Arachnids are arthropods in the class Arachnida (/əˈræknɪdə/) of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroons.
Adult arachnids have eight legs attached to the cephalothorax. In some species the frontmost pair of legs has converted to a sensory function, while in others, different appendages can grow large enough to take on the appearance of extra pairs of legs.
The Silurian (/sɪˈljʊəri.ən, saɪ-/ sih-LURE-ee-ən, sy-) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at 443.1 Ma (million years ago) to the beginning of the Devonian Period, 419.62 Ma. The Silurian is the third and shortest period of the Paleozoic Era, and the third of twelve periods of the Phanerozoic Eon. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.
One important event in this period was the initial establishment of terrestrial life in what is known as the Silurian-Devonian Terrestrial Revolution: vascular plants emerged from more primitive land plants, dikaryan fungi started expanding and diversifying along with glomeromycotan fungi, and three groups of arthropods (myriapods, arachnids and hexapods) became fully terrestrialized.
Shellfish, in colloquial and fisheries usage, are exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. In addition, a few species of land crabs are eaten, for example Cardisoma guanhumi in the Caribbean. Shellfish are among the most common food allergens.
Due to narrowing in the meaning of the English word fish over the centuries, shellfish no longer fall under what is usually considered fish. Most shellfish are low on the food chain and eat a diet composed primarily of phytoplankton and zooplankton. Many varieties of shellfish, and crustaceans in particular, are actually closely related to insects and arachnids; crustaceans make up one of the main subphyla of the phylum Arthropoda. Molluscs include cephalopods (squids, octopuses, cuttlefish) and bivalves (clams, oysters), as well as gastropods (aquatic species such as whelks and winkles; land species such as snails and slugs).
Ecdysozoa (/ˌɛkdɪsoʊˈzoʊə/) is a group of protostome animals, including Arthropoda (insects, chelicerates (including arachnids), crustaceans, and myriapods), Nematoda, and several smaller phyla. The grouping of these animal phyla into a single clade was first proposed by Eernisse et al. (1992) based on a phylogenetic analysis of 141 morphological characters of ultrastructural and embryological phenotypes. This clade, that is, a group consisting of a common ancestor and all its descendants, was formally named by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes.
A large study in 2008 by Dunn et al. strongly supported the monophyly of Ecdysozoa.
Amblypygi is an order of arachnids also known as whip-spiders or tailless whip-scorpions, not to be confused with whip-scorpions or vinegaroons that belong to the related order Thelyphonida. The name "amblypygid" means "blunt tail", a reference to a lack of the flagellum that is otherwise seen in whip-scorpions. Amblypygids possess no silk glands or venom. They rarely bite if threatened but can grab fingers with their pedipalps, resulting in thorn-like puncture-injuries.
As of 2023, five families, 17 genera, and around 260 species had been discovered and described. They are found in tropical and subtropical regions worldwide, mainly in warm and humid environments. They like to stay protected and hidden within leaf litter, caves, or underneath bark. Some species are subterranean; all are nocturnal. Fossilized amblypygids have been found dating back to the Carboniferous period, such as Weygoldtina.
Humans make many uses of arthropods, including as food, in art, in stories, and in mythology and religion. Many of these aspects concern insects, which are important both economically and symbolically, from the work of honeybees to the scarabs of Ancient Egypt. Other arthropods with cultural significance include crustaceans such as crabs, lobsters, and crayfish, which are popular subjects in art, especially still lifes, and arachnids such as spiders and scorpions, whose venom has medical applications. The crab and the scorpion are astrological signs of the zodiac.