POLYPS {CUPLIKE ANIMALS) 



23 



like tentacles compare with the length of the hydra's body ? 

 About how many tentacles are on a hydra's body ? Do all 

 have the same number of tentacles ? Are the tentacles 

 knotty or smooth ? (Fig. 35.) The hydra is usually ex- 

 tended and slender ; sometimes it is contracted and rounded. 

 In which of these conditions is the base (the foot) larger 

 around than the rest of the body ? (Fig. 34.) Smaller ? 

 How many openings into the 

 body are visible ? Is there a 

 depression or an eminence at 

 the base of the tentacles ? For 

 what is the opening on top of 

 the body probably used ? Why 

 are the tentacles placed at the 

 top of the hydra's body ? Does 

 the mouth have the most con- 

 venient location possible ? 



The conical projection bear- 

 ing the mouth is called Jiypo- 

 stome (Fig. 34). The mouth 

 opens into the digestive cavity. 

 Is this the same as the general 

 body cavity, or does the stomach have a wall distinct from 

 the body cavity ? How far down does the body cavity 

 extend ? Does it extend up into the tentacles ? (Fig. 39.) 



If a tentacle is touched, what happens? Is the body ever bent? 

 Which is more sensitive, the columnar body or the tentacles? In 

 searching for hydras would you be more likely to find the ten- 

 tacles extended or drawn in? Is the hypostome ever extended 

 or drawn in? (Fig. 34.) 



Locomotion. — -The round surface, or disk, by which the 

 hydra is attached, is called its foot. Can you move on 

 one foot without hopping ? The hydra moves by alter- 



Fig. 35. — Hydra (much 

 enlarged). 



