201 
THE OCEAN WORLD. 
habiting brooks filled with herbage — attaching itself particularly to the 
duckweed of stagnant ponds, and more especially to the under surface ol 
the leaf. The animal is reduced to a small greenish tubular sac, closed 
at one of its extremities, open at the other, and bearing round this open' 
ing from six to ten appendages, very slender, and not exceeding a line 
in breadth. The tubulous sac is the body of the animal (Fig. 87)- 
Fig. 86. Hydra vulgaris. 1. Hydra with ova and young, unhatched. 2. Egg ready to burst 
its shell. 3. Hydra of natural size attached to a piece of floating wood. 
The opening is at once its mouth and the entrance to the digestif 
canal ; the appendages, the tentacula or arms. 
The Hydras have no lungs, no liver, no intestines, no nervous systeiW 
no heart. They have no organ of the senses, except those which exist 
in the mouth and the skin. The arms or branches are hollow int er ' 
nally, and communicate with the stomach. They are provided with 
vibratile cells, furnished with a great number of tuberosities disposed 
spirally, and containing in their interior a number of capsules provided 
