54 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



cytolysis upon contact; it is characteristic of predatory holotrichs 

 of the Order Gymnostomatida (Dragesco, 1952c). A third (on 

 the basis of electron-microscope studies) has been described only 

 from a cryptomonad flagellate (Dragesco, 1951). 



The undischarged non-toxic trichocyst of the Paramecium type 

 consists of a homogeneous spindle- or carrot-shaped body of 

 low density, situated with its long axis perpendicular to the pellicle 

 surface. Upon discharge it becomes a long, uniform or tapering 

 shaft with a conspicuous cross-banding of about 55 rmi, capable 

 of fraying into striated longitudinal filaments. A few micrographs 

 of high resolution show a two- or four-band striation within the 

 major period, and a super-period encompassing four of the major 

 periods often appears. General similarity to the structure of 

 collagen fibrils (Hodge, 1960) is striking. Early studies of whole 

 discharged trichocysts indicated that the striated structure was a 

 hollow cylinder, since it appeared very flat in the dried specimens. 

 However, subsequent studies of sectioned material suggest that 

 the shaft is solid, with its characteristic periodicity observable 

 throughout; apparently the extreme flattening of dried shafts 

 reflects a high water content. Jakus's (1945) extensive investiga- 

 tions showed that the trichocyst material was a protein similar to 

 collagen in structure and extensibility, in swelling properties at 

 pH's far from its isoelectric range, and in resistance to tryptic 

 digestion. But other reactions were not typical of pure collagen, 

 and the 55-m^ periodicity of trichocysts is significantly smaller 

 than the 64-m/x period of collagen. 



Since unextruded trichocyst bodies clearly do not contain 

 folded-up, preformed trichocyst shafts, the problem of discharge 

 is particularly significant. The process occurs in milliseconds. 

 Rouiller and Faure-Fremiet (1957a) succeeded in obtaining thin 

 sections of Frontonia showing trichocysts in the process of dis- 

 charging internally. Earliest stages show, within the homogeneous 

 matrix, islands of organization where periodicity of 12 to 15 m^ 

 is observable. In later stages, the periphery of the shaft is very 

 regularly striated at about 24 m/x, and increasing zones within 

 the matrix show the finer periodicity. Parts of a single trichocyst 

 reach the mature, 55-m/x period while other parts are still organiz- 

 ing. The process involves (1) the unmasking or creation of 

 molecular groupings capable of binding heavy metals such as 



