Answer: Entamoeba coli trophozoites and cysts. Also present is a trophozoite of Dientamoeba fragilis.
This case from Florida Fan nicely shows the diagnostic features of E. coli, including the large size and >4 nuclei in the mature cyst stage:
As I mentioned above, there was also a D. fragilis trophozoite present. Because of its small size and indistinct chromatin features, this organism is significantly more difficult to identify. Here is the current case (bottom, left), as well as an idealized photo from my own collection (bottom, right). Note that the trophozoites have 1 to 2 nuclei with a 'fragmented' or 'fractured' chromatin pattern. The chromatin may also have a smudgy appearance.