224 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



filamentous layers of the ophryoscolecids seem possible candidates 

 for the distinction of being contractile. Most of these are con- 

 tinuous with tracts or bundles that depart from the layers, and in 

 several instances aggregations of tiny membranous bodies are 

 described as following the contours of the filamentous structures. 



Of all of these filamentous organelles, only the ecto-endoplasmic 

 boundary of astomes and some of the body myonemes of peritrichs 

 have been described as directly and consistently associated with 

 kinetosomes. Oddly enough, for several of the others, no inser- 

 tion on any other body component has yet been detected. 



The conclusion that some of these filamentous organelles are 

 contractile seems logical and inevitable, but just as inevitably it 

 must be recognized that these cannot be the only organelles 

 specialized for contraction in the protozoa. Unless secondary 

 filaments or the matrix in flagella are the exclusive sites of mole- 

 cular shortening or sliding, the tubular fibrils must be involved. 

 Other organelles composed of tubular fibrils are equally suspect. 



We may consider first the well-documented morphology of 

 Stentor (see also the thoughtful discussion in the paper by Randall 

 and Jackson, 1958). Observers of living and stained cells in the 

 light microscope have identified as myonemes the structures 

 recognized by electron microscopy as kinetodesma. Both 

 M-bands and kinetodesma, as stated earlier, remain straight under 

 conditions of body contraction and maximal extension. Slippage 

 against each other of the overlapping fibrils in each row, firmly 

 attached to kinetosomes at one end and individually free on the 

 opposite side of the kinetodesmos at the other, could result in 

 very significant shortening and lengthening of the whole fiber, 

 although the availability to each fibril of only two others to hang 

 on to makes the situation rather different from that in striated 

 muscle. Perhaps filaments of two different kinds are essential for 

 the slipping-zipper model, but since we know nothing whatever 

 of the chemical composition of kinetodesma, we can conveniently 

 shrug off this difficulty for now. 



In Spirostomum and Condylostoma — both contractile — only 

 kinetodesma, and no M-bands, have been reported to date, but 

 more detailed study of both genera is needed. The suggestion of 

 Randall and Jackson that the two systems (filamentous and 

 tubular) may complement and reinforce each other to accomplish 



