256 THE HYDRA 



food for a time, its tentacles and mouth begin to react in a charac- 

 teristic fashion with the result that the prey shortly disappears 

 into the mouth and the hydra resumes its watchful waiting, 

 although it is now less responsive to such stimulation until it 

 again becomes " hungry." The water-fiea might as well have 

 gripped an electric wire for all the help it receives from its pro- 

 tective skeleton or the hairs and spines upon its appendages. In 

 spite of its apparent lack of weapons, the hydra has easily over- 

 powered its armored prey. These deadly effects are produced by 

 the hypnotoxin, which is brought into contact with the prey by the 

 nematocyst threads in the manner described. Thus the hydra is a 

 terrible enemy for such animals as water-fleas, although at first 

 glance it appears wholly incapable of offensive or defensive actions. 

 Because of its nematocysts, it is well equipped for this phase of its 

 struggle for existence. 



Such stinging capsules are found in all but one group of the 

 coelenterates. In some of the jelly fishes their poison is so strong as 

 to produce a violent reaction in the human skin. In all cases they 

 are used for paralyzing active prey and incidentally for de- 

 fense; so that a jellyfish, in spite of its soft body, may overpower 

 active animals of small size, and even animals that are protected 

 by external skeletons and defensive organs. When a nema- 

 tocyst is discharged it is of no further use, and both capsule 

 and thread are soon detached from the hydra. Often the cnido- 

 blast cell is also torn from its position and lost. There are several 

 types of nematocysts in hydra and many types in different coelen- 

 terates, but these must be studied at first hand to be understood. 

 However, they are all constructed and seem to function in the 

 same general manner. That the nematocysts are not " cells " is 

 obvious. By suitable methods they may be extracted from a 

 hydra without being discharged, and kept for days, to be dis- 

 charged later upon appropriate stimulation. There are also some 

 animals, like the flat worm Microstoma and certain molluscs, 

 that feed upon coelenterates without causing a discharge of 

 the nematocysts, and later use these stinging capsules for their 

 own defense. 



There is another feature in the behavior of the cnidoblasts with 

 their contained nematocysts that presents further complications. 

 If the various regions of a hydra are examined, almost no nemato- 

 cysts in the developmental stages are represented upon the tenta- 



