Onychophora
27-554: Hexapoda Myriapoda Crustacea Chelicerata Uniramia ( uni – one, ramus – branch, i.e. single-branches) is a group within the arthropods . In the past this group included the Onychophora , which are now considered a separate category. The group is currently used in a narrower sense. Uniramia is one of three subphyla in the Arthropoda classification suggested by Sidnie Manton . This classification divided arthropods into
54-522: A flash in the Ediacaran biota ; filling the Cambrian explosion with the results, and a more nuanced understanding of animal evolution suggests gradual development of body plans throughout the early Palaeozoic . Recent studies in animals and plants started to investigate whether evolutionary constraints on body plan structures can explain the presence of developmental constraints during embryogenesis such as
81-447: 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 phyla . Haeckel's animal phyla were coelenterates , echinoderms , and (following Cuvier) articulates, molluscs, and vertebrates. Stephen J. Gould explored
108-766: A three-phyla polyphyletic group, with phylum Uniramia including the Hexapoda (insects), Myriapoda (centipedes and millipedes) and the Onychophora (velvetworms). The discovery of fossil lobopods , determined to be intermediate between onychophorans and arthropods led to the splintering of the Lobopoda and Onychophora into separate groups. This redefined the Uniramia as strictly "true" arthropods with exoskeletons and jointed appendages. Uniramians have strictly uniramous appendages. Systematics can result in rival taxonomies, and this seems to have happened to Uniramia. The name Uniramia
135-506: A various number of annuli. This type of antenna is therefore called annulated antenna. Johnston's organ , which is found on the pedicel, is absent in the Entognatha. The thorax is composed of three segments, each of which bears a single pair of legs. As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods and with gill on
162-489: Is a set of morphological features common to many members of a phylum of animals . The vertebrates share one body plan, while invertebrates have many. This term, usually applied to animals, envisages a "blueprint" encompassing aspects such as symmetry , layers , segmentation , nerve , limb , and gut disposition. Evolutionary developmental biology seeks to explain the origins of diverse body plans. Body plans have historically been considered to have evolved in
189-536: Is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura), followed by six segments, all closely fused together, with the following appendages: The mouth lies between the fourth and fifth segments and is covered by a projection from the sixth, called the labrum (upper lip). In true insects (class Insecta) the mouthparts are exposed or ectognathous , while in other groups they are enveloped or endognathous . Similar appendages are found on
216-583: Is described as replacing Uniramia in early twentieth-century texts (Heymons, 1901), where it was the preferred name for the category uniting the Hexapoda (insects) + Myriapoda ; but depending on the source, the term Atelocerata may have replaced Mandibulata , be an infraphylum beneath Mandibulata, or may no longer be a valid category after closer, cladistics -based genetic study. Arachnida Merostomata Pycnogonida Trilobitomorpha Symphyla Chilopoda Diplopoda Entognatha Ectognatha Crustacea The Crustacea were generally considered
243-532: Is far from exhaustive of the possible patterns for life: the Precambrian Ediacaran biota 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. Genes , embryos and development together determine
270-610: The Devonian period. This fossil may help to fill the arthropod gap from 385 million to 325 million years ago, although some researchers oppose this view and suggest that the fossil may instead represent a decomposed crustacean or other non-insect. In 2023, a hexapod-like arthropod fossil from the Ordovician marine fossil site Castle Bank was reported, although further study is needed. Body plan A body plan , Bauplan ( pl. German : Baupläne ), or ground plan
297-406: The abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment. The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity. These were then considered subclasses of a subphylum called Uniramia or Atelocerata. In the first decade of the 21st century, however, this
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#1733115129017324-420: The abdominal segments of some immature aquatic insects. In most insects the second and third thoracic segments also support wings. It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves. The abdomen follow epimorphic development, where all segments are already present at the end of embryonic development in all
351-541: The animal kingdom into four body plans. Taking the central nervous system as the main organ system which controlled all the others, such as the circulatory and digestive systems, Cuvier distinguished four body plans or embranchements : Grouping animals with these body plans resulted in four branches: vertebrates , molluscs , articulata (including insects and annelids ) and zoophytes or Radiata . Ernst Haeckel , in his 1866 Generelle Morphologie der Organismen , asserted that all living things were monophyletic (had
378-567: The animals into quadrupeds , birds , "amphibians" (including tortoises , lizards and snakes ), fish , "insects" (Insecta, in which he included arachnids , crustaceans and centipedes ) and "worms" (Vermes). Linnaeus's Vermes included effectively all other groups of animals, not only tapeworms , earthworms and leeches but molluscs , sea urchins and starfish , jellyfish , squid and cuttlefish . In his 1817 work, Le Règne Animal , French zoologist Georges Cuvier combined evidence from comparative anatomy and palaeontology to divide
405-509: The closest relatives of the Uniramia, and sometimes these were united as Mandibulata . However, the competing hypothesis — that Crustacea and Hexapoda form a monophyletic group, the Pancrustacea , to which the Myriapoda are the closest relatives — has support from molecular and fossil evidence. Hexapoda The subphylum Hexapoda (from Greek for 'six legs') or hexapods comprises
432-476: The form of an adult organism's body, through the complex switching processes involved in morphogenesis . Developmental biologists seek to understand how genes control the development of structural features through a cascade of processes in which key genes produce morphogens , 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
459-408: The heads of Myriapoda and Crustacea , although the crustaceans have secondary antennae . Collembolans and diplurans have segmented antenna; each segment has its own set of muscles. The antennea of insects consist of just three segments; the scape, the pedicel and the flagellum. Muscles occur only in the first two segments. The third segment, the flagellum, don't have any muscles and is composed of
486-523: The hexapod groups except for Protura, which has an anamorphic development where the hatched juveniles has an incomplete complement of segments, and goes through a post-embryonic segment addition with each molting before the final adult number of segments is reached. All true insects have eleven segments (often reduced in number in many insect species), but in Protura there are twelve, and in Collembola only six (sometimes reduced to only four). The appendages on
513-513: The hexapods diverged from their sister group, the Anostraca (fairy shrimps), at around the start of the Silurian period 440 million years ago , coinciding with the appearance of vascular plants on land. Since then remipedians have been revealed as closest living relative of hexapods. Several hypotheses about their internal relationships have been suggested over the years, with proturans as
540-462: 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 punctuated equilibrium . 20 out of the 36 body plans originated in the Cambrian period, in the " Cambrian explosion ". However, complete body plans of many phyla emerged much later, in the Palaeozoic or beyond. The current range of body plans
567-441: The largest clade of arthropods and includes most of the extant arthropod species. It includes the crown group class Insecta (true insects), as well as the much smaller clade Entognatha , which includes three classes of wingless arthropods that were once considered insects: Collembola (springtails), Protura (coneheads) and Diplura (two-pronged bristletails). The insects and springtails are very abundant and are some of
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#1733115129017594-568: The most important pollinators , basal consumers , scavengers / detritivores and micropredators in terrestrial environments. Hexapods are named for their most distinctive feature: a three-part body plan with a consolidated thorax and three pairs of legs . Most other arthropods have more than three pairs of legs. Most recent studies have recovered Hexapoda as a subgroup of Crustacea . Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into an anterior head, thorax , and posterior abdomen. The head
621-471: The phenomenon referred to as phylotypic stage . Among the pioneering zoologists , 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. In his 1735 book Systema Naturæ , Swedish botanist Linnaeus grouped
648-535: The sister group to the other hexapods and collembolans and diplurans belonging together in Antennomusculata as the latest suggestion: The following cladogram is given by Kjer et al. (2016): Collembola (springtails) Protura (coneheads) Diplura (two-pronged bristletails) Archaeognatha (jumping bristletails) Zygentoma (silverfish) Pterygota (winged insects) An incomplete possible insect fossil, Strudiella devonica , has been recovered from
675-611: Was called into question, and it appears the hexapods' closest relatives may be the crustaceans . The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha , or as several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than to the Collembola (springtails). A 2002 molecular analysis suggests that
702-456: Was temporarily rejected as a polyphyletic group, but when used now refers to the subphylum consisting of the insects + myriapods. Subphylum Uniramia is characterized by uniramous (single-branching) appendages, one pair of antennae and two pairs of mouthparts (single pairs of mandibles and maxillae ). Their body forms and ecologies are diverse. While most unirames are terrestrial, "some are aquatic for part or all of their life cycles." Atelocerata
729-424: Was the existence of groups of homeobox genes , 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 evolutionary developmental biology ('Evo Devo'), which studies
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