34 



HANDBOOK OF INVERTEBRATE ZOOLOGY. 



f. Make a drawing of a hydranth, showing all these 

 points. 



IV. Examine a tentacle with a higher power, and notice 

 that it is made up, like the hydroraulus and hydranth, of 

 three layers. 



a. The ectoderm forms a thin, highly clastic, and con- 

 tractile layer (Fig. 19, ) which contains numbers of 

 small oval bodies, the nematocysts or lasso cells. 



b. The supporting layer (Fig. 19, b) forms a thin 

 transparent line between the ectoderm and endoderm. 



c. The endoderm (Fig. 19, c) completely tills the 

 centre of the tentacle, and consists of large cells with 

 a distinct cell wall and a large granular central nucleus, 

 which is attached to the wall by irregularly branched 

 protoplasmic threads. 



-^ F i<;. 10. Part of a tentacle, magnified two hundred and 



fifty diameters. 



a. Ectoderm. 6. Supporting layer, c. Endoderm. 



d. In a surface-view notice the longitudinal 

 muscular fibres, which lie just outside the sup- 

 porting layer of the tentacle. 



e. In a living specimen notice that the tenta- 

 cles are retracted and extended by contractions 

 of the elastic substance of the body, and not by 

 the folding of the tentacle upon itself. 



f. Make a drawing of a tentacle, showing these 

 points. 



g. Crush a portion of a tentacle, by pressure 

 on the cover-glass, and examining it with a high 

 power, notice the nematocyst* which are thus set 

 free. When fully extended, each (Fig. 20) con- 

 no, in. g j stg Q f a sma ]i ova ] ca p su le, and a very long 



slender filament, with three barbs or hooks near its prox- 

 imal end. 



