328 APPLIED BIOLOGY 



the distribution of digested food, oxygen, and excretions in 

 a frog ( 52). 



Excretion is accomplished by osmosis of waste matters from 

 the cells to the surrounding water. Probably the ectoderm 

 cells do the larger share of this work. 



Movement is caused partly by contraction of cells and partly 

 by that of muscle-processes which are extensions of some cells 

 in both inner and outer layers. 



Irritability is well developed in Hydra. It is in part a 

 function of all the constituent cells ; but some cells believed 

 to be simple nerve-cells are near the nettle-cells, which must 

 be extremely sensitive in order to discharge the nettle-threads 

 when stimulated by other small animals. 



Reproduction of Hydra is both asexual and sexual. When 

 there is plenty of food and other favorable conditions, asexual 

 budding occurs. A small elevation appears on the side of a 

 hydra, and this soon grows into a small hydra which be- 

 comes detached and lives independently. When the animals 

 are growing rapidly, there may be several buds on one 

 animal at the same time (Fig. 97). 



Under certain unfavorable conditions, such as lack of food, 

 stagnation of water, etc., hydras form reproductive organs, 

 ovaries and spermaries. These appear as transparent 

 thickenings of the ectoderm, the spermaries near the tentacles, 

 the ovaries near the base of the animal. Any individual 

 may form both kinds of sex-organs; that is, each is both 

 male and female; but usually both kinds do not mature 

 on a given individual at the same time. An animal which 

 can develop both ovaries and spermaries is known as her- 

 maphroditic. Likewise a flower with both stamens and pistil 

 is sometimes called hermaphroditic. Sponges, many worms, 

 many snails, and some other lower animals also exhibit 

 hermaphroditism. 



When the spermaries of a hydra are mature, the sperm-cells 

 escape and swim freely in the water. A single ovum or egg- 



