Velvet worm

Onychophora (from Ancient Greek, onyches, 'claws'; and pherein, 'to carry'), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, Peripatus), is a phylum of elongate, soft-bodied, many-legged panarthropods. In appearance they have variously been compared to worms with legs, caterpillars, and slugs. They prey upon smaller animals such as insects, which they catch by squirting an adhesive slime. Approximately 200 species of velvet worms have been described, although the true number of species is likely greater. The two extant families of velvet worms are Peripatidae and Peripatopsidae. They show a peculiar distribution, with the peripatids being predominantly equatorial and tropical, while the peripatopsids are all found south of the equator. It is the only phylum within Animalia that is wholly endemic to terrestrial environments. Velvet worms are considered close relatives of the Arthropoda and Tardigrada, with which they form the taxon Panarthropoda. This makes them of palaeontological interest, as they can help reconstruct the ancestral arthropod. In modern zoology, they are particularly renowned for their curious mating behaviour and for bearing live young. Velvet worms are segmented animals with a flattened cylindrical body cross-section and rows of unstructured body appendages known as oncopods or lobopods (informally: stub feet). The animals grow to between 0.5 and 20 cm (.2 to 8 in), with the average being about 5 cm (2  in), and have between 13 and 43 pairs of legs. Their skin consists of numerous, fine transverse rings and is often inconspicuously coloured orange, red or brown, but sometimes also bright green, blue, gold or white, and occasionally patterned with other colours. Segmentation is outwardly inconspicuous, and identifiable by the regular spacing of the pairs of legs and in the regular arrangement of skin pores, excretion organs and concentrations of nerve cells. The individual body sections are largely unspecialised; even the head develops only a little differently from the abdominal segments. Segmentation is apparently specified by the same gene as in other groups of animals, and is activated in each case, during embryonic development, at the rear border of each segment and in the growth zone of the stub feet. Although onychophorans fall within the protostome group, their early development has a deuterostome trajectory, (with the mouth and anus forming separately); this trajectory is concealed by the rather sophisticated processes which occur in early development. The stub feet that characterise the velvet worms are conical, baggy appendages of the body, which are internally hollow and have no joints. Although the number of feet can vary considerably between species, their structure is basically very similar. Rigidity is provided by the hydrostatic pressure of their fluid contents, and movement is usually obtained passively by stretching and contraction of the animal's entire body. However, each leg can also be shortened and bent by internal muscles. Due to the lack of joints, this bending can take place at any point along the sides of the leg. In some species, two different organs are found within the feet: On each foot is a pair of retractable, hardened (sclerotised) chitin claws, which give the taxon its scientific name: Onychophora is derived from the Greek onyches, 'claws'; and pherein, 'to carry'. At the base of the claws are three to six spiny 'cushions' on which the leg sits in its resting position and on which the animal walks over smooth substrates. The claws are used mainly to gain a firm foothold on uneven terrain. Each claw is composed of three stacked elements, like Russian nesting dolls. The outermost is shed during ecdysis, which exposes the next element in — which is fully formed, so does not need time to harden before it is used. (This distinctive construction identifies many early Cambrian fossils as early offshoots of the onychophoran lineage.) Apart from the pairs of legs, there are three further body appendages, which are at the head and comprise three segments: The surface of the mandibles is smooth, with no ornamentation. The cuticle in the mandibles (and claws) is distinct from the rest of the body. It has an inner and outer component; the outer component has just two layers (whereas body cuticle has four), and these outer layers (in particular the inner epicuticle) are dehydrated and strongly tanned, affording toughness. On the third head segment, to the left and right of the mouth, are two openings designated 'oral papillae'. Within these are a pair of large, heavily internally branched slime glands. These lie roughly in the centre of the body and secrete a sort of milky-white slime, which is used to ensnare prey and for defensive purposes. Sometimes the connecting 'slime conductor' is broadened into a reservoir, which can buffer pre-produced slime. The slime glands themselves are probably modified crural glands. All three structures correspond to an evolutionary origin in the leg pairs of the other segments. Unlike the arthropods, velvet worms do not possess a rigid exoskeleton. Instead, their fluid-filled body cavity acts as a hydrostatic skeleton, similarly to many unrelated soft-bodied animals that are cylindrically shaped, for example sea anemones and various worms. Pressure of their incompressible internal bodily fluid on the body wall provides rigidity, and muscles are able to act against it. The body wall consists of a non-cellular outer skin, the cuticula; a single layer of epidermis cells forming an internal skin; and beneath this, usually three layers of muscle, which are embedded in connective tissues. The cuticula is about a micrometer thick and covered with fine villi. In composition and structure, it resembles the cuticula of the arthropods, consisting of α-chitin and various proteins, although not containing collagen. It can be divided into an external epicuticula and an internal procuticula, which themselves consist of exo- and endo-cuticula. This multi-level structure is responsible for the high flexibility of the outer skin, which enables the velvet worm to squeeze itself into the narrowest crevices. Although outwardly water-repellent, the cuticula is not able to prevent water loss by respiration, and, as a result, velvet worms can live only in microclimates with high humidity to avoid desiccation. The surface of the cuticula is scattered with numerous fine papillae, the larger of which carry visible villi-like sensitive bristles. The papillae themselves are covered with tiny scales, lending the skin a velvety appearance (from which the common name is likely derived). It also feels like dry velvet to the touch, for which its water-repellent nature is responsible. Moulting of the skin (ecdysis) takes place regularly, around every 14 days, induced by the hormone ecdysone. The inner surface of the skin bears a hexagonal pattern. At each moult, the shed skin is replaced by the epidermis, which lies immediately beneath it; unlike the cuticula, this consists of living cells. Beneath this lies a thick layer of connective tissue, which is composed primarily of collagen fibres aligned either parallel or perpendicular to the body's longitudinal axis. The colouration of Onychophora is generated by a range of pigments. The solubility of these pigments is a useful diagnostic character: in all arthropods and tardigrades, the body pigment is soluble in ethanol. This is also true for the Peripatidae, but in the case of the Peripatopsidae, the body pigment is insoluble in ethanol.