HistoryAmong the pioneering s, Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognised by modern zoologists has risen to 36.
Linnaeus, 1735In his 1735 book ', Swedish grouped the animals into , , "amphibians" (including , and ), , "insects" (Insecta, in which he included , and ) and "worms" (Vermes). Linnaeus's Vermes included effectively all other groups of animals, not only , and but , and , , and .
Cuvier, 1817In his 1817 work, ''Le Règne Animal'', French zoologist combined evidence from comparative anatomy and to divide the animal kingdom into four body plans. Taking the as the main organ system which controlled all the others, such as the circulatory and digestive systems, Cuvier distinguished four body plans or ''embranchements'':De Wit, Hendrik Cornelius Dirk De Wit. ''Histoire du Développement de la Biologie'', Volume III, Presses Polytechniques et Universitaires Romandes, Lausanne, 1994, p. 94-96. Grouping animals with these body plans resulted in four branches: , , (including and ) and zoophytes or .
Haeckel, 1866, in his 1866 ''Generelle Morphologie der Organismen'', asserted that all living things were (had a single evolutionary origin), being divided into plants, protista, and animals. His protista were divided into moneres, protoplasts, flagellates, diatoms, myxomycetes, myxocystodes, rhizopods, and sponges. His animals were divided into groups with distinct body plans: he named these . Haeckel's animal phyla were , , and (following Cuvier) articulates, molluscs, and vertebrates.
Gould, 1979explored the idea that the different phyla could be perceived in terms of a Bauplan, illustrating their fixity. However, he later abandoned this idea in favor of .
Origin20 out of the 36 body plans originated in the period, in the "", However, complete body plans of many emerged much later, in the or beyond. The current range of body plans is far from exhaustive of the possible patterns for life: the includes body plans that differ from any found in currently living organisms, even though the overall arrangement of unrelated modern taxa is quite similar. Thus the Cambrian explosion appears to have more or less completely replaced the earlier range of body plans.
Genetic basiss, s and development together determine the form of an adult organism's body, through the complex switching processes involved in . Developmental biologists seek to understand how genes control the development of structural features through a cascade of processes in which key genes produce s, chemicals that diffuse through the body to produce a gradient that acts as a position indicator for cells, turning on other genes, some of which in turn produce other morphogens. A key discovery was the existence of groups of , which function as switches responsible for laying down the basic body plan in animals. The homeobox genes are remarkably conserved between species as diverse as the fruit fly and humans, the basic segmented pattern of the worm or fruit fly being the origin of the segmented spine in humans. The field of animal ('Evo Devo'), which studies the genetics of in detail, is rapidly expanding with many of the developmental genetic cascades, particularly in the fruit fly ', catalogued in considerable detail.
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