[CHAPTER X]

HYDRA

LABORATORY EXERCISE

Technical Note.—Hydra lives in fresh water, attached to stones, sticks, or decayed leaves. It can be found in most open fresh-water ponds not too stagnant, often attached to Chara. There are two species occurring commonly, H. viridis, the green Hydra, and H. fuscus, the brown or flesh-colored Hydra. Both are very small forms and have to be looked for carefully. Specimens should be brought to the laboratory, put into a large dish of water and left in the light. Hydra is best studied alive. Place a living specimen attached to a bit of weed in a watch-crystal filled with water or on a slide with plenty of water and examine with the low power of the microscope.

Note the cylindrical body (fig. [7], A, B) with its flat basal attachment and radial tentacles (varying in number) which crown the upper end and surround the centrally located mouth. Note the movements of Hydra, its powers of contraction, and method of taking in food.

Technical Note.—To feed Hydra, place very small "water-fleas" (Daphnia sp.) in the water with it.

Observe the method by which "water-fleas" are taken into the mouth. Food is caught on stinging cells (to be studied later) and conveyed to the mouth by the tentacles. Note that the cylindrical body encloses a cavity, the digestive cavity. How is this connected with the exterior? If Hydra captures prey too large or is no longer hungry, the prey is released.