172 Practical Zoology. 



3. That the body wall consists of two layers, which are con- 

 tinuous with the corresponding layers of the tentacles. How do 

 these layers differ from each other ? 



The body is, then, a double-walled sac, and the tentacles are 

 simply extensions of this sac. Watch the movements of the 

 different parts of the body. Can hydras move from place to 

 place? If so, how is this accomplished? Look in the body 

 cavity for foreign matter which has been taken in through the 

 mouth as food. Look also for minute particles obtained by the 

 digestion of such food matter. These particles may often be 

 seen in motion, caused by contractions of the body walls, or by 

 the action of flagella lining the body cavity. Look for knoblike 

 extensions of the side of the body. Buds are formed as out- 

 growths of the body walls with a cavity continuous with the 

 body cavity. Place in a dish by itself with some aquatic plants, 

 a hydra bearing buds, and watch from day to day the develop- 

 ment of the bud into the form of the parent. Observe the free 

 circulation of food material from the parent to the bud. Watch 

 the formation of tentacles. Look also for a thinning away of 

 the free end of the bud. 



What is the greatest number of buds found on any one speci- 

 men ? Are buds borne on buds ? By means of a pipette trans- 

 fer a hydra in a large drop of water to a slide. Cut two strips 

 of paper a quarter of an inch long and one sixteenth of an inch 

 wide, and lay one on each side of the drop of water. Carefully 

 place the cover slip on the water, with its edges resting on the 

 papers so as not to crush the specimen. 



Examine now with a quarter or one-fifth inch objective. Ob- 

 serve the cells of which the body walls are composed. Note 

 the knotty appearance of the tentacles. In these projections 

 of the tentacles and in the walls of the body are certain distinct 

 oval cells, the thread cells. Place a drop of acetic acid on the 

 slide at one edge of the cover slip, and touch the opposite edge 

 of the cover slip with a piece of blotting paper, meanwhile watch- 

 ing the specimen closely. Examine carefully to see the thread 



