How To Study Living Animals


Proximal end,

the end by which an animal is attached to an object.

Distal end,

the end opposite the proximal end.


slender projections around the distal end.


the opening through the distal end, into the central cavity herunterladen.


a small hydra or other cœlenterate growing out from the wall of the parent.


a thin, gluey partition, without wandering cells, between the ectoderm and the endoderm.

Nettle cells,

very small cells, chiefly in the tentacles, easily identified in permanent preparations as clear cells with small hairs projecting from them herunterladen. See text-books for details of their structure.


the region or organ where the sperm cells are formed.


the region or organ where the egg cells are formed.

Cœlenterates (hollow bowels),

sac-shaped animals, the digestive tract having only one opening; the body wall is of two layers microsoft office kostenlos downloaden vollversion deutsch chip.


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 class 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 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?