like tentacles compare with the length of the hydra's body? About how many tentacles are on a hydra's body? Do all have the same number of tentacles? Are the tentacles knotty or smooth? (Fig. 35.) The hydra is usually extended and slender; sometimes it is contracted and rounded. In which of these conditions is the base (the foot) larger around than the rest of the body? (Fig. 34.) Smaller? How many openings into the body are visible? Is there a depression or an eminence at the base of the tentacles? For what is the opening on top of the body probably used? Why are the tentacles placed at the top of the hydra's body? Does the mouth have the most convenient location possible?
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Fig. 35.—Hydra (much enlarged).
The conical projection bearing the mouth is called hypostome (Fig. 34). The mouth opens into the digestive cavity. Is this the same as the general body cavity, or does the stomach have a wall distinct from the body cavity? How far down does the body cavity extend? Does it extend up into the tentacles? (Fig. 39.)
If a tentacle is touched, what happens? Is the body ever bent?
Which is more sensitive, the columnar body or the tentacles? In
searching for hydras would you be more likely to find the tentacles
extended or drawn in? Is the hypostome ever extended
or drawn in? (Fig. 34.)
Locomotion.—The round surface, or disk, by which the
hydra is attached, is called its foot. Can you move on
one foot without hopping? The hydra moves by alter-
