Coelenterata. 173 



cells which have been discharged as a result of the irritating 

 acid. Small animals coming in contact with the tentacles are 

 paralyzed by means of these thread cells which are suddenly 

 shot out ; the tentacles then carry the victim to the mouth, and 

 it is swallowed. 



Note the simplicity of the structure of hydra the absence of 

 any distinct nervous system, and all special organs of circulation 

 and respiration. On the side of a hydra, near the base, may 

 sometimes be seen a conical elevation, the ovary, in which the 

 eggs are produced. Also on the side of the body, but near the 

 tentacles, may sometimes be found several elevations, the sper- 

 maries, in which the sperm cells are produced. 



THE SEA ANEMONE. 



Look for sea anemones attached to rocks. The beginner in 

 this sort of collecting and observation is usually not prepared 

 for what he sees ; he does not usually realize that the name " sea 

 anemone " is exceedingly appropriate, and he is not likely to 

 look for brilliant forms, like sunflowers, asters, and chrysanthe- 

 mums. Watch them both in their expanded and in their con- 

 tracted condition. When they are expanded gently touch them. 

 Are they firmly or loosely attached ? Do they ever move about ? 

 Have they any means of getting food ? 



In its general form the sea anemone resembles a hydra, having 

 a cylindrical, hollow body attached by one end to some foreign 

 object, and at the free end a mouth surrounded by tentacles. 

 In its internal structure, however, the sea anemone presents 

 some new features. The mouth, instead of opening directly into 

 the body cavity, as in the hydra, opens into an esophagus which 

 hangs like a bag suspended in this cavity ; the esophagus has no 

 bottom, but at its lower end communicates freely with the body 

 cavity. 



The body wall and esophagus may be represented by a glove 

 finger with its tip cut off and the open end turned back part 

 way into the larger part of the finger. 



