86 THE BIOLOGY OF THE ANIMAL 



2. Influence of light {heliotropism). — Put several 



hydras into a beaker or a tumbler, give them 

 time to become fixed in position, mark on the 

 glass the position of each individual, then 

 cover the top and sides of the beaker with 

 thick opaque paper or cloth. At one side 

 make a hole about one-half inch in diameter 

 through the paper to the glass, so as to admit 

 the light. Set the beaker in a well -lighted 

 place, but not in direct sunlight, and examine 

 after a few hours' exposure. Have the hydras 

 changed their positions? If so, what relation 

 does their present position bear to that of the 

 hole in the paper ? Are hydras sensitive to a 

 small amount of light ? 



3. Co-ordination. — Do the movements of the hydra 



seem to be made at random or for a purpose ? 

 Do any of your observations lead you to think 

 that the animal performs intelligent actions ? 



Micyi'oscojpio Structure. — Put on the high power and re- 

 view all of the doubtful points of morphology 

 and physiology, and in addition investigate the 

 following topics : Is the stomach cavity or en- 

 teron continuous throughout the body proper ? 

 Can you discover an intestine? Does the enteron 

 extend into the tentacles ? Is there an opening 

 in the foot ? Can you find any opening, the 

 anus, through which waste matter may leave 

 the body ? Study the structure of the body-wall, 

 and note the two cell-layers, the outer or ecto- 

 derm and the inner or entoderm, and between 

 the two the supporting layer or mesogloea. 

 Does the ectoderm cover the entire surface of the 



