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 outside? What do you think is its use?
  2. In the body wall, identify the endodermal and ectodermal layers of cells, separated by the mesoglea, which is usually stained more deeply. Study these cell layers carefully. What work ought each to do? What can you discover 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? Why, or why not?

Suggested drawings.
  1. Hydra undisturbed, and hydra after being touched or shaken.
  2. A hydra in successive poses to show its flexibility.
  3. A hydra taking food.
  4. Hydras to show reproduction in one or both ways.
  5. A section of hydra, showing details.
Comparative Study of Cœlenterates
Materials.

Various cœlenterates, such as hydroids, hydro-medusæ, jellyfishes, sea anemones, corals, sea fans, etc. Since nearly all the cœlenterates except hydras are marine forms, these will usually have to be dead specimens, preserved in formalin or alcohol, or put up as permanent preparations for the microscope.