168 PRACTICAL ZOOLOGY. 



of the body walls, or by the action of cilia lining the 

 body cavity. Look for knob-like extensions of the 

 side of the body. Buds are formed as outgrowths 

 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 development 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 specimen ? Are buds borne on buds ? By means 

 of a pipette transfer a hydra in a large drop of water 

 to a slide. Cut two strips of thick 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 coverslip on the water, with its edges rest- 

 ing on the papers so as not to crush the specimen. 



Examine now with a quarter or one-fifth inch 

 objective. Observe 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 coverslip, and touch 

 the opposite edge of the coverslip with a piece of 

 blotting paper, meanwhile watching the specimen 

 closely. Examine carefully to see the thread-like 

 prolongations of the thread cells which have been 

 discharged as a result of the irritating acid. Small 

 animals coming in contact with the tentacles are 



