194 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



a system of intracellular myonemes that presumably function in 

 body contraction. Such myonemes occur in most peritrichs ; near 

 the oral end of the cell they lie immediately below the ectoplasm ; 

 aborally they converge to insert on the scopula in Opercularia, to 

 insert on skeletal elements of the basal disc in mobile forms (see 

 Trtchodinopsis y below), or to continue into the stalk in vorticellids 

 (Rouiller, Faure-Fremiet, and Gauchery, 1956c; Faure-Fremiet 

 and Rouiller, 1958b; Sotelo and Trujillo-Cenoz, 1959). In all 

 instances they are composed of very fine filaments, permeated by 

 a system of fine branching canaliculi and small vesicles. The 

 periphery of the myoneme bundle is never completely bounded 

 by membranes, but often is partially sheathed by discontinuous 

 flattened or inflated membranous sacs that on their cytoplasmic 

 side may be studded with Palade particles. The membrane 

 systems within and around the myonemes appear to be at least 

 intermittently in continuity with each other and also with 

 endoplasmic reticulum of the general cytoplasm. Both the French 

 group and Sotelo and Trujillo-Cenoz point out the parallelism 

 between this and the sarcoplasmic reticulum of vertebrate striated 

 muscle (a canalicular system around and in the Z-bands of muscle 

 fibers). 



Faure-Fremiet, Rouiller, and Gauchery (1956b) have observed 

 myonemes, with the same structure, associated with the cyto- 

 plasmic collar at the rim of the peristome, or with the peristomal 

 disc itself. The same authors state that restoration of body shape 

 following contraction is assured by a resistant cuticle underlain 

 by annular fibers, visible in appropriate thin sections as single, 

 regularly spaced, slender, dense threads. 



This survey indicates that a fairly impressive system of co- 

 ordinated contractile structures and skeletal elements is present in 

 most peritrichs. But what has been glimpsed so far is simple by 

 comparison with the incredibly elaborate architecture of a mobile 

 peritrich, Trtchodinopsis paradoxa, investigated by Faure-Fremiet, 

 Rouiller, and Gauchery (1956a). This species has a basal disc 

 supported by a complex skeletal armature, consisting of circular 

 and radial elements, and bordered by a delicate cytoplasmic rim, 

 the velum, and several rows of locomotor cilia. The prominent 

 circular skeletal component is made up of heavy, dense, homo- 

 geneous plates overlapping to form an articulated ring. Overlying 



