CHAPTER X 

 HYDRA 



LABORATORY EXERCISE 



TECHNICAL NOTE. Hydra lives in fresh water, attached to stones, 

 sticks, or decayed leaves. It can be found in most open fresh-water 

 ponds not too stagnant, often attached to Chara. There are two 

 species occurring commonly, H. iriridis, the green Hydra, and H. 

 fuscus, the brown or flesh-colored Hydra. Both are very small 

 forms and have to be looked for carefully. Specimens should be 

 brought to the laboratory, put into a large dish of water and left 

 in the light. Hydra is best studied alive. Place a living specimen 

 attached to a bit of weed in a watch-crystal filled with water or on 

 a slide with plenty of water and examine with the low power of the 

 microscope. 



Note the cylindrical body (fig. 7, A, E) with its flat 

 basal attachment and radial tentacles (varying in number) 

 which crown the upper end and surround the centrally 

 located mouth. Note the movements of Hydra, its powers 

 of contraction, and method of taking in food. 



TECHNICAL NOTE. -To feed Hydra, place very small " water- 

 fleas" (Daphuia sp. ) in the water with it. 



Observe the method by which " water-fleas " are taken 

 into the mouth. Food is caught on stinging cells (to be 

 studied later) and conveyed to the mouth by the tenta- 

 cles. Note that the cylindrical body encloses a cavity, 

 the digestive cavity. How is this connected with the ex- 

 terior ? If Hydra captures prey too large or is no longer 

 hungry, the prey is released. 



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