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? 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?