128 AN INTRODUCTION TO ZOOLOGY 



Suspension from the surface film may be compared with that of a 

 needle placed on the surface of the water (122). Threads of a 

 gelatinous substance, extending out from the basal disk, help 

 sustain the body, while in some cases a large air bubble attached 

 to the foot keeps the animal afloat (123). The position of rest 

 in every case gives the Hydra the greatest opportunities for cap- 

 turing food, since in this condition it has control of a large amount 

 of territory. 



FIG. 59. Spontaneous changes of positions in an undisturbed Hydra. Side 

 view. The extended animal (i), contracts (2), bends to a new position 

 (3), and then extends (4). (From Jennings.) 



SPONTANEOUS MOVEMENTS. All the movements of Hydra 

 are the result of the expansion or contraction of the muscle fibers, 

 and are produced by two kinds of stimuli, internal, or spontane- 

 ous, and external. Spontaneous movements may be observed 

 when the animal is attached and undisturbed. At intervals of 

 several minutes the body, or tentacles, or both, contract suddenly 

 and rapidly, and then slowly expand in a new direction. Hungry 

 specimens are more active than well-fed individuals. The result 



