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Information about Velvet Worm

Onychophora
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Velvet worm

Velvet worm
Scientific classification
Kingdom:Animalia
Phylum:Onychophora
Grube, 1853
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Global range of Onychophora: Peripatidae in green, Peripatopsidae in blue

Global range of Onychophora: Peripatidae in green, Peripatopsidae in blue
Extant families


Peripatidae
Peripatopsidae
Onychophora, the velvet worms (occasionally called walking worms or spitting worms), are segmented, caterpillar-like, terrestrial animals somewhat resembling both arthropods and annelid worms. They are generally regarded either as a class of arthropods or as a separate phylum (in which case they are the only animal phylum to include no known marine species). About 200 modern species are known separated into two families, with 10 genera — Peripatidae, found in many tropical and subtropical regions worldwide including Mexico, Central America, South America, and Africa, and Peripatopsidae, found in Chile, South Africa, Australia and New Zealand. Modern species are fairly small; at least one species reaches lengths of 20 cm (8 inches). The best known modern form is Peripatus which was described by Guilding in 1825.

Description

Onychophores are apparently descended from some segmented form but have lost their overt segmentation except for the head which is composed of three segments. There are two antennae on the first head segment and usually a pair of eyes. The second head segment has a mouth. Unlike arthropods, the animal does not have a stiff exoskeleton. Their skin (cuticle) is covered with fine hair (papillae) giving a velvet-like feel and accounting for their popular name of "velvet worms". The cuticle is shed periodically to allow growth. Although the body has several dozen repeated leg pairs, it is not actually segmented like annelids. The true coelom, like that of arthropods, is restricted almost entirely to the gonadal cavities. The hemocoel is also arthropod-like, being partitioned into sinuses, including a dorsal pericardial sinus. The entire structure is supported by blood pumped by a heart. Legs are tipped by chitinous claws and walking pads used on smooth surfaces. Onychophores breathe through passages in the skin called trachea which are always open. As a result, all known species require a humid environment to avoid desiccation.

Behaviour

Modern onychophores are predators that are able to immobilize animals several times their own size with an adhesive substance that they eject from glands in their head. They can immobilize targets up to 30 cm (12 in) away, and carry a quantity of fluid of as much as 10% of their body weight.

Velvet worms have an unusual method of transferring sperm. The male onychophore attaches a sperm packet to the female. They tend to be fairly indiscriminate where on the female they attach the packet. The tissue beneath the packet dissolves and the packet melts into the female's body. The sperm swim through the hemocoel to the ovaries. Velvet worms are live-bearers, and care for their young after birth.

Evolution

Onychophores are thought to be closely related to the arthropods. The structure of their brains is similar to spiders, raising the possibility that they are most closely related to arachnids, though molecular analysis contradicts this. Possible marine onychopores are known from the Lower Cambrian — Aysheaia, Hallucigenia — and possibly the late Pre-Cambrian — Xenusion. A single fossil terrestrial species, Helenodora inopinata, is known from the Pennsylvanian of Illinois, another one by the name of Cretoperipatus burmiticus from the Cretaceous of Myanmar and some Tertiary specimens are reported from Caribbean amber.

The similarity of these creatures to caterpillars is not purely coincidental; as with most creatures, the larval stage of moths and butterflies are thought to represent, however inconsistently, previous evolutionary stages. Arthropods are thought to have evolved from a multi-segmented animal not entirely unlike both onychophores and caterpillars. In fact, the characteristics of larval insects are part of the evidence cited to support models of insect ancestors, for example in justifying how many segments and legs such creatures are thought to have had, many of which evolved into other parts on modern insects, like antennae and mouthparts.

Classification

  • Phylum Onychophora
  • Class Onychophorida
  • Order † Paronychophora (extinct)
  • Family † Onychodictyidae
  • Genus † Onychodictyon
  • Order Euonychophora
  • Family Peripatidae
  • Genera: † Cretoperipatus, Eoperipatus, Epiperipatus, Heteroperipatus, Macroperipatus, Mesoperipatus, Oroperipatus, Peripatus, Plicatoperipatus, Speleoperipatus, Typhloperipatus
  • Family Peripatopsidae
  • Genera: Acanthokara, Aethrikos, Akthinothele, Anoplokaros, Austroperipatus, Baeothele, Centrorumis, Cephalofovea, Critolaus, Dactylothele, Dystactotylos, Euperipatoides, Florelliceps, Hylonomoipos, Konothele, Lathropatus, Leuropezos, Mantonipatus, Metaperipatus, Minyplanetes, Nodocapitus, Occiperipatoides, Ooperipatellus, Ooperipatus, Opisthopatus, Paraperipatus, Paropisthopatus, Peripatoides, Peripatopsis, Phallocephale, Planipallipus, Regimitra, Ruhbergia, Sphenoparme, Symperipatus, Tasmania, Tasmanipatus, Tetrameraden, Vescerro, Wambalana
  • Family incertae sedis
  • Genus † Helenodora

References

  • Richard C. Brusca & Gary J. Brusca (2003). Invertebrates. 2nd edition. Sinauer Associates, 936 pages. ISBN 0-87893-097-3. 

External links

Scientific classification or biological classification is a method by which biologists group and categorize species of organisms. Scientific classification also can be called scientific taxonomy, but should be distinguished from folk taxonomy, which lacks scientific basis.
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family (Latin: familia, plural familiae) is a rank, or a taxon in that rank. Exact details of formal nomenclature depend on the Nomenclature Code which applies.
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caterpillar is the larval form of a member of the order Lepidoptera (the insect order comprising butterflies and moths). They are mostly phytophagous in food habit, with some species being entomophagous.
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Terrestrial animals are animals that live predominantly or entirely on land, as compared with aquatic animals, which live predominantly or entirely in the water (e.g., fish, lobsters, octopuses), or amphibians, which rely on a combination of aquatic and terrestrial habitats (e.g.
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Arthropoda
Latreille, 1829

Subphyla and Classes
  • Subphylum Trilobitomorpha
  • Trilobita - trilobites (extinct)
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Annelida
Lamarck, 1809

Classes and subclasses

Class Polychaeta (paraphyletic?)
Class Clitellata*
   Oligochaeta - earthworms, etc.
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class is the rank in the scientific classification of organisms in biology below Phylum and above Order.

For example, Mammalia is the class used in the classification of dogs, whose phylum is Chordata (animals with notochords) and order is Carnivora (mammals that eat meat).
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Arthropoda
Latreille, 1829

Subphyla and Classes
  • Subphylum Trilobitomorpha
  • Trilobita - trilobites (extinct)
  • Subphylum Chelicerata

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phylum (Greek Φῦλον plural: Φῦλα phyla) is a taxon in the rank below kingdom and above class.
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species is one of the basic units of biological classification. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring.
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family (Latin: familia, plural familiae) is a rank, or a taxon in that rank. Exact details of formal nomenclature depend on the Nomenclature Code which applies.
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genus (plural: genera) is part of the Latinized name for an organism. It is a name which reflects the classification of the organism by grouping it with other closely similar organisms.
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tropics are the geographic region of the Earth centered on the equator and limited in latitude by the Tropic of Cancer in the northern hemisphere, at approximately 23°30' (23.5°) N latitude, and the Tropic of Capricorn in the southern hemisphere at 23°30' (23.5°) S latitude.
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Peripatus
Guilding, 1826

Peripatus is a genus of Onychophora, which is distinguished by its independent pairs of legs, a characteristic which many entomologists believe suggests an evolutionary link between arthropods and worms.
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18th century - 19th century - 20th century
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1822 1823 1824 - 1825 - 1826 1827 1828

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Antennae (singular antenna) are paired appendages connected to the front-most segments of arthropods. In crustaceans, they are biramous and present on the first two segments of the head, with the smaller pair known as antennules.
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cuticle or cuticula is given to a variety of tough but flexible, non-mineral outer coverings of an organism, or part of an organism, that provide protection. They are non-homologous, differing in their origin, structure and chemical composition.
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A papilla (plural: papillae) can be:
  • A small projection, such as a nipple-like projection on the skin, at the base of a hair or the root of a feather; the base of a new tooth.
  • A pimple or blister
  • An interdental papilla is the part of gingiva located between teeth.

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