120 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



web marking of ridges and grooves. Manton and Parke observed 

 occasional intracellular vesicles containing scales, and since 

 phagotrophy was never observed in this species they concluded 

 that scales are synthesized within the cell, perhaps in a prominent 

 group of vacuoles with heterogeneous contents found near the 

 mitochondrion. The regular imbrication of the superficial scales, 

 particularly conspicuous on the flagellum, could be explained by 

 the successive opening of vesicles, each containing one completed 

 scale, at the body surface around the flagellum base. 



In addition to scales, the flagellum of M. squamata often bears 

 short, curved mastigonemes ; these either are sporadic in occur- 

 rence or usually are detached from the flagellum in the process 

 of preparation. 



In Pedinomonas tuberculata (Manton and Parke, 1960), the cell 

 membrane is elevated at intervals over cone-shaped tubercles of 

 dense amorphous material; in addition, tiny flakes or spicules are 

 scattered over the surface, but these are not scales, nor do they 

 form a continuous wall. The living cell swims with its single 

 flagellum extending from the posterior pole. In this position the 

 single, large, curved chloroplast fills the anterior end of the cell. 

 In electron micrographs the plastid is seen to contain a dense 

 central pyrenoid and discontinuous peripheral discs converging 

 at the two ends of the organelle. Starch grains surround the 

 pyrenoid and appear elsewhere between discs. In the concavity 

 of the plastid surface are several small microtubular mitochondria, 

 and a conspicuous Golgi apparatus lies between these and the 

 kinetosome. The nucleus is much flattened against one side of 

 the body between the extremities of the chloroplast. Striated root 

 fibrils arise from the proximal end of the kinetosome and pass 

 beneath the cell membrane to unknown destinations. 



The polarity of P. tuberculata and the two Micromonas species 

 is of some interest. In saying that the flagellum arises antero- 

 laterally in M. squamata, laterally in M. pusilla, or posteriorly in 

 P. tuberculata, Manton and Parke are considering the end of the 

 cell directed forward most commonly in locomotion to be anterior. 

 However, the same cluster of organelles is found near the kineto- 

 some wherever it occurs : Golgi body, one or more mitochondria, 

 and nucleus, while the chloroplast occupies the opposite half or 

 more of the cell's volume. Thus it is justifiable to say that the 



