148 THE INVERTEBRATA 



glandular 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 contraction the "explosion" of the nematocyst is caused, an 

 action so violent that the whole cell may be cast out of the animal as 

 a consequence. The external part of the thread cell develops a short 



Fig. 109. 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; br.w. (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; cnc. cnidocil; fil. filament; Id. lid; la. lasso; m. muscular 

 fibrils ; nu. nucleus of thread cell. 



sensory process, the cnidocil, 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 

 a single cell we have the receptor and effector organs which are 

 necessary for a very remarkable reflex action independent of the 

 nervous system. Lastly, the nematocyst may be attached to the base 



