EFFECTOR MECHANISMS 371 



as soon as the cilia begin beating; conversely, the animals will come to a 

 dead stop as soon as the cilia become motionless. In the case of a larger 

 animal, 1 cm in diameter and moving with a speed of 1 cm/sec, it has been 

 calculated that the animal will glide some 20 cm before coming to rest 

 once the cilia stop beating. And from a position of rest it will take some 

 considerable time to attain maximal velocity (51). The absolute speed of 

 ciliated organisms is slow. Some examples are: Paramecium, 1-3 mm sec; 

 Monas, 0-26 mm sec. 



Ciliary locomotion is suitable for small animals which can get along at 

 low speeds. Small turbellarians and planktonic larvae suggest themselves 

 as examples. Streamlining, such as many ciliates possess, reduces the 

 water resistance which the animals encounter. For larger invertebrates, 

 such as ctenophores, to obtain equal facility of movement in all directions 

 a low specific gravity is necessary, and this is attained by high water content 

 (94-73 ° () in Pleurobrachia). Where range and power of movement are 

 required, cilia and flagella prove inadequate, and heavy animals depend 

 upon muscles for motive force (51, 72. 76. 101). 



Trichocysts in Protozoa 



Trichocysts are peculiar explosive bodies occurring in the ectoplasm 

 of ciliates. They are fusiform or succular in shape, with the long axes more 

 or less perpendicular to the external surface. When activated, each tricho- 

 cyst discharges a long filament through a pore in the pellicle. Trichocysts 

 are evenly distributed over the surface of the body, as in Paramecium; or 

 are restricted to special areas, the proboscis of Di/eptus, for example. 

 Ordinary trichocysts, such as those of Paramecium, have a uniform appear- 

 ance. In other ciliates, such as Prorodon, there are structures known as 

 cnidotrichocysts, containing an elongated coiled thread which can be 

 everted. 



Effective stimuli causing discharge of the trichocysts are chemical 

 (acid and alkali), mechanical (pressure) and electrical (discrete shocks). 

 Explosion takes place rapidly, within a few milliseconds. Owing to the 

 small size of these structures (each trichocyst is only a few micra in dia- 

 meter) little is known about the mechanism of discharge, but it is thought 

 that hydration of protein material or an osmotically controlled flow of 

 water into the trichocyst may be responsible. 



Several functions have been ascribed to these structures. In some species, 

 of which Paramecium is an example, they probably serve as organs of 

 attachment. But the trichocysts of predatory forms such as Dileptus are 

 employed in capturing food. An encounter with prey causes the trichocysts 

 to discharge and the prey is paralysed instantly, apparently by a toxin 

 associated with the discharged filaments. 



Trichocysts of typical pattern occur in many flagellates. In cryptomonads 

 the gullet or furrow is often lined with trichocysts which discharge their 

 threads when the organism is exposed to noxious stimuli. This is not a 

 feeding reaction, since these flagellates are holophytic or saprophytic. 



