THE FRESH WATER HYDRA II9 



ment, size and length of the tentacles. Between the ten- 

 tacles, in the central disk, find the mouth. Note its position, 

 size, shape, etc. Make a drawing of the specimen (XS)- 



(2) Under a microscope study the tentacles. Note the 

 large circular bodies dotting the surface. These are thread 

 cells or nematocysts. Note their size, number in various 

 parts of the tentacles, and apparent structure. Watch them 

 under the microscope using low power. Tap the slide with 

 pencil and note what follows. (This usually causes the 

 extension of the lasso threads from the cells). Study the 

 lasso threads and note their uses. In a watch crystal of 

 water containing fresh hydra put an infusion containing 

 Paramecium. If the hydra uses them for food notice the 

 method of capture. Draw a tentacle showing a lasso thread. 



(3) If a bud is found study its relation to the body. 

 How does it differ from a tentacle? It is a young hydra 

 growing out from the parent hydra. What will finally 

 become of it ? This form of reproduction is called budding. 

 Hydra reproduces also by eggs, which are developed in the 

 ectoderm of the body. Cut a hardened specimen length- 

 wise with a sharp scalpel or scissors. Do you find an inner 

 cavity? What relation does it have to the mouth? Has 

 it an opening into the body ? What becomes of the indigesti- 

 ble portions of the food of the hydra? Make a drawing of 

 the longitudinal section of the hydra (X5)- 



(4) In like manner using a hardened specimen make a 

 cross section of the body just back of the base of the tenta- 

 cles. Do you find the parts found in the longitudinal sec- 

 tion ? Draw ( X5) • What seems to be the office of the cells 

 on the outside of the body? Of the cells in the sac on the 

 inside of the body ? 



