1922] McDonald: On Balantidium coU and Balantidium suis 263 



in function. The cytoplasm is distinctly differentiated into ectoplasm 

 and endoplasm. The former constitutes a thin layer just underneath 

 the pellicle and in it is situated the basal apparatus of the cilia. The 

 layer of ectoplasm thickens greatly at the anterior end of the animal, 

 to form, as it were, a matrix for the cytostome and its accessory appa- 

 ratus. Within the ectoplasm, but not set off from it by a sharp line 

 of demarcation, is the endoplasm. In the endoplasm are numerous 

 food inclusions, often present in the form, of starch or paramylum 

 bodies. The macronucleus and the micronucleus are also within it, 

 but seem to have no constant position in the cell. The .macronucleus 

 is either bean-shaped (as in Balantidium coli) or elongate and sausage- 

 shaped (as in Balantidium suis). There are two contractile vacuoles, 

 the larger being situated anteriorly and the smaller posteriorly. They 

 lie closely beneath or may be entirely surrounded by ectoplasm, thus 

 belonging really within that layer. 



So far as can be determined the animals show no modification with 

 respect to a substratum, yet the lateral and posterior displacement of 

 the cytostome has lead to the designation of that side, toward which 

 displacement occurs, as ventral, and the opposite surface as dorsal. 

 This terminology is very nearly universal in the literature on Balan- 

 tidium, and the correlated terms of right and left are used in the 

 original description of the family Bursaridae (Stein, 1867) and the 

 genus Balantidium (Claparede and Lachmann, 1858). The dorsal 

 side may be somewhat more convex than the ventral ; this occurs not 

 infrequently in Balantidium suis, though, due to the plasticity of the 

 organism, this is by no means constant. No part of the body is differ- 

 entiated for skeletal purposes. The anal aperture or cytopyge is at 

 the posterior tip and may be present as an actual aperture or only 

 as an extreme thinning of the ectoplasm and pellicle at this point. In 

 connection with it there is usually a rectal vacuole which serves as a 

 storage reservoir for solid waste awaiting extrusion. 



ECTOPLASMIC STRUCTURES 



Pellicle. The entire body is covered by an extremely thin but 

 resistant pellicle (pel., figs. I and L). The pellicle seems to be some- 

 what thickened, as shown by a higher degree of refractibility, along 

 the margin of the lips of the cytostome where it turns in to form the 

 lining of the oesophagus and the groove in which the oral cilia are set ; 

 otherwise, it is nowhere noticeably specialized. It shows alternating 



