Animalcule is an older term for a microscopic animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

 or protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

n. Some better-known animalcules include:
  • Actinophrys
    The actinophryids are small, familiar group of heliozoan protists. They are the most common heliozoa in fresh water, and are especially frequent in lakes and rivers, but a few are found in marine and soil habitats as well...

    , and other heliozoa
    Heliozoa are phagotrophs. They are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. These give them the characteristic sun-like appearance for which they are named, and are variously used for capturing food,...

    , called sun animalcules
  • Amoeba
    Amoeba is a genus of Protozoa.History=The amoeba was first discovered by August Johann Rösel von Rosenhof in 1757. Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus, who could change his shape...

    , called proteus
    In Greek mythology, Proteus is an early sea-god, one of several deities whom Homer calls the "Old Man of the Sea", whose name suggests the "first" , as protogonos is the "primordial" or the "firstborn". He became the son of Poseidon in the Olympian theogony In Greek mythology, Proteus (Πρωτεύς)...

  • Noctiluca scintillans
    Noctiluca scintillans
    Noctiluca scintillans, commonly known as the Sea Sparkle, and also published as Noctiluca miliaris, is a free-living non-parasitic marine-dwelling species of dinoflagellate that exhibits bioluminescence. The bioluminescent characteristic of N...

    , commonly called the 'Sea Sparkles'
  • Paramecium
    Paramecium is a group of unicellular ciliate protozoa, which are commonly studied as a representative of the ciliate group, and range from about 0.05 to 0.35 mm in length. Simple cilia cover the body, which allow the cell to move with a synchronous motion at speeds of approximately 12 body...

    , called slipper animalcules
  • Rotifer
    The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

    , called wheel animalcules
  • Stentor, called trumpet animalcules
  • Vorticella
    Vorticella is a genus of protozoa, with over 16 known species. They are stalked inverted bell-shaped ciliates, placed among the peritrichs. Each cell has a separate stalk anchored onto the substrate, which contains a contractile fibril called a myoneme. When stimulated this shortens, causing the...

    , and other peritrich
    The peritrichs are a large and distinctive group of ciliate protozoa. They are usually bell or disc shaped, with a prominent paroral membrane arising from the oral cavity and circling counter-clockwise around the anterior of the cell, accompanied by a smaller series of membranelles...

    s, called bell animalcules

The term was also used by Anton van Leeuwenhoek
Anton van Leeuwenhoek
Antonie Philips van Leeuwenhoek was a Dutch tradesman and scientist from Delft, Netherlands. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist...

, the 17th-century preformationist
In the history of biology, preformationism is either the specific contention that all organisms were created at the same time, and that succeeding generations grow from homunculi, animalcules, or other fully formed but miniature versions of themselves that have existed since the beginning of...

 and discoverer of microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

s, to describe spermatozoa.

The word appears in the Major-General's Song
Major-General's Song
I Am the Very Model of a Modern Major-General is a patter song from Gilbert and Sullivan's 1879 comic opera The Pirates of Penzance. It is perhaps the most famous song in Gilbert and Sullivan's operas. It is sung by Major-General Stanley at his first entrance, towards the end of Act I...

, in which Major-General Stanley sings, "I know the scientific names
Binomial nomenclature
Binomial nomenclature is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages...

of beings animalculous..."
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