Correct Option: A

“Animalcule” is a old term for a microorganism; it just means “small animal”. A paramecium is shaped somewhat like a slipper (a soft shoe), so it is called the slipper animalcule. Other better-known animalcules include: Amoeba, called Proteus animalcule; Noctiluca scintillans, commonly called the ‘Sea Sparkles’; Rotifers, called wheel animalcules, etc.