CHAPTER VII 



THE HYDRAS, FRESH WATER JELLYFISHES 



• 



Coelenterata 



Form and habits of hydras. — The hydras (Fig. 87) are al- 

 most the only fresh water representatives of the great pre- 

 dominantly marine group of Coelenterata to which the jelly- 

 fishes, sea anemones, and corals belong. They are soft trans- 

 parent animals which can stretch themselves until they are 



Fig. 87. — Hydras hanging from plants in an aqua- 

 rium. 



half an inch long, or contract till they are as small as pinheads 

 (PI. IV). When a hydra is extended it looks like a piece of 

 string frayed out at one end. The frayed strands are a circlet 

 of tentacles which surround the mouth, and bring to it any 



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