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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.
Merostomata is a class of chelicerate arthropods that contains the extinct Eurypterida (sea scorpions) and the extant Xiphosura (horseshoe crabs). The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.
Harpactea sadistica is the first spider species – and the first member of the entire subphylum Chelicerata – found to use traumatic insemination. [2] The males have specialized genital structures at the pedipalps that are adapted to grip the female and inject the sperm, using a structure resembling a hypodermic needle.
Members of the genus Pycnogonum have squarish bodies with a tough integument and a few hairs. The cephalon (the anterior end of the body which is fused with the first segment of the trunk) has a long smooth proboscis and a low tubercle on which the eyes are set.
Sanctacaris is a Middle Cambrian arthropod from the Burgess Shale of British Columbia.It was most famously regarded as a stem-group chelicerate, a group which includes horseshoe crabs, spiders and scorpions, although subsequent phylogenetic studies have not always supported this conclusion. [1]
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.
While adults live freely and are often found wandering about, searching for small animals and insect eggs for food, the larvae try to find a host to attach themselves to, often an insect like a grasshopper or fly, but also arachnids like harvestmen or spiders.
Mimetids are usually yellow and brown and are usually 3 to 7 millimetres (1 ⁄ 8 to 1 ⁄ 4 in) long. Mimetids can be recognized by the rows of spine-like hairs on their long front legs; the rows consist of a long spine, followed by a series of progressively shorter ones.