HYDRA 



313 



is reattached. This process is consecutively repeated and is called 

 '^ looping." Occasionally the animal bends over, holds by the tentacles, 

 then turns a ''handspring" or ''somersault" to attach the basal disc 

 beyond the attachment by the tentacles. The fourth means by which 

 locomotion is effected is by dropping to the bottom, then secreting a 

 bubble of gas at the basal disc and floating back to the top on that. 



FI&. 110. — Locomotion in hydra. Succes'sive positions taken when progressing 

 by somersaults. (From Jennings, Behavior of the Lower Organisms, The Columbia 

 University Press.) 



External Anatomy 



Hydra is a macroscopic animal, but it is relatively small. Its body 

 is quite contractile, being able to extend from a contracted length 

 of two or three millimeters to a length of eighteen or twenty milli- 

 meters. The column or body is a tubular, cylindrical trunk which 

 ordinarily stands in a vertical position. In some forms the distal 



