24 HYDRA. 



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 singleT 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 supporting lamella 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 mesogloea 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 contractility is specially asso- 

 ciated with the supporting lamella ; the real con- 

 tracting elements are the muscular ' tails ' of the 

 ectoderm 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 dis- 

 charged in consequence of the irritation. Some 



