Is the hydra’s =body= round or two-sided? (Fig. 35.) What is its _general shape_? Does one individual keep the same shape? (Fig. 34.) How does the length of the threadlike _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? [Illustration: FIG. 34.--Forms assumed by Hydra.] [Illustration: 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 alternately elongating and rounding the foot. Can you discover other ways by which it moves? Does the hydra always stand upon its foot? [Illustration: FIG. 36.--NETTLING CELL. II. discharged, and I. not discharged.] =Lasso Cells.=--Upon the tentacles (Fig. 35) are numerous cells provided each with a thread-like process (Fig. 36) which lies coiled within the cell, but which may be thrown out upon a water flea, or other minute animal that comes in reach. The touch of the lasso paralyzes the prey (Fig. 37). These cells are variously called lasso cells, nettling cells, or thread cells. The thread is hollow and is pushed out by the pressure of liquid within. When the pressure is withdrawn the thread goes back as the finger of a glove may be turned back into the glove by turning the finger outside in. When a minute animal, or other particle of food comes in contact with a tentacle, how does the tentacle get the food to the mouth? By bending and bringing the end to the mouth, or by shortening and changing its form, or in both ways? (Fig. 34, _C_.) Do the neighboring tentacles seem to bend over to assist a tentacle in securing prey? (Fig. 34, _C_.) [Illustration: FIG. 37.--HYDRA capturing a water flea.]