Paramecium bursaria from the freshwater acidic kettle pond Chatfield's Hole situated in a white pine forest. They measure on average 190 um in length. I can see a contractile vacuole cycling through diastole and systole as well as the peniculid oral apparatus. Interestingly, this pond also harbors Homalozoon species, probably H. vermiculare, which also are loaded with Zoochlorellae.

Paramecium bursaria (Ehrenberg, 1831) Focke, 1836 Synonym
Loxodes bursaria Ehrenberg, 1831

Foot-shaped, more or less flattened; uniform ciliation except for a group of long caudal cilia; green with symbiotic zoochlorellae; a long broad vestibulum leads to the buccal cavity, the buccal ciliary apparatus is characterized by 2 "peniculi"; 1 compact micronucleus; 2 contractile vacuoles; numerous prominent trichocysts.
Measurements
Length 180-300 um. (ref. ID; 1219)