THE SIMPLEST MANY-CELLED ANIMALS 329 



cell develops in an ovary, and is fertilized by a single sperm- 

 cell. The fertilized egg-cell divides into a large number of 

 smaller cells, forming an embryo, and becomes surrounded 

 by a hard shell or cyst. The protected embryo falls to the 

 bottom of the pond. There it may remain for some time. 

 Dry dust scraped from bottoms of ponds during mid-summer 

 drought may contain embryos which will develop into hydras 

 soon after being placed in water. 



Hydras have another reproductive process which occurs 

 only when by some accident an individual is cut into two 

 or more pieces. The remarkable fact is that within a few 

 days each piece will grow into a perfect hydra. This is an 

 example of regeneration. Many other lower animals have the 

 same power of forming a perfect body from a part, and some 

 animals as high as frogs can regenerate small parts, such as 

 toes, if they happen to be destroyed. It should be noted that 

 regeneration is not a regular reproductive process in Hydra, 

 but provides against accidents. 



284. Division of Labor in Hydra. All cells must use 

 food, but only the endoderm cells are able to digest food. 

 Hence, ectoderm cells must depend upon the endoderm cells 

 for their food. On the other hand, the ectoderm cells form 

 the external protective covering of the animal, furnish the cells 

 which form the reproductive organs, catch the food, receive 

 impressions from the environment, and probably are the chief 

 causes of the movements of the animal. The tentacles are 

 specialized for collecting food, and their nettle-cells still 

 more highly adapted to a special purpose. The presence of 

 simple nerve-cells suggests the beginning of differentiation of 

 nervous organs. 



285. A Case of Mutual Aid : Symbiosis. The green 

 bodies in the endoderm cells of one species of Hydra have 

 been discovered to be simple, one-celled plants (one of the 

 Algae), which also live in tissues of some fresh-water sponge 

 colonies and in some other animals. Such a living together, 



