Why is paramecium called slipper animalcule?

Paramecium, or to give them their full Latin name, Paramecium tetraurelia, were called 'slipper animalcule' due to their slipper-like shape, more pronounced when paramecium were discovered in the early 17th Century, due to the lower-quality lenses available to scientists back then, as you can see from this drawing:

The above image was drawn by Frenchman Louis Joblot in 1718, and is one of the first recorded pictures of a paramecium. The resemblance to a slipper is more noticeable here than in a more recent picture:

The 'animalcule' comes from the old word for protozoa, single-celled organisms who were considered animals because they appeared to behave like animals in that they had independent movement, consumed food and excreted waste in a similar way.