CHAPTER XII 



PHYLUM III. CCELENTERATA (HOLLOW INSIDE). (HYDROIDS, CORALS, 



JELLY-FISHES, ETC.) 



LABORATORY EXERCISES 



217. Hydra. Hydras are small tubular animals found in 

 permanent fresh-water pools, attached to submerged leaves, 

 twigs, algae, etc. They are somewhat difficult to recognize 

 when disturbed because they contract into small rounded 

 masses, close against the supporting object. Promising ma- 

 terials should be collected from several ponds, and placed in 

 shallow vessels (a white-ware dish is good), and in a short 

 time the hydras will become extended. The green hydra 

 (H. viridis) is perhaps more common and hardier, but is not 

 so satisfactory for general laboratory work as the brown (H. 

 fusca), because it is less transparent. 



i. Study the living animal in a glass jar (tumbler). 



Is it free or attached? What happens if it is freed from its 

 attachment? Is it lighter or heavier than the water? Evi- 

 dences. Can it move from one portion of the vessel to another ? 

 If so, does it become detached ? Watch same individuals from 

 day to day. What is its position in the water? If the vessel 

 containing hydras be placed near the window for several days, 

 at which side of the vessel do the animals become collected? 

 When the animals are stretched out at their greatest length, 

 touch lightly the tip of one of the tentacles. Touch the body. 

 Repeat the experiment until you are sure of your results. Note 

 and explain as well as you can the results. Of what degree of 

 contraction is the animal capable ? Do you notice any contrac- 

 tions or motions of parts, when the hydra is undisturbed? 

 What seems to be the purpose of the motions? Evidences? 

 Bring a piece of meat the size of a pin-head, or a Daphnia or 

 Cyclops, in contact with the tip of a tentacle and note the results. 

 How do the other tentacles behave ? Place a food-particle di- 



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