Trichocysts and Nematocysts 655 



system, which some consider to have a nerve-net type of conductile function. 

 The mechanism of discharge probably involves either a hydration of some 

 protein within the trichocvst or a rapid osmotic inflow of water on excitation 

 of these organelles. 



The role of trichocysts in protozoans is still not definitelv established. 

 They have been considered by many to possess a protective function. This 

 appears doubtful in such species as Paramecium, which are readily ingested 

 by Didinium even after the Paramecium has extruded many of its tricho- 

 cysts. More probably the trichocysts function here as organs of attachment.^* 

 In ciliates such as Dileptiis and Actinoholina, prey coming in contact with a 

 region of the body bearing trichocysts appears to be paralyzed instantly, as if 

 a toxin were associated with the discharged trichocyst. 



Among the dinoflagellates, Polykrikos and I^ematodinium, there are tri- 

 chocysts that bear a very close resemblance structurally to the nematocysts 

 of coelenterates and are frequently called nematocysts. Trichocysts in va- 

 rious stages of differentiation have been found within these dinoflagellates. 



Fig. 247. Electron micrograph of discharged Parameciutn trichocysts, shadow-cast 

 with chromium. From Jakus and Hall.'" 



Another type of effector organelle very closely resembling the coelenteratc 

 nematocyst is the polar capsule of sporozoans of the groups Myxosporidia and 

 Actinomyxidia. Within the digestive tract of host organisms, the action of 

 the digestive juices induces these capsules to discharge their spirally-coiled 

 hollow thread after the fashion of a nematocyst. The discharged filament 

 serves as a means of temporary anchorage of the parasite to the host tissues. 



NEMATOCYSTS 



The nematocysts of coelenterates are extremely small intracellular struc- 

 tures of spherical, oval, or spindle form. They consist of a capsule within 

 which a hollow thread, continuous with an end, is introverted and coiled.'^- ^^ 

 The discharge of the nematocyst involves a rapid expulsion of the thread- 



