102 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



internal lamellae occur as single discs, not bands of discs, follow- 

 ing parallel contorted paths through the plastid matrix. As noted 

 in Chapter 2, this arrangement is more like that in the blue-green 

 algae than those of any other algae (Gibbs, 1960). A central 

 pyrenoid is traversed by a few sinuous lamellae, but is not asso- 

 ciated with starch grains which appear instead in the cytoplasm 

 outside the plastid, each grain surrounded by a membrane. In 

 the peripheral cytoplasm are assorted small membranous structures 

 identified as endoplasmic reticulum, and stacks of smooth lamellae 

 resembling Golgi elements, but no mitochondria. The nucleus 

 has a dense eccentric nucleolus and is surrounded by a double 

 membrane. The cell is bounded by a plasma membrane and 

 enveloped by a thick sheath with a finely fibrous texture. 



The absence of mitochondria in an aerobic, photosynthetic, 

 nucleated cell may be a distinctly primitive feature; bacteria and 

 blue-green algae lack mitochondria, and there is evidence to 

 suggest that mitochondrial enzymes in them are localized in the 

 cell membrane or elaborations of it (Robinow, 1960). A study of 

 the sites of oxidative activity in Porphyrtdium would seem to be 

 in order. Otherwise, Porphyridium appears to have a minimum 

 complement of all the conventional eucellular equipment. It may 

 be morphologically the simplest free-living eucell yet investigated. 



Interestingly enough, the red algae, which never bear flagella 

 at any time, also appear to lack centrioles, although reinvestigation 

 of a few reports of centriole-like granules in the literature of light 

 microscopy (see Fritsch, 1945) is needed. Unless they have 

 secondarily lost the kinetosome/centriole, we must assume either 

 that they developed nuclear membranes, chloroplasts and Golgi 

 structures independently of all other eucellular stocks, or else, 

 with Dougherty and Allen, we may suggest that the modern reds 

 descended from a Porphyridinm-Yikt archetypal eucell that also was 

 ancestral to other higher protists. 



Once the kinetosome/centriole evolved, diversification of 

 flagellate groups occurred on a prodigious scale. Perhaps this was 

 coincidental, or was consequent on the extent of dispersal and 

 subsequent isolation permitted by flagellar motility. But the 

 variety of intracellular structures that, as we shall see, are 

 intimately associated with kinetosomes suggests that this new 



