190 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



Metaradiophrya gigas is one of a group considered by de Puytorac 

 to be derived from thigmotrichs. It is a large, uniformly ciliated 

 organism bearing an anterior hooked holdfast apparatus and a 

 cortical system of longitudinal fibers assumed to be skeletal. It 

 has a silverline pattern of rather irregular polygonal plaques, and 

 a ridged ectoplasm. The troughs between ridges are, according 

 to de Puytorac, occupied by membrane-limited alveoli, overlain 

 by a continuous outer membrane. Silver is deposited, just as in 

 the tetrahymenids, along lines separating adjacent alveoli. 



Light-microscope study of Metaradiophrya shows that the 65 

 skeletal rods, which attach anteriorly to the holdfast, parallel the 

 kineties on their right sides and that each consists of a solid, heavy 

 fiber tapering as it extends posteriad and finally being replaced by 

 a bundle of smaller fibers. They are anisotropic, and are composed 

 of protein. Additional systems of fibers attached to the holdfast 

 are assumed to be contractile. The electron microscope reveals a 

 pattern of remarkable complexity. The posterior portions of the 

 skeletal fibers consist of dense fibrils arising individually from 

 kinetosomes along the kinety and passing to the right and anteriad 

 to join a loose bundle of overlapping units, exactly like the 

 kinetodesma of Paramecium and the tetrahymenids. In Meta- 

 radiophrya the tapering kinetodesmal fibrils are relatively long; 

 the bundle at any one level contains seven to 15 of them. Begin- 

 ning at a specific level, successive new kinetodesmal fibrils, 

 instead of remaining separate, fuse with each other to form a 

 homogeneous solid rod of steadily increasing diameter. For some 

 distance, the diminishing bundle of fibrils left over from more 

 posterior kinetosomes runs parallel to the widening solid rod. 

 Anteriorly, some of the kinetodesma reach the shaft of the 

 holdfast, which is composed of a similar dense material. Kineto- 

 desmal fibrils, the fused rod, and the holdfast all show a periodic 

 banding at about 20 m/x, but the relative width of light and dark 

 stripes within the period differs in the smaller and larger fibers. 

 The pellicle over the shaft and surrounding the protruding hook 

 of the holdfast is at least double-layered, and fine longitudinal 

 fibrils are arrayed against its inner surface. 



In addition to the kinetodesmal fibrils, each kinetosome is the 

 point of origin of other fibrous elements. In parts of the anterior 

 region of the body, these have the form of tubular fibrils, about 



