Myriapoda in the context of "Reginald Innes Pocock"

Crustaceans (from Latin word "crustacea" meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (/krəˈsteɪʃə/), a large, diverse group of mainly aquatic arthropods including the more familiar decapods (shrimps, prawns, crabs, lobsters and crayfish), seed shrimps, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata. It is now well accepted that the hexapods (insects and entognathans) emerged deep in the crustacean group, with the completed pan-group referred to as Pancrustacea. The three classes Cephalocarida, Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans (oligostracans and multicrustaceans).

The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crab with a leg span of up to 3.8 m (12.5 ft) and a mass of 20 kg (44 lb). Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. They are distinguished from other groups of arthropods, such as insects, myriapods and chelicerates, by the possession of biramous (two-parted) limbs, and by their larval forms, such as the nauplius stage of branchiopods and copepods.

The supraesophageal ganglion (also supraoesophageal ganglion, arthropod brain, or microbrain) generally consists of a set of three fused pairs of ganglia, which constitute the brain in most insect species and in some other closely related arthropods, such as myriapods and crustaceans. It receives and processes information from the first, second, and third metameres. The supraesophageal ganglion lies dorsal to the esophagus and consists of three parts, each a pair of ganglia that may be more or less pronounced, reduced, or fused depending on the genus:

The subesophageal ganglion continues the nervous system and lies ventral to the esophagus. Finally, the segmental ganglia of the ventral nerve cord are found in each body segment as a fused ganglion; they provide the segments with some autonomous control.

A spiracle or stigma is the opening in the exoskeletons of insects, myriapods, velvet worms and many arachnids to allow air to enter the trachea. Insect respiratory system differs from vertebrates'. The circulatory system plays a relatively minor role in circulating oxygen and removing carbon dioxide; instead, trachea and air sacs in the insect body allow direct gas exchange, and these tracheal tubes eventually connect to the external environment via spiracles.

In most species, the spiracles are controlled by motor neurons in the central nervous system. It can be opened and closed in an efficient manner to admit air while minimizing associated physiological costs, such as water loss during respiration. Many sensory stimuli can affect the control of spiracles in insects, e.g. chemosensory (carbon dioxide, oxygen, etc.) or mechanosensory (sound, touch, etc.). It has been shown that during metabolically intensive behaviors, such as flight, insects can dynamically modulate the spiracle opening size just so to meet the metabolic demand of flight, while not losing too much water.

Rhyniognatha is an extinct genus of arthropod of disputed placement. It has been considered in some analyses as the oldest insect known, as well as possibly being a flying insect. Rhyniognatha is known from a partial head with preserved mouthparts from the Early Devonian aged Rhynie chert around 400 million years ago, when Earth’s first terrestrial ecosystems were being formed. The type, and only species is R. hirsti, which was named and described in 1928. Other analyses have interpreted the specimen as a myriapod.

The clade Mandibulata constitutes one of the major subdivisions of the phylum Arthropoda, alongside Chelicerata. Mandibulates include the crustaceans, myriapods (centipedes and millipedes, among others), and all true insects. 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 and centipedes, among others) and Pancrustacea (including crustaceans and hexapods, the latter group containing insects). Molecular phylogenetic studies suggest that the living arthropods are related as shown in the cladogram below. Crustaceans do not form a monophyletic group as insects and other hexapods have evolved from within them.

Centipedes (from Neo-Latin centi-, "hundred", and Latin pes, pedis, "foot") are predatory arthropods belonging to the class Chilopoda (Ancient Greek χεῖλος, kheilos, "lip", and Neo-Latin suffix -poda, "foot", describing the forcipules) of the subphylum Myriapoda, an arthropod group which includes millipedes and other multi-legged animals. Centipedes are elongated segmented (metameric) animals with one pair of legs per body segment. All centipedes are venomous and can inflict painful stings, injecting their venom through pincer-like appendages known as forcipules or toxicognaths, which are actually modified legs instead of fangs. Despite the name, no species of centipede has exactly 100 legs; the number of pairs of legs is an odd number that ranges from 15 pairs to 191 pairs.

Centipedes are predominantly generalist carnivorous, hunting for a variety of prey items that can be overpowered. They have a wide geographical range, which can be found in terrestrial habitats from tropical rainforests to deserts. Within these habitats, centipedes require a moist microhabitat because they lack the waxy cuticle of insects and arachnids, causing them to rapidly lose water. Accordingly, they avoid direct sunlight by staying under cover or by being active at night.

Pancrustacea is the clade that comprises all crustaceans and all hexapods (insects and relatives). This grouping is contrary to the Atelocerata hypothesis, in which Hexapoda and Myriapoda are sister taxa, and Crustacea are only more distantly related. As of 2010, the Pancrustacea taxon was considered well accepted, with most studies recovering Hexapoda within Crustacea. The clade has also been called Tetraconata, referring to having a four-part cone in the ommatidium. The term "Tetraconata" is preferred by some scientists in order to avoid confusion with the use of "pan-" to indicate a clade that includes a crown group and all of its stem group representatives.