HYDR^, JELLYFISHES, SEA ANEMONES, CORAL POLYPS 49 



body through all parts of the surface and gives off car- 

 bonic acid gas from all of its cells. In other words, each 

 cell of the body performs the act of breathmg for itself, 

 independently of every other cell. 



How the hydra digests its food. — The food, as we have 

 seen, enters the mouth and passes directly into the large 

 digestive cavity, which is uniform throughout its whole 

 length and not differentiated into stomach, intestine, etc. 

 But some of the endoderm cells lining this cavity secrete 

 a digestive fluitl which breaks up and dissolves the food 

 to some extent. Probably particles of this food are then 

 taken up by other of the endoderm cells and digested 

 within them by their own protoplasmic mass. The ecto- 

 derm cells obtain their nourishment from the nutritive 

 matter in the endoderm cells. 



The hydra's manner of excretion. — There is only one 

 opening to the body of the hydra and all undigested por- 

 tions of food are passed outward through the mouth, the 

 same route through which they entered the botly. As we 

 have seen, carbonic acid gas is given off through all parts 

 of the surface. There are no special organs of excretion. 



Reproduction of the hydra. — These little animals repro- 

 duce by budding and by eggs. An individual is often found 

 with a smaller hydra growing out from the side of its body. 

 The smaller one arises by a process of budding. The body 

 wall of the parent bulges out at some point, forming a bud- 

 like protuberance which enlarges and develops at its free 

 end a mouth and tentacles, thus becoming a well-developed 

 hydra (Fig. 17). The body cavity of the young hydra is 

 continuous with that of the parent (Fig. 17). After a time 

 these bud hydrte become detached and they then pass a 

 separate existence. 



herhick's zool. ^ 



