STRUCTURE AND FUNCTION 57 



duced in the lower one, the ovary, and sperm cells in the upper one, 

 the spermary. Refer to your text-book for further details. 



Details of structure. 



1. Using an entire mounted specimen and a section of 

 hydra, identify the body wall and the central cavity. What 

 is the extent of the central cavity.? (Examine both the 

 body and the tentacles.) Where does it open to the out- 

 side } What do you think is its use .? 



2. In the body wall, identify the endodermal and ecto- 

 dermal layers of cells, separated by the mesoglea, which is 

 usually stained more deeply. Study these cell layers care- 

 fully. What work ought each to do .■' What can you dis- 

 cover in its structure which would fit each layer to do its 

 work } 



3. In the tentacles, identify the nettle cells. Where are 

 they .'' How are they arranged .' About how many of 

 them would be discharged if a small animal were to bump 

 into a tentacle .■• 



Summary of Important Points in the Study of Hydra 



1. Name the different kinds of cells in a hydra. Which 

 kind differs most from such a cell as the starfish egg .' 

 What work does this specialized cell do .■" 



2. How much of a hydra's body may be set in action by 

 touching a tentacle.? Contrast this with the sponge. 

 What do you infer concerning the nervous power of these 

 two animals .? 



3. Look back over your notes and list the different kinds 

 of work a hydra can do. 



4. Can it do any more kinds of work than a paramecium 

 or a sponge can .' If so, give further details. 



5. Can it do any of its work in any better way.' Would 

 you expect it to be able to 1 Why, or why not } 



