24 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



and a mouth of their own. Finally the buds become constricted 

 at the base and separate from the parent. The young Hydrae 

 thus produced soon attach themselves to some object and begin 

 their independent existence. This budding takes place more 

 commonly when food is plentiful and other conditions are 

 favorable. At other times reproductive organs, ovaries and 

 spermaries, may be formed, and the animal produces special 

 germ cells. The ovaries appear as thickenings of the body- 

 wall near the lower end of the body. In each ovary a single 

 ovum or egg develops. The spermaries appear as smaller 

 thickenings of the ectoderm near the tentacles. In them the 

 sperm cells develop. When these reproductive elements are 

 ripe they are cast out into the water where the ova are ferti- 

 lized by the spermatozoa. Thus Hydra is hermaphroditic, 

 that is, both sexes are represented in a single individual. 

 We will find that many worms, snails and some other ani- 

 mals are also hermaphroditic. In such animals various 

 methods are adopted to prevent self-fertilization. In Hydra 

 this is prevented by the ova and spermatozoa in any indi- 

 vidual usually ripening and being cast into the water at dif- 

 ferent times. The fertilized ovum soon divides into a large 

 number of cells, forming the embryo, and after becoming 

 surrounded by a hard shell or cyst drops to the bottom of 

 the pond where it may remain for some time before it goes 

 on with its development. 



Sometimes, through accident, a Hydra may be cut into two 

 or more pieces. Each part has the power of developing into 

 a new individual Hydra just like the original. This power of 

 developing anew parts of the body that may have been lost, 

 is possessed by many other animals, especially the lower ones, 

 and is known as regeneration. 



