CHAPTER IV 

 A STUDY OF HYDRA 



Frogs and grasshoppers and all the other vertebrates and 

 insects are very complex animals, having their bodies made up 

 of many highly specialized organs and tissues, each part 

 adapted to performing some special life process. There are 

 other many-celled animals much simpler in structure than 

 this, animals without specially developed digestive, nervous, 

 muscular or reproductive systems. Yet they are capable of 

 doing all of the essential things that the more complex animals 

 can do. They can take and assimilate food, respond to stimuli, 

 move, and reproduce their kind. Hydra is a good example 

 of such a simple animal, and as it is to be found in most open 

 fresh- water ponds, in water troughs and other places where the 

 water is not too stagnant, it may be taken as a type for study. 



Hydrae will often be found attached to a stone, stick or leaf, 

 or to the green, moss-like plants that are often found in stand- 

 ing water. They may be green or brownish in color and as 

 they are only about one-eighth of an inch long or even smaller, 

 they will have to be looked for very carefully. Once seen, 

 however, they may be easily recognized by the cylindrical body 

 attached by the base and with its free end crowned with a 

 circlet of slender tentacles or arms which lash about slowly 

 while the animal is extended. When touched or alarmed the 

 whole body is quickly contracted and the tentacles drawn in, 

 so that the animal now looks like a simple, small, round or oval 

 projection on the leaf or stone. Food is caught by the ten- 

 tacles and conveyed to the mouth, which lies in the center at 

 the base of the tentacles. Through the mouth the food passes 

 into a space, the digestive cavity, surrounded only by the body- 

 wall. The food is dissolved in this cavity and the waste parts 



