HYDRA 77 



with the high power. The nematocysts may be discharged 

 from the body and the thread thrown out, this serving to 

 secure prey or to protect the animal. The capsule of the 

 cell contains a fluid which escapes through the thread, and 

 kills or paralyzes small animals. By gently tapping on 

 the cover glass above the animal, or by introducing beneath 

 the cover some irritating fluid as dilute acetic acid or iodine, 

 one may cause a discharge of the nematocysts. Study 

 such discharged cells and observe the capsule, the thread 

 and the barbs. 



Drawings should be made to show the structures worked 

 out. 



H. Physiology. 



1. Irritability. Touch various regions of the body with 

 a needle and note results. Is hydra sensitive? Are the 

 several parts equally sensitive? Jar the table or the slide. 

 What does the hydra do? 



2. Contractility. Does the body of the animal show 

 spontaneous movements? Do the tentacles show the same 

 properties? Does the contraction of the tentacles and body, 

 or of all the tentacles, take place at the same time, or is 

 there independent movement and contraction? What 

 effects on the shape of the body do the various movements 

 produce? Do the movements seem to be definitely co- 

 ordinated or purposeful? 



3. Locomotion. Does the animal remain fastened in one 

 place in the aquarium or does it move about? On the out- 

 side of the aquarium make a mark to locate the position of 

 an individual. Make several observations after some hours 

 or days and determine whether the specimen has moved. 



4. Heliotropism, or movements in response to light. Ex- 

 amine hydra in the aquarium. Are specimens arranged 



