TEICHOCYSTS. 35 



(fig. 21), which on the slightest touch shrink into spiral folds, and 

 again straighten themselves to their full extent. The agent by which 

 this contraction is effected is a delicate spiral thread contained in the 

 interior of the flexible stem, regarded by Ehrenberg as a muscular 

 filament ; its muscular nature is, however, doubted by Dujardin, who 

 regards this as being one of the most inscrutable points connected with 

 their economy. That a central canal exists in the retractile stem is 

 generally admitted, and likewise that it contains a fleshy substance less 

 transparent than the rest of the tube ; but, according to M. Dujardin's 

 observations, it is the diaphanous substance around this central cord 

 that contracts, and as it forms a band one border of which is much 

 thicker than the other, the more powerful action of the thicker portion 

 gives that helical curvature to the stem which forms so remarkable a 

 feature in its movements. 



(70). When certain species of Infusoria (e.g.Bursaria leucas) are ex- 

 amined under a sufficiently high power, minute fusiform corpuscles may 

 be detected thickly imbedded in the integument. These bodies are per- 

 fectly colourless and transparent ; they are about -g^^-th f an i ncn 

 long, and may easily, even without any manipulation, be witnessed at 

 the margin, where they are seen to be arranged perpendicular to the 

 outline of the animalcule, while on the surface turned towards the 

 observer their extreme transparency and want of colour render them 

 invisible against the opake background, and it becomes necessary to 

 crush the animalcule beneath the covering-glass, so as to press out the 

 green globules which it contains, in order to bring the fusiform bodies 

 into view. To these bodies it has been proposed to give the name of 

 trichocysts. 



As long as the animalcule continues free from annoyance, the tricho- 

 cysts undergo no change ; but when subjected to external irritation, as 

 occurs during the drying away of the surrounding water, or the appli- 

 cation of acetic acid or other chemical irritant, or the too forcible action 

 of the compressor, they become suddenly transformed into long fila- 

 ments, which are projected from all parts of the surface of the animal- 

 cule ; these filaments have been mistaken for cilia by Cohn and Stein. 

 The rapidity with which their evolution is effected, joined with the 

 great minuteness and transparency of the object, renders it extremely 

 difficult to follow it. 



(71.) It is not difficult, by rapidly crushing the animalcule, to force 

 out some of these organs in an unchanged state. If the eye be now 

 fixed on one of the isolated trichocysts, it will most probably be seen, 

 after the lapse of a few seconds, to become all at once changed (with 

 a peculiar jerk, as if by a sudden release from some previous state of 

 tension) into a little spherical body. In this condition it will probably 

 remain for two or three seconds longer, and then a spiral filament will 

 become rapidly evolved from the sphere, apparently by the rupture of a 



D2 



