouth leads into a central cavity bounded by 

 the endoderm, and the cavity is called the gastric cavity, or 

 enteron. The structure is that of the gastrula, a stage in 

 development passed through by all the Metazoa. The gastrula 

 marks therefore an important event in the evolutionary 

 history of all kinds of Metazoa (phylogeny), as it does in the 

 embryological history of each individual (ontogeny). There 

 are allies of Hydra, as Microhydra, found in fresh and brackish 

 water, which are without tentacles, and thus have a typical 

 gastrula structure. Hydra departs from the structure in 



