STRUCTUEE OF TENTACLE 19 



smaller kinds occur in the body- wall, though 

 less abundantly than in the tentacles, 

 iv. The nerve-cells. Certain small steUate 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 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 tMchness 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 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 supporting lamella 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 supporting lamella is not. 

 This shows that the latter is the contractile 

 layer ; or at any rate that it contracts to a greater 

 extent than either ectoderm or endoderm. 



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 allozu it to run under 

 the cover-glass. 



As the reagent reaches the tentacles these 

 will be retracted and their nematocysts discharged 



c 2 



