Tetrapulmonata is a non-ranked supra-ordinal clade of arachnids. It is composed of the extant orders Uropygi (whip scorpions), Schizomida (short-tailed whip scorpions), Amblypygi (tail-less whip scorpions) and Araneae (spiders). It is the only supra-ordinal group of arachnids that is strongly supported in molecular phylogenetic studies. Two extinct orders are also placed in this clade, Haptopoda and Uraraneida. In 2016, a newly described fossil arachnid, Idmonarachne, was also included in the Tetrapulmonata; as of March 2016 it has not been assigned to an order.
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Tetrapulmonate in the context of Gill
A gill (/ɡɪl/ ) is a specialized respiratory organ that many aquatic animals use for aquatic gas exchange, i.e. to extract dissolved oxygen from water and to excrete carbon dioxide. Branchia (pl.: branchiae, from Ancient Greek βράγχια) is the zoologists' academic name for gills.
The gills of some semi-aquatic clades (e.g. crabs, terrestrial hermit crabs and amphibious fishes), have also adapted to allow air respiration (breathing) on land provided the gills are kept moist. In some terrestrial chelicerates species (tetrapulmonates such as spiders and scorpions), their ancestral gills have evolved into fully air-breathing book lungs. In some planktivorous bony fish species (e.g. silver and bighead carps), the gills are also used as a filter feeding organ via comb-like projections called gill rakers.
Tetrapulmonate in the context of Book lung
A book lung is a type of respiration organ used for atmospheric gas exchange present in some arachnids such as scorpions and tetrapulmonates (spiders and whip scorpions). It is a terrestrial adaptation of the book gills seen in aquatic chelicerates such as horseshoe crabs, where the gills are ventral appendages under the abdomen (opisthosoma) arranged into a series of page-like lamellae in order to maximize surface area for diffusion. In arachnids, each of these organs is housed inside an air-filled cavity (atrium) that opens to the surrounding atmosphere through a small opening (spiracle), which provides a controlled environment for respiration.