PROTOZOA AS CELLS 53 



its components have to recognize, among other things, which end 

 is up. The problem clearly is not limited to cells with flagella, or 

 even with centrioles, but it is significant to note that Gall (1961) 

 has found some evidence of a consistent relationship between 

 asymmetry and polarity in sperm centrioles. 



E/ Cetera 



Although trichocysts are hardly a common cell organelle, they 

 occur in many ciliates and flagellates. Like flagella, they attracted 

 the attention of early electron microscopists because they were 

 accessible to study by techniques other than sectioning. A review 

 of their structure here will provide a brief introductory notion of 

 the sorts of fibrous elements that protozoa are capable of elabora- 

 ting and that constitute an important aspect of differentiation 

 in many of them. 



Trichocysts are minute organelles present in the peripheral 

 cytoplasm; under the influence of many sorts of stimuli they 

 "explode", discharging long slender filaments through the cell 

 surface into the environment, or sometimes, abnormally, into the 

 internal cytoplasm. At least two kinds and perhaps three kinds 

 of true explosive trichocysts occur. The most familiar is the 

 conspicuous, non-toxic rod expelled by Paramecium and a 

 number of other holotrichous ciliates (Jakus, 1945; Jakus and 

 Hall, 1946; Pease, 1947; Knoch and Konig, 1951; Beyersdorfer 

 and Dragesco, 1 952a, 1 952b ; Dragesco, 1 952a ; Kriiger, Wohlfarth- 

 Bottermann, and PfefTerkorn, 1952; Nemetschek, Hofmann, and 

 Wohlfarth-Bottermann, 1953; Wohlfarth-Bottermann, 1953a; 

 Potts, 1955; Sedar and Porter, 1955; Rouiller and Faure-Fremiet, 

 1957a), and also found in a flagellate (Dragesco, 1952b). The 

 function of these trichocysts is enigmatic, in spite of considerable 

 study of their structure. Whether the dense fringe of discharged 

 trichocysts produced by Paramecium^ provides a protective cover 

 is a moot point. Some authors have suggested that they have 

 adhesive properties serving for attachment, but this has not been 

 shown in recent studies. In media of relatively high salt concen- 

 tration, salts are present as granular deposits in the shafts of 

 discharged trichocysts, and Wohlfarth-Bottermann (1953a) 

 suggests that they serve an osmoregulatory function. 



A second type of trichocyst is toxic, causing paralysis and some 



