60 
LABORATORY EXERCISES 
EXERCISE 64 
A FRESH WATER HYDRA 
The hydras are found in fresh water ponds and streams. Look 
for them on the under sides of submerged leaves and stems. 
Living ones may be brought into the laboratory and kept for ob¬ 
servation. Prepared microscopic and lantern slides will facilitate 
the study of these animals. 
I. Observe the hving animals which you have in the laboratory. 
Note their positions in the water. How long is the main part, the 
body, of the animal? Note the tentacles which you find branching 
out from one end. Compare the length of these with that of the 
body. Observe any movements of the animals. By what part 
of the body do they attach themselves to objects? Where are 
these animals most abundant? Determine the reason for this 
distribution. 
II. Place one of the hydras in a little water in a shallow dish. 
Examine it with a lens. Note the cylindrical form of the body. 
Note the basal part by which it attaches itself to objects. At the 
opposite end find the position of the mouth. This is surrounded by 
the tentacles. What are the arrangement and number of the ten¬ 
tacles? Within the body is the digestive cavity. The instructor 
may introduce a number of small crustaceans, such as Cyclops, 
into the water. Observe what happens when the hydra comes in 
contact with one of them. How does a hydra get its food? Draw 
the hydra as you see it. (See Waggoner, Fig. 175.) 
III. Examine a hydra which has been mounted under low power. 
(The cover glass should be supported so that the animal will not 
be crushed.) Observe that the body wall, as well as the wall of 
the tentacles, consists of two layers of cells, the ectoderm, or outer 
layer, and the endoderm, or inner layer. A. very thin gelatinous 
layer lies between. The body of the hydra is similar to a hollow, 
double-walled tube. The tentacles are merely hollow extensions 
of the tube, somewhat as the fingers of a glove are extensions of the 
