It4 THE OCEAN WORLD. 
stagnant ponds, and more especially to the under surface of the 
leaf. The animal is composed of a small greenish tubular sac, 
closed at one of its extremities, open at the other, and bearing 
round this opening from six to ten tentacles, very slender, some- 
times not exceeding a line in breadth. The tubulous sac is the 
Fig. 40.—Hydra vulgaris. 1. Hydra with ovum and young, unhatched.- 2. Hydra of natural size 
attached to a piece of floating wood. 3. Egg ready to burst its covering. 
body of the animal (Fig. 41), the opening is at once its mouth and 
the entrance to the digestive canal; the appendages, the tentacula 
or arms. 
The Hydre have no lungs, no liver, no intestines, no nervous 
system, no heart. They have no organ of the senses, except those 
which may exist in their mouth and skin. The arms or tentacles are 
