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Chelicerata split from Mandibulata by the mid-Cambrian, as evidenced by stem-group chelicerates like Habeliida and Mollisonia present by this time. [2] The surviving marine species include the four species of xiphosurans (horseshoe crabs), and possibly the 1,300 species of pycnogonids (sea spiders), if the latter are indeed chelicerates. On the ...
The chelicerae (/ k ə ˈ l ɪ s ər iː /) are the mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as "jaws", chelicerae may be shaped as either articulated fangs, or as a type of pincers.
Paired maxillae cut food and manipulate it during mastication. Maxillae can have hairs and "teeth" along their inner margins. At the outer margin, the galea is a cupped or scoop-like structure, which sits over the outer edge of the labium. They also have palps, which are used to sense the characteristics of potential foods.
Ecdysozoa (/ ˌ ɛ k d ɪ s oʊ ˈ z oʊ ə /) is a group of protostome animals, [8] including Arthropoda (insects, chelicerata (including arachnids), crustaceans, and myriapods), Nematoda, and several smaller phyla.
However, a 2022 analysis recovered the monophyly of Merostomata, as opposed to a monophyletic Arachnida, with Xiphosura as the only modern representative in a derived position, indicating the convergence of several characteristics that supposedly united the arachnids.
The name "Mandibulata" refers to the mandibles, a modified pair of limbs used in food processing, the presence of which are characteristic of most members of the group. The mandibulates are divided between the extant groups Myriapoda (millipedes & centipedes, among others) and Pancrustacea (including insects and crustaceans, among others).
The microscopic mite Lorryia formosa (). The mites are not a defined taxon, but is used for two distinct groups of arachnids, the Acariformes and the Parasitiformes.The phylogeny of the Acari has been relatively little studied, but molecular information from ribosomal DNA is being extensively used to understand relationships between groups.
It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects to an abdomen (and is often referred to as such), the opisthosoma is differentiated by its inclusion of the respiratory organs (book lungs or book gills) and the heart.