General Morphology o£ the Protozoa 31 



suggest that Euplotes harpa, and perhaps other Euplotidae, cannot con- 

 centrate small food particles very efficiently (147), although the peri- 

 stomial area is partially enclosed ventrally by an extension of the right 

 peristomial wall (Fig. 1,15, B). In cultures, E. harpa thrives best on food 

 particles larger than bacteria (147). 



In such ciliates as Chilodonella and Prorodon the pharynx is sur- 

 rounded by a conical or cylindrical "pharyngeal basket" (Fig. 1. 15, D) 

 which undergoes dilation during ingestion (179). The basket is com- 

 posed of rods which are probably protein in nature (153) and, in certain 

 species, may represent fused bundles of slender trichites (179). Conver- 

 gence and apparent fusion of the rods posteriorly may be noted, as in 

 Chilodofiella (153). There are circumpharyngeal trichites in Spathidium 

 (235) and Didiniwn (179) also, although a compact basket is lacking. 

 The paralysis of small ciliates after contact with the cytostomal region of 

 S. spathula (235) suggests that the trichites function much like tricho- 

 cysts of certain other ciliates. 



The pharyngeal-rod apparatus ("Staborgan") of Perayieyna and similar 

 holozoic Euglenida (Chapter IV) includes two longitudinal rods extend- 

 ing posteriorly from the cytostome and a smaller curved element at the 

 rim of the cytostome (23). The conical "siphon" of Ryitosiphon (23) is 

 possibly a derivative of the rod-apparatus but its homologies are un- 

 certain. 



VACUOLES OF PROTOZOA 



Contractile vacuoles 



Contractile vacuoles, characteristic of fresh-water species, are ab- 

 sent in most parasitic and marine Protozoa. The position, number, and 

 accessory structures of the contractile vacuoles vary in different Protozoa. 

 In such genera as Amoeba, the position of the vacuole changes with 

 movements of the organism. Differentiation of the cortical layer is gen- 

 erally accompanied by a relatively fixed position of the contractile 

 vacuole. 



The origin of a new vacuole after discharge (systole) usually involves 

 the appearance of a few minute vacuoles in the area where the new con- 

 tractile vacuole will develop. These small vacuoles fuse into a single 

 larger one. Later increase in volume (diastole) of the contractile vacuole 

 involves various processes. In certain ciliates, the young vacuole is fed by 

 one or more canals (Fig. 1, 16, F, G). The newly formed vacuole in 

 Spirostomum ambiguutn (42), for example, fuses with a long canal which 

 in turn receives fluid by fusion with small vacuoles. The new vacuole in 

 Paramecium multimicronucleatiim (119) is fed in similar fashion by sev- 

 eral canals. Growth of the new vacuole in Amoeba (80), Euplotes (210), 

 and many other Protozoa depends, to some extent at least, upon fusion 



