CHAPTER XXV 



THE FRESH-WATER POLYP AND SOME ALLIES: 



CCELENTERATA 



To-day the many-hued anemone 

 Waving, expands within the rock-pools green, 

 And swift, transparent creatures of the sea 

 Dart throu' the feathery sea-fronds scarcely seen. 



Sir Lewis Morris 



The Fresh- Water Polyp. There are several species of fresh- 

 water polyps (Fig. 139). These are so small that the casual 

 observer would seldom be aware of their existence, even 

 though they were abundant in his aquarium. Two common 

 species are Hy'dra viridis'sima, the green hydra, and Hydra 

 oligac'tis, the brown hydra. 



The green hydra lives among green fresh-water plants 

 in places where there is abundant sunlight. At rest, it 

 holds to a plant with its aboral surface, the remainder of 

 the body floating outward or downward. The body, which 

 is cylindrical in form, is about 3 mm. (■§- in.) long and 

 .4 mm. (^o in-) in diameter. A little practice will enable 

 a person to distinguish one in an aquarium, for speci- 

 mens frequently leave the green plants and crawl up the 

 side of the glass nearest the light. They can move either 

 by a slow, creeping movement on the aboral surface, or by 

 the process of "looping," like a measuring worm. Hydras 

 exist for a long time in an aquarium, provided there is an 

 abundance of their food animals, which are usually small 

 Crustacea. 



The number of tentacles which a specimen of the green 

 hydra may have depends upon its size, and also upon its age. 



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