114 



CCELENTERATA. 



processes. The muscular processes are pressed closely 

 against the mesogloea, to which their ends are probably 

 attached. In a general way the processes run parallel to 

 the long axis of the animal though they are somewhat 

 indefinite in arrangement. Similar processes of the endo- 

 derm cells run in a circular direction, in a transverse plane. 



Fig. 48. — An Ectoderm Cell, Endoderm Cell, and a 



Nerve Cell (After Jickeli) connected with 



A Nematocyst. 



Ectoderm Cell. 



Endoderm Cell. 



,.- Vacuole, 



■^ Nucleus. 



Nerve Cell. 



The interstitial cells lie at the base of the epithelial cells 

 between their tapering ends, They appear in sections as 

 simple rounded and nucleated cells. In the living animal 

 they may be amoeboid. All over the body, but especially 

 on the tentacles, the interstitial cells give rise to the 

 cnidohlasts. These grow outwards between the epithelial 

 cells till they reach the surface. They are large ovoid cells 

 which develop in their interior a cyst containing a long 

 thread with barbs at its base and a fluid. On stimulation 

 the cyst, or nematocyst^ discharges the thread or sting which 

 has a paralysing effect on small animals. 



Other interstitial cells accumulate in a mass to form 



the germ- cells. The testis is a mass of these germ-cells 



... covered by epitheUal cells and situated under 

 Reproductive. . i ^ n., • • -i 



the tentacles. The ovary is a similar mass 



towards the aboral end of the animal. The spermatozoa 



are produced in great numbers by division of the germ-cells 



