110 



COLLEGE ZOOLOGY 



DIschorged thread 

 Dissolved chitin 

 Chitin 



Cnidoblast 

 Fibril 



Undischarged 

 nematocyst 



Nucleus 



Nematocyst penetrating the sclerotized 

 covering of an insect 



Nematocyst discharged 

 Figure 54. Penetrant nematocyst of the hydra. Note cnidoblast on right. 



tube which is coiled within is then everted. 

 First the base of the tube with the large 

 spines appears, and then the rest of the tube 

 rapidly turns inside out. Penetrants are able 

 to penetrate the tissues of other animals 

 only when they are discharged with great 

 speed, and before eversion is completed. 

 Even the extremely firm sclerotized covering 

 of insects may be punctured by these struc- 

 tures. 



For a long time, touching the cnidocil 

 was considered the cause of the explosion of 

 the nematocysts, and for this reason the 

 cnidocil is known as the trigger. One can 

 easily prove, however, that mechanical 

 shocks have no influence upon the nemato- 



cysts. The discharge of the capsule is prob- 

 ably triggered by a suitable stimulus, pre- 

 sumbly chemical, applied to the cnidocil. 

 However, the mechanism of setting off the 

 explosion is still not understood, but once 

 the process has started, the energy for ever- 

 sion of the thread is provided by the progres- 

 sive swelling of the shaft wall itself. It has 

 been clearly shown that as the thread everts, 

 it not only increases in diameter, but also 

 markedly elongates. In the discharging ne- 

 matocyst, the inverted filament comes into 

 immediate contact with external water only 

 at the advancing tip of the shaft. In isolated 

 and dried nematocysts, discharge will not 

 proceed unless the advancing tip is supplied 



