CHAPTER 19 



TRICHOCYSTS 



Trichocysts and Nematocysts 



T 



■ HE ECTOPLASM OF numcrous species of ciliates contains extrusible 

 I bodies known as trichocysts.^- ^^- ^ These are fusiform bodies oriented 

 m obliquely or at right angles to the surface. Among various proto- 

 zoans, they may be uniformly distributed over the surface, as in Paramecium, 

 or restricted to certain regions, as in the proboscis of Dileptus. They may be 

 projected outward on the ends of long tentacles in feeding specimens of 

 Actinoholina. 



The trichocysts appear to have their origin within or close to the macro- 

 nucleus in Frontonia.-^ From here they make their way to their definitive 

 position in the ectoplasm, completing their differentiation as they migrate. 

 In Prorodon, the fully formed trichocyst appears to have the form of a 

 cylindrical sac containing an elongated, coiled filament. In Paramecium the 

 undischarged trichocysts may be seen to possess an oval body proper, about 

 2-3 IX long by 2/3 /a in diameter, and a cap-covered tip of slightly smaller 

 diameter. On discharge, the tip with its covering cap is separated from the 

 body proper through the elongation of a shaft. ^■''' ^^- " 



The trichocysts can readily be induced to discharge their filaments through 

 chemical (acid or base), mechanical (pressure), or electrical (condenser dis- 

 charge or induction shock) stimulation. With electrical stimulation there is 

 an increase in the number discharged as the strength of the stimulus is in- 

 creased.-- The total discharge is very rapid, occurring in a matter of a few 

 milliseconds. The discharged trichocyst is needle-like in general form, being 

 ten or more times as long as the undischarged body. The discharged tricho- 

 cysts of Paramecium may be 40 /u, long. The trichocysts of Paramecium show 

 birefringence, indicating an orientation of elongated submicroscopic particu- 

 lates in the long axis of the organelle.-^ A study of the extended threads of 

 discharged trichocysts of Paramecium, with the electron microscope reveals 

 them to be composed of a shaft showing a periodic transverse banding at 

 intervals of 600 to 650 a (Fig. 247). The characteristics of the shaft indi- 

 cate it to be a thin cylindrical membrane composed of elongated protein 

 chains showing a periodic structure somewhat resembling that of collagen 

 fibers. The over-all striation of the organelle would then be a consequence 

 of the alignment of these fibers in phase with one another. At the tip of the 

 shaft there is a relatively opaque, thornlike body indicative of the presence of 

 a dense proteinaceous structure or of elements of high atomic mass.^- '^ 



Little or nothing is known of the mechanism of trichocyst discharge or 

 to what extent trichocysts are activated other than in direct response to en- 

 vironmental stimuli. The trichocysts appear connected with the silver-line 



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