CONCLUSIONS 225 



the extraordinarily dramatic changes of form in Stentor is attractive. 

 But Blepharisma has similar kinetodesma. Perhaps Blepharisma is 

 contractile and doesn't know it — no less silly explanation 

 suggests itself. 



The sucking tentacles of Ephe/ota are said to be extensile and 

 contractile. They lack the filamentous axis of the prehensile 

 tentacles and have instead an orderly arrangement of tubular 

 fibrils in sheets and crests. The haptonema of Chrysochromulina 

 are, without question, contractile; no structure other than mem- 

 branes and tubular fibrils has been discerned in them. The 

 accessory fibers of the Peranema rodorgan, strongly suspected of 

 moving that body, are sheets that appear fibrillar. The contractile 

 axostyle of Pjrsonympha consists exclusively of tubular fibrils 

 resembling in arrangement the kinetodesmos of Stentor (but the 

 significance of the adjacent, filamentous paraxostyle is unknown). 

 Only tubular fibrils have been found associated with ciliate 

 contractile vacuoles. Other functions might be suggested for 

 these fibrils, but something has to hold injector and ejector canals 

 closed while ampullae or vacuoles are filling ; simple elasticity can 

 hardly suffice. Retrociliary fibers of ophryoscolecids are reason- 

 ably suspected of being contractile (for organisms with relatively 

 rigid bodies, they are remarkably well supplied with possible 

 contractile systems, like protozoan arthropods). 



None of the striated fibrils can be seriously suspected of being 

 actively contractile on present evidence — even though Stentor 

 kinetodesma appear to arise as banded fibrils. Most of them fit 

 agreeably enough into a provisional category labeled "elastic 

 supporting and anchoring structures". 



Many of the tubular-fibril bodies certainly do not themselves 

 contract. Often these are composed of fibrils in precise geometric 

 array — unlike the slightly curved laminae of the Pyrsonympha 

 axostyle and the heterotrich kinetodesmos. Such are the skeletal 

 protein fibers of the gymnostome pharyngeal basket, the rods of 

 the Peranema rodorgan, the membranelle roots of Stentor. The 

 retrociliary fibers of ophryoscolecids are likewise precisely packed, 

 and this might constitute an argument against the assumption that 

 they are contractile, were not actin and myosin filaments in some 

 muscles hexagonally packed. 



The peripheral fibrils of Tetramitus, euglenoids, trypanosomids, 



