Coelenterata. 317 



outstretched tentacles, with their many stinging cells, it 

 has a trap continually set for the unwary small swimmers 

 that abound in such places as the hydra inhabits. 



Recovery after Mutilation. One very remarkable fact 

 about hydra is its ability to live and grow after injury. If 

 cut into several pieces, each grows into a complete hydra. 



Multiplication by Budding. Hydra multiplies by bud- 

 ding. A small part of the wall bulges out and forms a 

 cylindric branch, with a double wall continued from the 

 two layers of the body, and the hollow of the branch is 

 continuous with that of the body (see Figs. 179 and 180). 

 After a while, a circle of tentacles is formed at the end of 

 the branch, and an opening appears at the end for a mouth. 

 Finally the base of the branch is constricted and the branch 

 separates and is an independent hydra. 



Multiplication by Eggs. Among the cells of the ecto- 

 derm, we noticed that certain cells are smaller and lie 

 deeper than the others. Some of these cells develop into 

 egg cells, or eggs. They are covered by the outer cells, 

 but make a marked bulging. This is the ovary (from ova, 

 eggs) and is usually found at about one third of the length 

 from the foot of the hydra (see Fig. 180). A somewhat 

 similar growth occurs near the tentacles, but the contained 

 cells are different. Instead of spherical cells like the egg 

 cells, here are produced cells resembling a miniature tad- 

 pole, with an oval head and a vibrating tail. These are 

 the sperm cells, or sperms, and the enlargement in which 

 they are developed is called a spermary. 



CLASS I. HYDROZOA. 



Hydroids. Most of the members of this class are called 

 hydroids, and, as the name implies, they are hydralike. 



