POLYPS (CUPLIKE ANIMALS) 



25 



long in the stomach, and the nutritive part is absorbed 

 by the lining cells, or endoderm (Fig. 39). The indiges- 

 tible remnants go out through the mouth. The hydra is 

 not provided with a special vent. Why could the vent not 

 be situated at the end opposite the mouth ? 



Circulation and Respiration. — Does water have free 

 access to the body cavity ? Does the hydra have few or 

 nearly all of its cells exposed to the water in which it 

 lives ? From its structure, decide whether it can breathe 

 like a sponge or whether 

 special respiratory cells are 

 necessary to supply it with 

 oxygen and give off carbon 

 dioxid. Blood vessels are 

 unnecessary for transfer- 

 ring oxygen and food from 

 cell to cell. 



Reproduction. — Do you 

 see any swellings upon the 

 side of the hydra ? (Fig. 34, A.) If the swelling is near 

 the tentacles, it is a spermary ; if near the base it is an 

 ovary. A sperm coalesces with or fertilizes the ovum after 

 the ovum is exposed by the breaking of the ovary wall. 

 Sometimes the sperm from one hydra unites with the ovum 

 of another hydra. This is called cross-fertilization. The 

 same term is applied to the process in plants when the 

 male element, or pollen, of the flower unites with the 

 ovules, or female element, of the flower on another plant. 

 The hydra, like most plants and some other animals, is 

 hermaphrodite, that is to say, both sperms and ova are 

 produced by one individual. In the autumn, eggs are 

 produced with hard shells to withstand the cold until 

 spring. Sexual reproduction takes place when food is 



Fig. 38. — Hydras on pond weed. 



