|Representatives of the four extant myriapod classes. Clockwise from top left: Chilopoda, Diplopoda, Symphyla, and Pauropoda.|
|Subphylum:|| Myriapoda |
Myriapods (from Ancient Greek μυρίος (muríos) 'ten thousand',and πούς (poús) 'foot') are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. 
The fossil record of myriapods reaches back into the late Silurian, although molecular evidence suggests a diversification in the Cambrian Period,  and Cambrian fossils exist which resemble myriapods.  The oldest unequivocal myriapod fossil is of the millipede Pneumodesmus newmani , from the late Silurian (428 million years ago). P. newmani is also important as the earliest known terrestrial animal.   The phylogenetic classification of myriapods is still debated.
The scientific study of myriapods is myriapodology, and those who study myriapods are myriapodologists. 
Myriapods have a single pair of antennae and, in most cases, simple eyes. Exceptions are the two classes symphylans and pauropods, and the millipede order Polydesmida and the centipede order Geophilomorpha, which are all eyeless.  The house centipedes (Scutigera) on the other hand, have large and well-developed compound eyes.  The mouthparts lie on the underside of the head, with an "epistome" and labrum forming the upper lip, and a pair of maxillae forming the lower lip. A pair of mandibles lie inside the mouth. Myriapods breathe through spiracles that connect to a tracheal system similar to that of insects. There is a long tubular heart that extends through much of the body, but usually few, if any, blood vessels. 
Malpighian tubules excrete nitrogenous waste into the digestive system, which typically consists of a simple tube. Although the ventral nerve cord has a ganglion in each segment, the brain is relatively poorly developed. 
During mating, male myriapods produce a packet of sperm, or spermatophore, which they must transfer to the female externally; this process is often complex and highly developed. The female lays eggs which hatch as much-shortened versions of the adults, with only a few segments and as few as three pairs of legs. With the exception of the two centipede orders Scolopendromorpha and Geophilomorpha, which have epimorphic development (all body segments are formed segments embryonically), the young add additional segments and limbs as they repeatedly moult to reach the adult form. 
The process of adding new segments during postembryonic growth is known as anamorphosis, of which there are three types: euanamorphosis, emianamorphosis, and teloanamorphosis. In euanamorphosis, every moult is followed by addition of new segments, even after reaching sexual maturity; in emianamorphosis, new segments are added until a certain stage, and further moults happen without addition of segments; and in teloanamorphosis, where the addition of new segments stops after the adult form is reached, after no further moults occur. 
Myriapods are most abundant in moist forests, where they fulfill an important role in breaking down decaying plant material,  although a few live in grasslands, semi-arid habitats or even deserts.  A very small percentage of species are littoral (found along the sea shore).   The majority are detritivorous, with the exception of centipedes, which are chiefly nocturnal predators. A few species of centipedes and millipedes are able to produce light and are therefore bioluminescent.  Pauropodans and symphylans are small, sometimes microscopic animals that resemble centipedes superficially and live in soils. Millipedes differ from the other groups in having their body segments fused into pairs, giving the appearance that each segment bears two pairs of legs, while the other three groups have a single pair of legs on each body segment.
Although not generally considered dangerous to humans, many millipedes produce noxious secretions (often containing benzoquinones) which in rare cases can cause temporary blistering and discolouration of the skin.  Large centipedes, however, can bite humans, and although the bite may cause intense pain and discomfort, fatalities are extremely rare. 
There has been much debate as to which arthropod group is most closely related to the Myriapoda.  Under the Mandibulata hypothesis, Myriapoda is the sister taxon to Pancrustacea, a group comprising the Crustacea and Hexapoda (insects and their close relatives). Under the Atelocerata hypothesis, Hexapoda is the closest, whereas under the Paradoxopoda hypothesis, Chelicerata is the closest. This last hypothesis, although supported by few, if any, morphological characters, is supported by a number of molecular studies. 
A 2020 study found numerous characters of the eye and preoral region suggesting that the closest relatives to crown myriapods are the extinct Euthycarcinoids.  There are four classes of extant myriapods, Chilopoda (centipedes), Diplopoda, Pauropoda and Symphyla, containing a total of around 12,000 species.  While each of these groups of myriapods is believed to be monophyletic, relationships among them are less certain. 
Centipedes make up the class Chilopoda. They are fast, predatory and venomous, hunting mostly at night. There are around 3,300 species,  ranging from the diminutive Nannarrup hoffmani (less than 12 mm or 1⁄2 in in length)  to the giant Scolopendra gigantea , which may exceed 30 centimetres (12 in).
Millipedes form the class Diplopoda. Most millipedes are slower than centipedes, and feed on leaf litter and detritus. Except for the first segment called collum, which don't have any appendages, and the next three segments with a single pair of legs each, they are distinguished by the fusion of each pair of body segments into a single unit, giving the appearance of having two pairs of legs per segment. It is also common for the sternites, pleurites and tergites to fuse into rigid armour rings.  The males produce aflagellate sperm cells, unlike the rest of the myriapods which produce flagellated sperm.  Around 12,000 species have been described, which may represent less than a tenth of the true global millipede diversity.  Although the name "millipede" is a compound word formed from the Latin roots millia ("thousand") and pes (gen. pedis) ("foot"), millipedes typically have between 36 and 400 legs. In 2021, however, was described Eumillipes persephone, the first species known to have 1,000 or more legs, possessing 1,306 of them.  Pill millipedes are much shorter, and are capable of rolling up into a ball, like pillbugs.
Symphylans, or garden centipedes, are closely related to centipedes and millipedes.   They are 3 to 6 cm long, and have 6 to 12 pairs of legs, depending on their life stage.    Their eggs, which are white and spherical and covered with small hexagonal ridges, are laid in batches of 4 to 25 at a time, and usually take up to 40 days to hatch.    There are about 200 species worldwide. 
Pauropoda is another small group of small myriapods. They are typically 0.5–2.0 mm long and live in the soil on all continents except Antarctica.[ citation needed ] Over 700 species have been described.  They are believed to be the sister group to millipedes, and have the dorsal tergites fused across pairs of segments, similar to the more complete fusion of segments seen in millipedes. 
Arthropleurideans were ancient myriapods that are now extinct, known from the late Silurian to the Permian. The most famous members are from the genus Arthropleura , which was a giant, probably herbivorous, animal that could be up to 2.63 metres (8 ft 8 in) long,  but the group also includes species less than 1 cm (0.39 in). Arthropleuridea was historically considered a distinct class of myriapods, but since 2000 scientific consensus has viewed the group as a subset of millipedes, although the relationship of arthropleurideans to other millipedes and to each other is debated.  
A variety of groupings (clades) of the myriapod classes have been proposed, some of which are mutually exclusive, and all of which represent hypotheses of evolutionary relationships. Traditional relationships supported by morphological similarities (anatomical or developmental similarities) are challenged by newer relationships supported by molecular evidence (including DNA sequence and amino acid similarities).  
Millipedes are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda, the name derived from this feature. Each double-legged segment is a result of two single segments fused together. Most millipedes have very elongated cylindrical or flattened bodies with more than 20 segments, while pill millipedes are shorter and can roll into a tight ball. Although the name "millipede" derives from Latin for "thousand feet", no species was known to have 1,000 or more until the discovery of Eumillipes persephone, which can have over 1,300 legs. There are approximately 12,000 named species classified into 16 orders and around 140 families, making Diplopoda the largest class of myriapods, an arthropod group which also includes centipedes and other multi-legged creatures.
Centipedes are predatory arthropods belonging to the class Chilopoda of the subphylum Myriapoda, an arthropod group which includes millipedes and other multi-legged animals. Centipedes are elongated segmented (metameric) creatures with one pair of legs per body segment. All centipedes are venomous and can inflict painful bites, injecting their venom through pincer-like appendages known as forcipules. Despite the name, no centipede has exactly 100 pairs of legs; they can have a varying number of legs, ranging from 15 pairs to 191 pairs, always an odd number. They are predominantly carnivorous.
Pauropods are small, pale, millipede-like arthropods. Around 830 species in twelve families are found worldwide, living in soil and leaf mold. They look rather like centipedes, or millipedes, and may be a sister group of the latter. However, this is controversial, as a close relationship with Symphyla has also been posited.
Symphylans, also known as garden centipedes or pseudocentipedes, are soil-dwelling arthropods of the class Symphyla in the subphylum Myriapoda. Symphylans resemble centipedes, but are very small, non-venomous, and only distantly related to both centipedes and millipedes. They can move rapidly through the pores between soil particles, and are typically found from the surface down to a depth of about 50 centimetres (20 in). They consume decaying vegetation, but can do considerable harm in an agricultural setting by consuming seeds, roots, and root hairs in cultivated soil.
Pauropodidae is a family of pauropods. It contains over 20 genera and 650 species, as well as the only known fossil pauropod, Eopauropus. Like most adult pauropods in the order Tetramerocerata, most adults in this family have 9 pairs of legs, but adults in one genus, Cauvetauropus, have only 8 pairs of legs, and female adults in another genus, Decapauropus, have either 9 or 10 pairs of legs. The first pauropod discovered with more than 9 pairs of legs was the species D. cuenoti, first described with 10 pairs in 1931.
Arthropleuridea is an extinct subclass of myriapod arthropods that flourished during the Carboniferous period, having first arose during the Silurian, and perishing in the Early Permian. Members are characterized by possessing diplosegement paranotal tergal lobes separated from the body axis by a suture, and by sclerotized plates buttressing the leg insertions. Despite their unique features, recent phylogenetic research suggests Arthropleuridea be included among millipedes in the class Diplopoda. The subclass contains three recognized orders, each with a single genus.
Glomerida is an order of pill-millipedes found primarily in the Northern Hemisphere. Also known as northern pill millipedes, they superficially resemble pill-bugs or woodlice, and can enroll into a protective ball. They have twelve body segments, 17 to 19 pairs of legs, and males have enlarged rear legs involved in mating. The order includes about 30 genera and at least 280 species, including Glomeris marginata, the common European pill-millipede. The order contains members in Europe, South-east Asia and the Americas from California to Guatemala. Although historically considered closely related with the similar sphaerotheriidans that also enroll, some DNA evidence suggest they may be more closely related to glomeridesmidans, a poorly known order that does not enroll.
Arthropods are invertebrate animals in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a segmented body, and paired jointed appendages. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. They are an extremely diverse group, with up to 10 million species.
The Geophilidae are a polyphyletic, cosmopolitan family of soil centipedes in the superfamily Geophiloidea containing the mostly defunct clades Aphilodontidae, Dignathodontidae, Linotaeniidae, Chilenophilinae, and Macronicophilidae.
Himantariidae is a monophyletic family of centipedes in the order Geophilomorpha and superfamily Himantarioidea, found almost exclusively in the Northern Hemisphere. The number of leg-bearing segments in this family ranges from 47 to 181. These centipedes are very elongated with a high mean number of trunk segments and great variability in this number within species. This family contains these genera:
Polyxenida is an order of millipedes readily distinguished by a unique body plan consisting of a soft, non-calcified body ornamented with tufts of bristles – traits that have inspired the common names "bristly millipedes" or "pincushion millipedes". There are at least 86 species in four families worldwide, and are the only living members of the subclass Penicillata.
Oryidae is a monophyletic family of soil centipedes belonging to the superfamily Himantarioidea.
Dendrothereua is a genus of house centipedes in the family Scutigeridae. There are at least three described species in Dendrothereua, found in the southern United States and the Neotropics.
Mecistocephalidae is a monophyletic family of centipedes in the order Geophilomorpha. It is the only family in the suborder Placodesmata. Most species in this family live in tropical or subtropical regions, but some occur in temperate regions. This family is the third most diverse in the order Geophiliomorpha, with about 170 species, including about 130 species in the genus Mecistocephalus.
Schendylidae is a family of centipedes in the order Geophilomorpha.
Kampecaris is an extinct genus comprising the Kampecarida, an enigmatic group of millipede-like arthropods, from the Silurian and early Devonian periods of Scotland and England. They are among the oldest known land-dwelling animals. They were small, short-bodied animals with three recognizable sections: an oval head divided along the midline, ten limb-bearing segments forming a cylindrical trunk that tapered slightly towards the front, and a characteristic swollen tail formed by a modified segment that tapers at its rear into an "anal segment". The cuticle forming their exoskeletons was thick, heavily calcified, and composed of two layers.
Ballophilidae is a monophyletic family of centipedes belonging to the order Geophilomorpha and superfamily Himantarioidea. Some authorities dismiss this group as a family, citing phylogenetic analysis, and instead refer to this clade as Ballophilinae, a possible subfamily within the family Schendylidae. The number of legs in this clade varies within species and ranges from 37 to 113 pairs of legs. Species in this clade tend to have more leg-bearing segments and greater intraspecific variability in this number than generally found in the family Schendylidae.
Gonibregmatidae are a paraphyletic family of soil centipedes belonging to the superfamily Geophiloidea.
The centipedes or Chilopoda are divided into the following orders.