BEHAVIOR OF CCELENTERATA 



195 



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we shall see later, under continued injurious stimulation of different 

 kinds. 



In speaking of righting reactions, it is often said that the organism 

 is forced by the different irrita- 

 bilities of diverse parts of the 

 bodv to take a certain orienta- 

 tion with reference to gravity 

 or to the surface of contact 

 (see, for example, Loeb, 1900, 4mToHH^ 

 p. 184). The facts just MMMiM$ 

 brought out show that we can 

 in Hydra consider this orienta- 

 tion forced only in the general 

 sense that all things which 

 occur may be considered 

 forced. Man takes sometimes 

 a sitting position, sometimes a FlG . I22 .-_ Pos ; tion taken by Cerianthus after it 



Standing one, Sometimes a re- nas been placed on its side on a wire mesh. After Loeb. 



clining one, depending upon ' 



his "physiological state" and past history, and the facts are quite 

 parallel for Hydra. So far as objective evidence shows, the behavior is 

 not forced in Hydra in any other sense than it is in man. The animal 



takes that position which seems best adapted to 

 the requirements of its physiological processes; 

 these requirements vary from time to time. 



In the sea anemone Cerianthus the conditions 

 for retaining a certain position are somewhat more 

 complex than- in Hydra, according to the account 

 given by Loeb (1891). The animal is usually 

 Fig. 123. — Cerianthus found in an upright position, occupying a mucus- 



which has woven itself through .. . . . , , T1 . . , . , , 



a meshwork, as a result of re- hned tube in the sand. If placed head down- 

 peatediy inverting the latter. warc j i n a test-tube, it rights itself in the same 



After Loeb (1S91). TT . . . ° . . . . . 



way as Hydra, freeing the head, bringing the 

 foot into contact, and straightening the body (Fig. 121). But in this 

 animal, gravity clearly plays a part in the behavior. Loeb placed the 

 animal on its side on a wire screen of large mesh. Thereupon it bends 

 its foot down through the meshes, lifts up its head, and takes its usual 

 position in line with gravity (Fig. 122). If now the screen is turned 

 over, the animal again directs its head upward, its foot downward — as 

 a human being under similar circumstances would do if possible. It 

 may thus weave itself in and out through the meshes (Fig. 123). 



But to be in line with gravity, with head free, is not the only require- 





