1922] McDonald: On Balantidium coli and Balantidium suis 293 



SUMMARY 



1. Pigs are very generally infected with balantidia as shown by 

 the findings in previous investigations, many of which were carried 

 on in foreign countries, and by the finding of 68 per cent infection 

 among the two hundred pigs examined during the present investi- 

 gation. 



2. There are two species of the genus Balantidium that are para- 

 sitic in the intestinal tract of pigs, namely, Balantidium coli and 

 Balantidium suis (sp. nov.). 



3. Balantidium coli is the species first described by Malmsten 

 (1857) from man, and later by Leuckart (1861) as a parasite in pigs. 



4. Balantidium suis (sp. nov.) has not hitherto been distinguished 

 from Balantidium coli. The former differs from the latter in being 

 more elongate and being broadest anterior instead of posterior to the 

 equatorial plane; in having a more slenderly proportioned macro- 

 nucleus ; and in having the mouth displaced ventrally, instead of being 

 almost terminal, which causes the plane of demarcation between ecto- 

 plasm and endoplasm in this region to slant posteriorly toward the 

 ventral surface instead of being perpendicular to the longitudinal 

 axis, as is the case in Balantidium coli. 



5. So far as recorded facts will justify conclusions, it seems un- 

 likely that Balantidium suis occurs as a parasite of man, but instead 

 that Balantidium coli is the cause of balantidiasis. Whether or not such 

 is the case, it is very desirable that the occurrence of the two species 

 in pigs be taken into account in future work on experimental infection, 

 for to a failure to distinguish between the two species may be due 

 the seemingly conflicting results of previous experiments. 



6. The cilia of the two species are homologous. Variation occurs 

 in size and relative position of parts only. The basal apparatus is 

 essentially diplosomic, consisting of a basal granule connected by a 

 ciliary rootlet to a secondary enlargement. The former is situated just 

 beneath the pellicle ; the latter lies in the plane of demarcation between 

 ectoplasm and endoplasm. The adoral cilia are largest, the basal granule 

 and secondary enlargement are farthest removed from one another, as 

 the ectoplasmic thickening is greatest in this region, and the ciliary 

 rootlet may extend far into the endoplasm. The cilia of the apical 

 cone intergrade between the adoral cilia and the body cilia. As one 



