3. A STUDY OF CŒLENTERATES

To show cells working together more definitely than in Sponges

A Study of Hydra
Materials.

Living hydras in permanent aquaria, undisturbed. Living hydras in small aquaria, i.e. tumblers, test tubes, watch glasses, etc., with pieces of water weed and if possible some of the microscopic animals found in water where hydras are abundant. If kept cool, hydras may live several days in such aquaria. Permanent slides of hydras; some whole, some in sections, and some showing the organs of reproduction.

Definitions.

Proximal end, the end by which an animal is attached to an object. Distal end, the end opposite the proximal end. Tentacles, slender projections around the distal end. Mouth, the opening through the distal end, into the central cavity. Bud, a small hydra or other cœlenterate growing out from the wall of the parent. Mesoglea, 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. See text-books for details of their structure. Spermary, the region or organ where the sperm cells are formed. Ovary, 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.

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?
  6. How can it capture them?
  7. What motions may a hydra perform, while remaining attached by its base? What are the results of these movements?
  8. 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?
  9. 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?
  10. 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?

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.

Definitions.

Colony, as used in this group, a number of individuals descended by budding from an original one, and remaining connected.
Polyp, an individual cœlenterate; 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 partitions.
  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?
  6. What is each kind best fitted to do? What is the probable result of this differentiation?
  7. What kinds of reproduction, if any, does the specimen you are studying show?
  8. Find out from books what other forms of reproduction are sometimes used by this animal.
Suggested drawings.
  1. At least one drawing of each cœlenterate you study.
Summary of the Comparative Study of Cœlenterates
  1. How may polyps in colonial forms differ from polyps which live singly?
  2. What variations in methods of reproduction are shown in this group?
  3. Which of the polyps you have studied shows the greatest differentiation? In what ways?
  4. What characteristic do you find common to all the cœlenterates you have studied?
Review and Library Exercise on Cœlenterates
  1. What are the characteristics which distinguish cœlenterates?
  2. Give the classes of cœlenterates, with the characteristics and an example of each.
  3. What enables a hydra to stick to a support by its foot?
  4. What are the processes in a hydra by which food is captured, swallowed, and digested?
  5. What is the chief fact of interest about Hydra viridis?
  6. Why do hydras reproduce all summer by budding and in the late fall by eggs?
  7. What change would have developed a hydra and its offspring into a plant-like colony instead of into a group of individuals?
  8. Why are ctenophores more easily seen in the night than other cœlenterates are?
  9. What relations may exist between hydroids and hydro-medusæ?
  10. What are the advantages of a sedentary life? Of a locomotory one?
  11. What is meant by the expression "alternation of generations"? Which animals are likely to develop alternation of generations, sedentary ones or locomotory ones? Why?
  12. Give at least two differences between hydro-medusæ and true jellyfishes.
  13. In the association between a hydractinia colony and a hermit crab, what advantages are derived by the hydractinia? by the crab? Define symbiosis. Give another illustration of it.
  14. How are new coral colonies started? How are large colonies formed?
  15. What are the conditions of life under which corals can grow vigorously?
  16. Where are corals most abundant?
  17. Note.—Show by coloring the regions on a blank map of the world.
  18. How may corals form a reef? Why do they, as a rule, form a reef instead of adding directly to the mainland?
  19. Give Darwin's theory regarding the way a coral atoll may have been formed.
  20. Where are fossil corals found in abundance? What does their presence prove?
  21. What is polymorphism? Give an illustration. What may be a disadvantage of polymorphism? What may be an advantage?
  22. In what ways is this group of economic importance?