CONCLUSIONS 217 



electron micrographs of multiple surface membranes requires a 

 considerable measure of luck as well as high resolution, and is 

 easier if pellicle fragments have been examined as well as sections ; 

 it is hardly surprising that investigators not specifically looking 

 for this sort of evidence have given ambiguous reports. 



But the classical ciliate silverline system is not a mirage. The 

 alveoli of the hymenostomes make morphological sense, and the 

 most reasonable supposition at present is that a like situation will 

 be found to obtain in other forms with equally conspicuous 

 patterns demonstrated by the same methods. 



At the moment, the only function known for the silverline 

 system is to aid the protozoologist in identification and develop- 

 mental studies of his cells, but presumably the ciliate has some 

 less altruistic reason for preserving it. The pellicular alveoli 

 spread out between meridional rows of organelles — cilia and 

 trichocysts or mucigenic bodies — emerging from the cell 

 surface. In Paramecium they are transversely dissected into orderly 

 compartments corresponding to ciliary units ; in the tetrahymenids 

 they extend longitudinally past several to many ciliary units along 

 each kinety, transverse interruptions being apparently random and 

 variable in time. Their general distribution clearly is dictated by 

 the disposition of kineties. Yet something else is involved in the 

 specific silverline pattern, for in many species it assumes the form 

 cf a network in areas between kineties, or shows highly charac- 

 teristic configurations in parts of the body that lack ciliature and 

 infraciliature. Specific variations conceivably may be as incidental 

 and non-adaptive as are some color variants in other organisms, 

 but the alveolar system itself must have a highly significant 

 adaptive value. 



The alveoli may contribute to the rigidity of the pellicle. Para- 

 mecium and the tetrahymenids upon appropriate treatment readily 

 yield ghosts or extensive surface fragments, attesting to a con- 

 siderable structural integrity in the pellicle. Stentor and Spiro- 

 stomum, by contrast, do not; their surfaces have to be highly 

 deformable to permit the extreme contraction or extension of the 

 cell. Randall and Jackson (1958) showed that very minutely 

 dissected alveoli or strings of vesicles are present under the 

 superficial unit membrane in some parts of Stentor; typical 

 Paramecium-like alveoli occur only near the adoral zone. 



