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The authors favoured relationships shown by more slowly evolving genes, which demonstrated the monophyly of Chelicerata, Euchelicerata and Arachnida, as well as of some clades within the arachnids. The diagram below summarizes their conclusions, based largely on the 200 most slowly evolving genes; dashed lines represent uncertain placements.
The group's origins lie within an arachnid sub-group defined by the presence of book lungs (the tetrapulmonates); [1] [2] the arachnids as a whole evolved from aquatic chelicerate ancestors. More than 45,000 extant species have been described, organised taxonomically in 3,958 genera and 114 families. [3] There may be more than 120,000 species. [3]
Opiliones' own divergence is dated to 414 million years ago, which arachnid are estimated to have originated during the late Cambrian to early Ordivician. [ 10 ] Genetic analysis performed on a modern Phalangium opilio specimen found that a suppressed gene that, if active, would generate a second pair of eyes at the lateral location, providing ...
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As in other arachnids, the nervous system is highly concentrated in the cephalothorax, but has a long ventral nerve cord with segmented ganglia which may be a primitive trait. [49] The pedipalp is a segmented, clawed appendage used for prey immobilization, defense and sensory purposes. The segments of the pedipalp (from closest to the body ...
However, in sea spider and arachnids, the pedipalps are more or less specialized for sensory [12] or prey-catching function [10] – for example scorpions have pincers [20] and male spiders have bulbous tips that act as syringes to inject sperm into the females' reproductive openings when mating.
Close-up of the chelicerae of a harvestman. The chelicerae consist of three segments, with the distal two forming a pincer [3] equipped with a row of teeth. Based on developmental gene expression data, the proximal segment is thought to be homologous to the femur and patella of the legs, the middle segment to the tibia through the tarsus of the legs, and the distal segment to the apotele (the ...
Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two segments: the cephalothorax (2) and the abdomen (3). The ventral side of a brown widow spider. The epigastric plates and furrow are visible, as well as the hourglass marking on the underside of the abdomen, which is a characteristic feature of widow ...