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A small phylum of caterpillar-like invertebrates – the velvet worms – that inhabit moist dark terrestrial habitats, such as forest litter and caves, in tropical and warm regions. The thin chitinous cuticle, which bears numerous papillae and sensory hairs giving it a velvety feel, is periodically moulted. The 110 or so known species, which include Peripatus, are generally small, with brownish bodies, although some are more brightly coloured. Sizes range from 14 mm to 200 mm in length, with females larger than males, and there may be from 14 to more than 40 pairs of unjointed hollow legs. Onychophorans capture prey, such as spiders and termites, by entangling them in a sticky secretion squirted from adhesive glands opening beside the mouth. The sexes are separate; some species lay eggs, whereas others are ovoviviparous or viviparous, the latter nourishing embryos internally via a placenta analogous to that of mammals. Onychophorans are closely related to arthropods and are thought to have descended from extinct marine forms that flourished in the Cambrian, such as Aysheaia and Hallucigenia, found in the Burgess shale deposits. Such fossil forms are now sometimes placed with modern velvet worms in the phylum Lobopodia.

Subjects: Biological Sciences.

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