1903.] On the Nematocysts of Solids. 485 



The immunity of Solids towards the nematocysts of Ccelenterates is 

 not complete. It is probably due largely to the secretion of mucus, 

 which either acts simply as a shield, or actually prevents the discharge 

 of nematocysts. 



Even without the application of mucus, the nematocysts of a 

 hydroid are often not discharged when a polyp is plucked off the stock, 

 so that they may be swallowed by the ^Eolid intact. Or the gastric 

 fluids which are strong enough not to give up water to an undischarged 

 nematocyst, may extract water from a discharged one and so cause 

 the reinversion of the thread. 



Development of Cnidocysts. The cnidocysts are derived each from two 

 or more cells of two kinds ; (a) a single cnidoblast or central cell which 

 ingests and arranges the nematocysts; (&) an undetermined number 

 of interstitial cells which probably secrete the membranous cyst. In the 

 adult cnidocyst both kinds of cells undergo more or less degeneration, 

 leaving practically only a membranous bag containing nematocysts. 



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