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 hydrocaulus and hydranth, of 

 three layers. 



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

 tractile layer (Fig. 19, a) 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 fills 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. 



-T Fig. 19. Part of a tentacle, magnified two hundred and 



Ni fifty diameters. 

 a. Ectoderm. 



h. 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. 



/. 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 nematocysts which are thus set 

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

 sists of a small oval eapsule, and a very long 

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

 imal end. 



FlO. 19. 



