Zoology (ζῷον from Greek words zoon – animal, and λόγος, logos – speech) is the science that studies animals. Multi-disciplines and using many techniques, this science has slowly developed over the centuries since prehistoric times. Historically, the first scientific reflections on zoology, which have been transmitted, are those of Aristotle. The attempts to classify species were numerous and often revised since that time.
The border between zoology, the study of animals, and botany, studying plants, has been and still is controversial. Some living beings, considered plants, have proved to be of animals, for some others it is still a subject to discussions at the dawn of the twenty-first century. For these atypical living beings, belonging to one or other of the sciences has also changed with advanced and technical or scientific discoveries (including microscopy and DNA analysis).
While most metazoans have always been unquestionably placed among the animals, in the nineteenth century, some lower metazoans were still placed in a special category called “Zoophytes” (etymologically: animal-plants). This group traditionally included sponges, cnidarians, ctenophore, and bryophytes. Carl Linnaeus classified molluscs such as Cuttlefish, the Aplysia, the sea cucumber, but also echinoderms (sea urchins and starfish ) in this category.
It was not until 1744 that Jean André Peyssonnel recognized coral as an animal, and similarly, the sponges were recognized as animals only in 1825.
The case of protozoa is even more problematic. The study of unicellular beings reveals ambiguous forms where the distinction between animal and plant is not absolute. Some of them, such as Euglena or some Péridiniens that may have autotrophic or heterotrophic nutrition, have long been placed at the boundary between the two disciplines. And some unicellular organisms having chlorophyll (characteristic of the “vegetable state”), are mobile and have a flexible cell membrane (characteristic of “animal state”).
In the second half of the nineteenth century, Ernst Haeckel, who believed that the coexistence of these characteristics was inherited from the ancestors common to animals and plants, which were unicellular organisms of the reign of Protista in 1866. Protistology, scientific study of protists, then found itself attached to both zoology and botany. However, there remained some traditional dichotomy: zoologists studied the forms of “animal affinity” and botanists studied the forms of “plant affinity.” Boundaries and classifications of protists proposed by scholars therefore differ considerably depending on the discipline. Researchers have attempted to delineate the two kingdoms of animals and plants, which tend to merge in the flagellates by the distinction of zooflagellates and phytoflagellates.
At the turn of the 21th century, the data of molecular biology used to assess more reliably kinship between the lines of living organisms.
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