A Guide for the Study of Animals Part 18

_Directions._

Take a small aquarium to your table, set it down carefully and leave it undisturbed. Identify a hydra and watch it for some time.

_Observations on the living animals._

1. Describe the size and shape of a hydra when expanded. Disturb it slightly by shaking the aquarium a little, and describe its shape when contracted. Notice also the flexibility of the body. What do you infer concerning the hydra's possession of a skeleton? What advantage can it be to have a body so flexible?

2. How many tentacles has the hydra that you are studying? What does the hydra do with these tentacles when it is expanded? What is the probable object of such actions?

3. How does a hydra respond to contact? What seems to be the object of such a response?

4. Notice the location of the hydras in the large, undisturbed aquaria. Where are they placed as regards the light side of the aquarium? Of what value is such a response to light in their case?

5. How can a hydra locate the small animals which are its food?

How can it capture them?

6. What motions may a hydra perform, while remaining attached by its base? What are the results of these movements?

7. If you have happened to see a hydra move from one place to another, describe the process. If not, give the facts which lead you to believe that it is able to do so. Suggest all the methods you think it may be able to use. What is your opinion of the hydra's power of locomotion? Of what use is it in getting food; in escaping enemies; in following the fluctuations of the water supply? If you had to cla.s.s the hydra as either one, would you call it a stationary or a locomotory animal?

8. Study budding hydras. Compare the bud with the parent hydra as to size, form and number and size of tentacles. Notice whether the bud moves independently or only with the parent. When does it separate from the parent?

9. In hydras collected late in the fall you may see another method of reproduction. If such material is at hand, notice small swellings near the proximal end and others near the tentacles.

Eggs are produced 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 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 b.u.mp 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._

a. Hydra undisturbed, and hydra after being touched or shaken.

b. A hydra in successive poses to show its flexibility.

c. A hydra taking food.

d. Hydras to show reproduction in one or both ways.

e. A section of hydra, showing details.

Comparative Study of Clenterates

_Materials._

Various clenterates, such as hydroids, hydro-medusae, jellyfishes, sea anemones, corals, sea fans, etc. Since nearly all the clenterates 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.

_Definitions._

_Colony_, as used in this group, a number of individuals descended by budding from an original one, and remaining connected.

_Polyp_, an individual clenterate; one of the individuals in a colony.

_Observations._

1. How large is an individual specimen in the form you are studying?

If the form is colonial, how large is the colony or portion of a colony you are studying? Estimate the number of individuals in it. Is the colony free-swimming or attached? If attached, to what is it usually fastened?

2. Compare the individual you are studying with a hydra, as to size and shape of the body, the location of the mouth, and the size, number, and arrangement of the tentacles.

3. Is there a skeleton? If so, describe it. What appears to be its use? In corals, notice the radiating part.i.tions.

4. Has the specimen any nettle cells? If so, where are they located?

5. Are all the polyps of the colony alike? If not, how many kinds are there? How do they differ?

What is each kind best fitted to do? What is the probable result of this differentiation?

6. What kinds of reproduction, if any, does the specimen you are studying show?

Find out from books what other forms of reproduction are sometimes used by this animal.