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The mandibulates are divided between the extant groups Myriapoda (millipedes & centipedes, among others) and Pancrustacea (including insects and crustaceans, among others). Molecular phylogenetic studies suggest that the living arthropods are related as shown in the cladogram below.
It contains a vast total of named species, second only to the insects in overall number. The fossil history of this class goes back to the Late Cambrian . As of 2017 [update] , 721 families of gastropods are known, of which 245 are extinct and appear only in the fossil record, while 476 are currently extant with or without a fossil record.
These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented. [53] Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla. [54]
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These minute arthropods are apterous, unlike some orders of insects that have lost their wings secondarily (but are derived from winged ancestors). Their mouthparts are enclosed within a pouch in the head capsule, called the gnathal pouch, so only the tips of the mandibles and maxillae are exposed beyond the cavity. [1]
Aquatic arthropods generally possess some form of gills in which gas exchange takes place by diffusing through the exoskeleton. Others may breathe atmospheric air while remaining submerged, via breathing tubes or trapped air bubbles, though some aquatic insects may remain submerged indefinitely and respire using a plastron. A number of insects ...
The winged insects are nocturnal and provide food for night-flying birds, bats, small mammals, amphibians and arthropods. The larval stage lasts much longer, often for one or more years, and has a bigger impact on the environment. [20] They form an important part of the diet of fish such as the trout. The fish acquire them by two means, either ...
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