A researcher was observing a slide of Paramecium caudatum and a slide of Streptococcus faecalis. He labeled the first slide as eukaryote and the second as prokaryote and classified these two in separate kingdoms. What could be the reason for classifying these two organisms in separate kingdoms?

The answer is: Paramecium caudatum has a nucleus and organelles inside its cell and Streptococcus faecalis don't.

The main difference between a eukaryotic and a prokaryotic cell is the presence of a nucleus and organelles in eukaryotic cell and their absence in prokaryotic cell. Both P. caudatum and S. faecalis are unicellular organisms. P. caudatum has two nuclei, one necessary for survival and one necessary for reproduction, and organelles. S. faecalis, like other Prokaryotes, has plasmids without a true membrane.