STEUCTUEE OF TENTACLE 19 



as nerve-cells. He has traced a connection 

 between them and the cells enclosing the 

 nematocysts, and suggests that they are con- 

 cerned in the discharge of the latter. 



b. The endoderm consists of a single layer of large 



granular cells lining the cavity of the tentacle. 

 Several of these bear flagella, by which currents 

 are caused along the tentacle. 



Focus the middle of the thickness of the tentacle : note 

 the movements of the flagella of the endoderm cells, and the 

 stream of nutrient particles up and down the cavity of the 

 tentacle. 



c. The mesoderm is the thin structureless layer between 



the ectoderm and endoderm. 



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

 and the mesoderm will be seen as a thin transparent line of 

 demarcation betioeen the other two layers. 



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 mesoderm is not. This 

 shows that the mesoderm is the contractile layer. 



d. Discharge of the nematocysts, 



Fociis one of the tentacles with the high power : then place 

 a drop of magenta on the slide and allow it to run under the 

 cover-glass. As the magenta reaches the tentacles these will 

 be retracted and their nematocysts discharged in consequence 

 of the irritation. Some of the nematocysts will remain in 

 the ectoderm with their threads projecting freely ; others will 

 separate completely. Examine, and draw them, showing the 

 shape of the capsules, the threads with their enlarged basal 

 portions, and the barbs. Search for specimens ivith partially 

 discharged threads ; and note that th& threads are turned 

 inside out in the process of discharge. 



c2 



