170 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



each kinety in most, but probably not all, ciliates ; it typically lies 

 to the ciliate's right (clockwise to an observer looking on the 

 anterior pole from the outside of the cell) of the kinetosomes. 

 This observed asymmetric relationship, stated by Chatton as the 

 "rule of desmodexy" (Lwoff, 1950), confers asymmetry and 

 polarity upon the kinety and upon the cell. 



Many functions have been ascribed to these various structures, 

 including those implied by the terms "neuromotor" and "neuro- 

 formative". We shall see that the evidence does not yet justify 

 any general conclusions in this regard; some possibilities, still 

 lacking experimental testing, are mentioned later. 



None of the ciliates considered to be most primitive has been 

 studied in detail with the electron microscope. We therefore shall 

 select as ciliate types some representatives of the moderately 

 advanced holotrich Order Hymenostomatida. As the most 

 familiar of all ciliates and the one that has received most attention 

 from electron microscopists, Paramecium might appropriately 

 introduce this section. Its cortical morphology, however, is 

 somewhat easier to understand if we consider first a group of 

 related hymenostomes that are almost as familiar, Tetrahymena, 

 Colpidium, and Glaucoma. The tetrahymenids not only are typical 

 of the order but are considered by Faure-Fremiet (1950) and 

 Corliss (1956) to represent a possible stem group important in the 

 ancestry of higher ciliates. 



Subclass Holotrichia 



Order Hymenostomatida 



Tetrahymena, Colpidium, and Glaucoma, and species within the 

 genera, differ in the number and arrangement of kineties and 

 silverlines, readily demonstrated by light microscopy. At the 

 electron-microscope level, however, no important differences are 

 observed; the ultrastructural basis of cortical patterns is the same 

 in all three. Early electron-microscope studies were those by 

 Wohlfarth-Bottermann and Pfefferkorn (1953) on dried, exploded 

 cells of Colpidium, and by Metz and Westfall (1954) on pellicle 

 fragments of Tetrahymena pyriformis, obtained by ultrasonic 

 bombardment of lightly fixed cells. More recently, Elliott and 

 Tremor (1958) have reported briefly on the contact zone in 



