the window. They hold on to the glass by one end, and all around the other end, which is wider, are a number of long threads called tentacles, hanging down gracefully in the water. At first you might think them whiskers, as they grow out around the mouth, but tentacle means a feeler or holder, and with its tentacles you will see how our little friend feels and holds its food, and carries it to his mouth, almost as you use you fingers. These little animals are called hydræ, because if you cut them up each piece will grow again, as did the heads of the old Greek monster. If you look at your hydra under the microscope, you will find all these parts: first, there is the part by which it holds on; it is round and hollow, something like the bottom of a fly's foot, and it changes its size whenever the body of the hydra changes its form. When the hydra is stretched out full length, the foot is smaller than the body; but, when the hydra shrinks up against the glass, it
Fig. 35. Green Hydra.
seems to be all foot. When the body is stretched out, it is round and hollow like a pipe-stem, or more like a very slender funnel, and the opening at the large end surrounded by tentacles or feelers is the mouth.
The hydra's feelers are not all the same length; some of them are prettily colored, and all are filled with wavy knobs or knuckles along the sides (Fig. 35). The bag or body of the hydra is made of two
