262 University of California Publications in Zoology [ V OL. 20 



Measurements of balantidia as they occur free in the human intes- 

 tine were made possible through the kindness of Dr. E. L. Walker, 

 of the Hooper Institute of Medical Research, who loaned me several 

 slides which he had prepared while in the Philippine Islands. This 

 material had been stained with haematoxylin. Using the same pre- 

 cautions as in previous work, a total of 100 individuals was measured. 

 The data were handled as before and the resulting graph is shown in 

 figure C. In comparison with previous graphs it will be noted that 

 this graph closely approaches coincidence with the curves representing 

 Balantidium coli, for its mode is at 1.3 while the extreme portion of 

 length and breadth is 1.5. 



In addition to slides of human material, there was also loaned 

 material from one pig and from one monkey. It is interesting that 

 each showed a pure infection with Balantidium coli. The monkey 

 (Monkey No. 10, Table I; Walker, 1913) had been experimentally 

 infected by feeding it cysts from a pig, but not the pig from which 

 the above-mentioned material was taken. Therefore this material 

 yields no evidence regarding the validity of the specific differentiation 

 nor the possibility of Balantidium suis becoming established in mon- 

 keys or in man. 



There is no likelihood of confusing the new species, Balantidium 

 suis with Balantidium minutum (Schaudinn, 1899). The differences 

 are very marked. The body of the latter is oval, pointed anteriorly, 

 more like Balantidium coli. The peristome reaches to the equatorial 

 plane. There is but a single vacuole while there are two in each of 

 the species considered here. The macronucleus is spherical, whereas 

 it is elongate in both Balantidium coli and Balantidium suis. 



MORPHOLOGY 



Balantidium coli (Malmsten) and Balantidium suis sp nov. are 

 ciliated protozoans, barely visible to the unaided eye, and are in a 

 general way sac-shape (balantidium, little bag). Viewed through the 

 microscope they appear grayish green in color. The homogeneity of 

 the cell contents is broken by the presence of the nuclei, the contractile 

 vacuoles, the food vacuoles, and sometimes by the presence of highly 

 refractile bodies, the paramylum bodies. The entire surface of the 

 body, except that of the oral plug, is covered with fine cilia. The cell 

 contents are retained by a thin transparent pellicle which is protective 



