198 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



low magnifications of the light microscope, it is a choice subject 

 for combined experimental and ultrastructural studies. The first 

 necessary approach has been made in the critical, detailed electron- 

 microscope investigation of S. polymorphus by Randall and 

 Jackson (1958). 



Some species of Stentor are colored by pigment granules 

 distributed in regular longitudinal rows. S. polymorphus, lacking 

 pigment, has alternating granular ridges and clear furrows in 

 which lie the kineties. When attached to a substrate by its 

 posterior end and fully extended in the familiar elongate trumpet 

 shape, Stentor may reach a length of 2 mm. When detached and 

 swimming, its shape is conical and its length about 0-7 mm. 

 Maximal contraction reduces it to a spheroidal body about 

 0-25 mm in length. Its cell volume in the superextended state 

 may be four times that at full contraction. According to Randall 

 and Jackson, chemicals that inhibit contraction also interfere with 

 adequate fixation, so all electron micrographs are of sections of 

 animals at least partly contracted by the initial impact of the 

 fixative. 



The cell surface, in addition to being longitudinally ridged, is 

 horizontally wrinkled in fixed cells. In most areas it consists of at 

 least two unit membranes, or possibly of a single one underlain 

 by a layer of very small flattened alveoli (smaller and less uniform 

 than those composing the silverline mosaic of the hymenostomes ; 

 unlike the latter, the Stentor pellicle disintegrates completely under 

 sonic bombardment or detergent treatment). Near the buccal 

 cavity, however, two outer membranes are elevated in scallops 

 above the ridged ectoplasmic surface which is bounded by one, 

 or possibly two, additional membranes. The picture in transverse 

 sections here is very like Paramecium. Another figure reveals the 

 presence of a parasomal sac adjacent to a cilium base. The 

 cytoplasm of Stentor is rather hyaline, with scattered small 

 particles and very abundant vesicles and vacuoles. Many of these, 

 ranging up to 2 /x in size and bounded by unit membranes, fill 

 much of the endoplasm. Just below the buccal cavity is a zone 

 packed with vacuoles some of which have two parallel limiting 

 membranes. These vacuoles increase in size some distance below 

 the buccal cavity; Randall and Jackson suggest that they may be 

 formed at that surface and migrate inward. Considerable intake 



