276 University of California Puhlications in Zoology [Vol.20 



seems to be its more normal proportions, however, it is approximately 

 pear-shaped with the stem end of the pear directed ventrally. In 

 Balantidium coli the adoral zone commonly includes the most anterior 

 point or apex of the animal — at least its dorsal margin passes through 

 this point.' In Balantidium suis, the anterior tip of the animal lies 

 wholly outside of this zone, the latter having migrated too far ventrally 

 to include it. 



The cytostome {cytst., fig. J) does not occupy the whole interior 

 of the peristome, but is situated at its ventral end. There is some 

 indication that this aperture may be completely closed by the oral 

 plug {or. pi., fig. I) which comprises the rest of the peristome within 

 the adoral row of cilia. This oral plug bears no cilia. It is exceed- 

 ingly mobile, adapting itself readily to the almost constantly changing 

 shape of the apical cone. It lies dorsal to the cytostome and is not 

 exactly bilaterally symmetrical since it is pushed somewhat to the left 

 to make room for the oesophagus. It extends inward, thinning as 

 it does so, until it teniiinates about the beginning of the endoplasm. 

 It is ectoplasmic, but very finely granular as compared with the rest 

 of the ectoplasm. Mallory's connective tissue stain ordinarily gives 

 it a decidedly bhiish tinge with slight spots of red only where there 

 are certain neuromotor fibers. Its action in feeding is very hard to 

 follow, but its high degree of mobility impresses one when watching 

 the activities of the organism, and may be demonstrated with fixed 

 material by its extreme protrusion (pi. 28, fig. 14). In addition to 

 this, the fact that it is intimately connected with the neuromotor 

 apparatus would indicate that it functions in selective feeding. The 

 oesophagus {oes., fig. I; pi. 27, figs. 4-8) beginning at the cj-tostome, 

 passes inwardly, not quite radially but swerving slightly to the right. 

 It maj^ be followed definitely through the ectoplasm and for a very 

 short way into the endoplasm where it ends blindly. So far as I 

 have been able to determine, it is a unifonn tube-like opening without 

 evident enlargements or constrictions. Prowazek (1913), however, 

 gives in part the following description, " . . . . es sehnt sich jedoch 

 nicht direct trichterformig in die Tiefe, da man von der drei scharfe 

 Konturen noch naehweisen kann (fig. 1)." While studying living 

 forms stained with neutral red, I have often observed specimens in 

 the exact position of the one shown by Prowazek (1913, p. 7, text 

 fig. 1). The lines shown by him were easily recognizable, deeply 

 stained with the neutral red, but I could interpret them onlj^ in the 

 following way. The most anterior line which he shows seems to be 



