X ELEUTHEROBLASTEA HYDRA 2 5 5 



cells of both layers. The statement made by Trembley, that 

 when a Hydra is turned inside out it will continue to live in 

 the introverted condition has not been confirmed, and it seems 

 probable that after the experiment has been made the polyp 

 remains in a paralysed condition for some time, and later reverts, 

 somewhat suddenly, to the normal condition by a reversal of the 

 process. There is certainly no substantial reason to believe that 

 under any circumstances the ectoderm can undertake the function 

 of the endoderm or the endoderm the functions of the ectoderm. 



One of the characteristic features of Hydra is the .slightly 

 expanded, disc-shaped aboral extremity usually called the " foot," 

 an unfortunate term for which the word " sucker " should be sub- 

 stituted. Tliere are no root-like tendrils or processes for attach- 



Fia. 127. — A series of drawings 

 of llydra^ showing the atti- 

 tudes it assumes during one 

 of the more rapid movements 

 from place to place. 1, Tiie 

 Hydra bending over to one 

 side ; 2, attaching itself to 

 the support by the mouth 

 and tentacles ; .3, drawing 

 the sucker up to the mouth ; 

 4, inverted ; 5, refi,\ing the 

 sucker ; 6, reassuming the 

 erect posture. (After Trem- 

 bley.) 



ment to the support such as are found in most of the solitary 

 Gymnoblastea. The attachment of the body to the stem or weed 

 or surface-film by this sucker enables the animal to change its 

 position at will. It may either progress slowly by gliding along 

 its support without the assistance of the tentacles, in a manner 

 similar to that observed in many Sea-anemones ; or more rapidly 

 by a series of somersaults, as originally described by Trembley. 

 The latter mode of locomotion has been recently described as 

 follows : — ■" The body, expanded and with expanded tentacles, bends 

 over to one side. As soon as the tentacles touch the bottom 

 they attach themselves and contract. Now one of two things 

 happens. The foot may loosen its hold on the bottom and the 

 body contract. In this manner the animal comes to stand on its 

 tentacles with the foot pointing upward. The body now bends 

 over again until the foot attaches itself close to the attached 

 tentacles. These loosen in their turn, and so the Hydra is again 



