202 



THE OCEAN WORLD. 



habiting brooks filled with herbage attaching itself particularly to the 

 duckweed of stagnant ponds, and more especially to the under surface of 

 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 

 opening 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, unliatd-ed. 2. Hydra of natural size 

 attached to a piece of floating wood. 3. hgg icady to burst its shell. 



The opening is at once its mouth and the entrance to the digestive 

 canal ; the appendages, the tentacula or arms. 



The Hydras have no lungs, no liver, no intestines, no nervous system, 

 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 inter- 

 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 



