COELENTERATA 



127 



undif^erentmtedinterstittalceW^ : part of the cytoplasm becomes gland- 

 ular and forms a large vacuole, filled with a poisonous fluid and lined 

 with a chitinous membrane, of complicated structure. The whole of this 

 secreted body is called the nematocyst. Another part of the cytoplasm 

 round the nematocyst develops muscular fibrillae, and by their con- 

 traction the "explosion" of the nematocyst is caused, an action so 

 violent that the whole cell may be cast out of the animal as a con- 

 sequence. The external part of the thread cell develops a short sensory 



ctn. 



Id. 



bar. _ _ _ 



.>/*'^- 



Fig. 107. Nematocysts of Hydra, large penetrating type. A, Undischarged. 

 B, Discharged. From H. attenuata. After P. Schulze. C, Discharged but re- 

 tained within its thread cell. After Will. bar. barbs; hr.zv. (stippled) wound 

 in chitin {ctn.) of prey possibly caused by mechanical action of smaller barbs, 

 the continuation {con.) being due to the solvent action of a fluid from within 

 the nematocyst; c.nc. cnidocil; fil. filament; Id. lid; la. lasso; m. muscular 

 fibrils; nil. nucleus of thread cell. 



process, the cnidocily which bores through the cuticle of the musculo- 

 epithelial cell in which it lies and comes into contact with the water. 

 The stimulation of these cnidocils, for example if the animal is 

 touched by the appendage of a wandering crustacean, causes a dis- 

 turbance which is transmitted through the body of the cnidoblast to 

 the muscle fibres of the nematocyst to cause explosion, so that within 



^ This is a type of cell which preserves an embryonic character and may 

 develop into germ cells and musculo-epithelial cells as well as cnidoblasts. 



