46 HYDR^, JELLYFISHES, SEA ANEMONES, CORAL POLYPS 



anterior end and is surrounded by five to eight slender ap-. 



pendages, the tentacles. These are often as long as the 



body (Fig. 16). 

 Movements and locomotion of the hydra. — Although the 



hydra, for the major part of its time, remains fixed by its 



posterior end to some 

 submerged object, 

 yet, even in this 

 position it is capa- 

 ble of considerable 

 movement. For ex- 

 ample, the body can 

 be contracted until 

 it appears hke a tiny 

 ball or extended until 

 it reaches half an inch 

 in length. Likewise, 

 the tentacles may be 

 contracted until they 

 appear as small knobs 

 about the mouth or 

 extended imtil they 

 become very slender 

 and as long as the 

 body. Moreover, the 

 body and tentacles 

 sway about through 



^ ^ , the water in search 



Fig. 16. — Fresh-water hydra fully expanded. . . , 



of food. 

 It must be remarked that the hydra has no muscle fibers 

 like those of the higher animals with which movement is 

 brought about. But many of the ectoderm cells have their 



