Onychophora

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Onychophora

(ŏn'əkŏf`ərə), small phylum with about 70 species of animals that are often called "missing links" between annelids (phylum AnnelidaAnnelida
[Lat., anellus=a ring], phylum of soft-bodied, bilaterally symmetrical (see symmetry, biological), segmented animals, known as the segmented, or annelid, worms.
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) and arthropods (phylum ArthropodaArthropoda
[Gr.,=jointed feet], largest and most diverse animal phylum. The arthropods include crustaceans, insects, centipedes, millipedes, spiders, scorpions, and the extinct trilobites.
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). The thin cuticle and wormlike form of onychophorans are reminiscent of annelids, while the tracheal system and details of embryonic development foreshadow similar developments in arthropods. The unsegmented, stumpy legs with arthropodlike claws are unique. Onychophorans live inconspicuously in surface litter and under fallen logs, where humidity is high and protection against drying is greatest. Ancient onychophorans were marine. Modern species live in widely scattered wet, warm to temperate habitats in South Africa, Central America, and New Zealand, suggesting a much wider distribution of terrestrial forms in the past. The 10 genera are often referred to collectively as Peripatus.

Onychophora

The only living animal phylum with true lobopods (annulate, saclike legs with internal musculature). There are about 70 known living species in two families, Peripatopsidae and Peripatidae. These terrestrial animals are frequently referred to as Peripatus. Onychophora comprise a single class or order of the same name. They were once considered a missing link between annelid worms and arthropods, but are best considered to be aligned with the arthropods.

They have a cylindrical body, 0.5–6 in. (1.4–15 cm) long, with one antennal pair, an anterior ventral mouth, and 14–43 pairs of stubby, unsegmented legs ending in walking pads and paired claws. Mandibles are present as modified tips of the first appendage pair. The body surface has a flexible chitinous cuticle. The body wall has three layers of smooth muscle, as in annelids, but the coelom is reduced to gonadal and nephridial cavities; the body cavity has an arthropodlike partitioned hemocoel; the heart is tubular with metameric ostia; and the nephridia are segmental. Gas exchange takes place by means of tracheae; spiracles are minute and numerous, located between skin folds. Slow locomotion is effected by legs and body contractions; the animals can squeeze into very tight spaces. The eyes, located at the antennal base, are the direct type with a chitinous lens and retinal layer. The sexes are separate; the testes and ovaries are paired; and the genital tracts open though the posterior ventral pore. Onychophora are oviparous, ovoviviparous, or viviparous.

The Onychophora are predatory, feeding on small invertebrates. They are largely nocturnal, occurring in humid habitats in forests.

Onychophora

 

a subphylum of moisture-loving terrestrial invertebrates. The animals are closely related to annelids and arthropods in terms of basic structure. Some zoologists classify Onychophora as an independent phylum. The subphylum contains the single class Protracheata, which embraces several dozens of species. The body, which resembles that of a caterpillar, is up to 10 cm long and is covered with a soft cuticle having numerous transverse folds. The head is not marked off from the body and bears a pair of antennae and a pair of ocelli. The oral cavity contains a pair of hooklike jaws. The trunk has 14 to 43 pairs of unsegmented legs, which are outgrowths of the body wall. The dermomuscular sac consists of annular external and smooth, longitudinal internal muscles. The respiratory organs are bundles of tracheae whose spiracles are distributed over the entire body surface. Excretory organs of the coelom-tubule type open at the bases of the legs. The digestive system consists of a tubule that extends the length of the body. It is made up of a long midgut, a short esophagus, and a short posterior hindgut. The circulatory system is a tubular dorsal vessel (heart) that has openings through which hemolymph enters the heart from the mixed body cavity. The nervous system is scalene.

The sexes are separate in Onychophora. The male attaches spermatophores to the skin of the female, and the spermatozoa travel through cracks in the skin to the egg in the ovary. The male may deposit the the spermatophores on a substrate, and the female picks them up with the genital pore. Fertilization occurs in the body of the female. Onychophora of the genus Ooperipatus deposit eggs; the remaining species are ovoviviparous (for example, in the genus Eoperipatus) or viviparous (for example, in the genera Peripatopsis and Peripatus). In ovoviviparous forms, the eggs hatch in the genital passages of the female. In viviparous species, a placenta forms in the uterus, through which nutrients enter the embryo from the female’s hemolymph.

All Onychophora are predators. To catch their prey, they eject a sticky mucus through the openings of special glands, which discharge from the oral papillae.

Onychophora are distributed on all continents of the southern hemisphere, on the islands of the Malay Archipelago, in New Guinea, and in New Zealand. In the northern hemisphere the invertebrates are found only in the Himalayas and Mexico.

M. S. GILIAROV

Onychophora

[‚än·ə′käf·ə·rə]
(invertebrate zoology)
A phylum of wormlike animals that combine features of both the annelids and the arthropods.
References in periodicals archive ?
Their current research, however, focuses on velvet worms (onychophorans), which can be thought of as worms with legs and are closely related to arthropods.
Even though velvet worms occur in several areas of the world, the Costa Rican species are of particular scientific importance because they have been studied in fields other than taxonomy.
Comparative anatomy of slime glands in Onychophora (velvet worms).
Slime protein profiling: a non-invasive tool for species identification in Onychophora (velvet worms).
Position and development of oocytes in velvet worms shed light on the evolution of the ovary in Onychophora and Arthropoda.
Neural development in Onychophora (velvet worms) suggests a step-wise evolution of segmentation in the nervous system of Panarthropoda.
A revision of brain composition in Onychophora (velvet worms) suggests that the tritocerebrum evolved in arthropods.
Geographic variation of habitats in Costa Rican velvet worms (Onychophora: Peripatidae).
Reproductive trends, habitat type and body characteristics in velvet worms (Onychophora).
Disparity, decimation and the Cambrian "explosion": comparison of early Cambrian and present faunal communities with emphasis on velvet worms (Onychophora).