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Onychophora

life science Maturity 11-13 Next article →

Onychophora is a phylum of soft-bodied, many-legged animals.

Velvet worm
Velvet worm

They are commonly known as velvet worms due to their velvety texture. Some people also call them peripatus. This name comes from the first genus described, Peripatus. These animals are unique among living animals. They are the only phylum in the kingdom Animalia that is entirely terrestrial. This means they live only on land. They are closely related to arthropods and tardigrades. Scientists group these three together in a taxon called Panarthropoda.

Onychophora global distribution map
Onychophora global distribution map

The anatomy of a velvet worm is quite specialized. Their bodies are segmented and shaped like flattened cylinders. They possess many rows of unstructured appendages called lobopods or oncopods. These are often described as "stub feet." The number of leg pairs varies by species. For example, Ooperipatellus nanus has only 13 pairs. However, Plicatoperipatus jamaicensis can have as many as 43 pairs.

Euperipatoides claw pair
Euperipatoides claw pair

Their skin features fine transverse rings. While many are orange, red, or brown, some species show bright colors like blue, gold, or green.

Velvet worms have a fascinating method for hunting and defense. They use oral papillae, which are modified limbs on the sides of the head.

Ooperipatellus viridimaculatus 34495501
Ooperipatellus viridimaculatus 34495501

Inside the body, large, branched slime glands connect to these papillae. These glands can be more than half the length of the worm's body. When threatened or hungry, the worm squirts adhesive slime through the papillae. The system uses a geometric amplifier to turn slow muscle contractions into fast squirts.

Peripatopsis overbergiensis 45218773
Peripatopsis overbergiensis 45218773

The slime is ejected in two streams at speeds of 5 to 10 meters per second. These streams cross in mid-air to create a disordered net of threads. This net can reach distances of up to ten centimeters.

Peripatopsis capensis internal anatomy IMG 0781a
Peripatopsis capensis internal anatomy IMG 0781a

Producing this slime requires a massive amount of biological energy. The slime is 90% water, but its dry residue is mostly protein, specifically a collagen-type protein. It also contains lipids and a surfactant called nonylphenol. This is the only known organism that produces nonylphenol. This substance may prevent the slime from sticking to the worm's own ejection channels. It may also slow the drying process. Because it takes about 24 days to replenish the slime, velvet worms often eat their own dried slime. This slime can account for up to 11% of the organism's total dry weight.

The legs, or lobopods, function differently than the jointed legs of insects. They are hollow, conical appendages that lack joints. Instead, they maintain rigidity through hydrostatic pressure from internal fluids. Animals move these legs by stretching or contracting their entire bodies. They can also use internal muscles to bend the legs at any point. Each leg features a pair of retractable, hardened claws.

Euperipatoides claw pair
Euperipatoides claw pair

These claws are made of sclerotised α-chitin reinforced with phenols and quinones. The claws are built like nesting dolls with three stacked elements. This allows the worm to shed the outer layer during ecdysis while keeping a fully formed layer underneath.

Velvet worms have several specialized sensory organs to navigate their world. On the first head segment, they have slender antennae for sensory perception. Behind these antennae, most species have a simple eye called an ocellus. This eye consists of a chitinous ball lens, a cornea, and a retina. The retina uses photoreceptors that are modified flagellated cells.

Their bodies are also covered in papillae, which are warty protrusions. These papillae are responsive to touch and carry mechanoreceptive bristles. This allows the velvet worm to sense its immediate environment through physical contact.

Onychophorans are of great interest to paleontologists. They help scientists reconstruct the ancestral arthropod. Only two fossil species are confidently assigned to this phylum. Antennipatus lived during the Late Carboniferous period.

Helenodora
Helenodora

Cretoperipatus lived during the Late Cretaceous period and belonged to the Peripatidae family. The distribution of living species is also very distinct. The Peripatidae family is mostly found in equatorial and tropical regions. In contrast, the Peripatopsidae family is found entirely south of the equator.

Onychophora global distribution map
Onychophora global distribution map
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🖼️ Images & Media (22)
File:Peripatoides novaezealandiae 272115998.jpg
Peripatoides novaezealandiae 272115998.jpg
File:Ooperipatellus viridimaculatus 34495501.jpg
Ooperipatellus viridimaculatus 34495501.jpg
File:Peripatopsis overbergiensis 45218773.gif
Peripatopsis overbergiensis 45218773.gif
Euperipatoides claw pair.tif
File:Peripatopsis capensis internal anatomy IMG 0781a.JPG
Peripatopsis capensis internal anatomy...
File:Dissected Euperipatoides.jpg
Dissected Euperipatoides.jpg
File:Onychophora_global_distribution_map.jpg
Onychophora_global_distribution_map.jpg
File:Peripatoides 226306189.gif
Peripatoides 226306189.gif
File:New Zealand peripatus, Wellington, NZ imported from iNaturalist photo 601149090.jpg
New Zealand peripatus, Wellington, NZ...
File:Eoperipatus_totoro.jpg
Eoperipatus_totoro.jpg
File:Velvet worm.png
Velvet worm.png
File:SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2 (white background).png
SEM image of Milnesium tardigradum in...

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