20 HYDEA. 



smaller kinds occur in the body-wall, though 

 less abundantly than in the tentacles, 

 iv. The nerve- cells. Certain small stellate cells 

 found in the ectoderm are described as nerve- 

 cells. A connection between, them and the cells 

 enclosing the nematocysts has been traced, and 

 it is possible that they are concerned in the dis- 

 charge of the latter. 



b. The endoderm consists of a single layer of large 



granular and vacuolated cells lining the cavity of 

 the tentacle. Many of these bear flagella. 

 Focus the middle of the thickness of the tentacle : note the 

 flagella of the endoderm cells, and the stream of nutrient 

 particles up and down the cavity of the tentacle. 



c. The mesogloea is the thin gelatinous layer between 



the ectoderm and the endoderm. 

 Focus the side of the tentacle at the middle of its thickness, 

 and the mesoglcea will be seen as a thin transparent layer 

 between the ectoderm and the endoderm. 



Sharply tap the microscope so as to cause the Hydra to 

 retract its tentacles. 



In the contracted condition of the tentacle 

 the ectoderm and endoderm are both strongly 

 wrinkled transversely, but the mesogloea is not. 

 This shows that contractihty is specially asso- 

 ciated with the mesogloea ; the real contracting 

 elements are the muscular ' tails ' of the ecto- 

 derm cells applied to or imbedded in its 

 substance. 



d. Discharge of the nematocysts. 



Focus one of the tentacles with the high power : then 

 place a drop of acetic acid on the slide and allow it to run 

 under the cover -glass. 



As the reagent reaches the tentacles these 

 will be retracted and their nematocysts discharged 

 in consequence of the irritation. Some of the 



