Velvet worms, constituting the phylum Onychophora (hence their other name, onychophorans), are soft-bodied, segmented invertebrates named for their velvety texture (due to the scaly papillae on their skin) and worm-like appearance. Sometimes, they are ambiguously called the peripatus from their first described genus, Peripatus. They possess 13 to 43 pairs of conical walking legs called oncopods or lobopods.
They are most abundant in the tropics and the temperate zone of the Southern Hemisphere and are the only animals to be endemic to terrestrial environments. When attacked, these animals squirt glue-like slime in defense from special projections called oral papillae near their mouth.
Over 200 species of velvet worms are currently divided into two extant families: Peripatidae and Peripatopsidae. Though the number of legs remains fixed in peripatopsid species, they may be variable in peripatid species.
They range between 0.04 to 8.7 in (0.1 to 22 cm) in length. One of the smallest velvet worms, Ooperipatellus nanus, measures approximately 0.4 in (10 mm), whereas Solórzano’s velvet worm (Mongeperipatus solorzanoi), the largest, reaches 8.7 in (22 cm).
Velvet worms possess several conical appendages along their bodies, known as oncopods or lobopods (commonly referred to as stub feet). Their skin, though not easily noticeable, features numerous transverse rings, and their segmentation is identified by the regular arrangement of pores, legs, nerve cells, and excretory organs.
They generally exist in shades of orange, red, and brown but are sometimes patterned with green, blue, gold, or white.
The body wall of these invertebrates comprises a non-cellular outer layer called the cuticula, a cellular middle layer of the epidermis, followed by three consecutive layers of muscles embedded in a connective tissue layer.
Like in arthropods, the cuticula in Velvet worms is composed of proteins like α-chitin. The cuticula is further subdivided into an external epicuticula and an internal procuticula. Its surface is dotted with a number of fine papillae, often bearing villi-like sensitive bristles. It is the papillae that confer a velvety look to these animals.
Beneath the epidermal layer lies a thick layer of connective tissue composed mainly of collagen fibers arranged parallelly or perpendicularly to the longitudinal axis of the body. This tissue layer, in turn, has three continuous layers of smooth muscles. The outermost layer has annular muscles, while the innermost has longitudinal muscles. Sandwiched in between is a middle layer of thin diagonal muscles arranged spirally.
Along the length of their bodies are rows of conical, baggy appendages or legs lacking joints. Although these appendages are internally hollow, the hydrostatic pressure of the body fluid adds rigidity to them. They are often equipped with different organs, like coxal vesicles and crural glands. While the former is found on the belly side of the legs and helps in water absorption, crural glands, located at the base of the legs, secrete pheromones for chemical messaging.
Each foot bears a pair of hard, sclerotized claws (composed of α-chitin, phenols, and quinones), at the base of which lies three to six spiny cushion-like structures.
The head of a velvet worm’s body has three distinct segments, each containing specialized body appendages that serve functions that are not necessarily locomotory.
The body cavity (hemocoel) contains a blood-like liquid called the hemolymph. This liquid lacks pigments and is primarily used for the transport of nutrients rather than oxygen. It contains two types of blood cells: amoebocytes and nephrocytes. While amoebocytes have roles in immunity and reproduction, the latter absorbs toxins and breaks them into simpler forms for excretion.
The circulatory system is open and characterized by a tube-like heart. The hemocoel is divided by a diaphragm into two parts. The first part, the pericardial sinus, is at the back and encloses the heart, while the second part, the perivisceral sinus, lies along the belly. The heart comprises annular muscles consisting of epithelial tissues and bears two lateral openings called ostia per segment. Apart from the fine vessels running through the muscular layers of the body wall and the arteries supplying the antennae, there are no other blood vessels.
Although gaseous exchange takes place through diffusion from the body surface, most of it occurs through fine, unbranched tubes, the tracheae, which provide oxygen to all organs, including the heart.
The walls of the trachea are less than 3 μm thick and are covered by a thin membrane through which oxygen diffuses easily. The tracheae originate from tiny openings called spiracles, which are grouped in dent-like recesses of the outer skin, known as atria. There are around 75 trachea bundles per body segment.
Unlike arthropods, which can control the tracheal openings, the tracheae of velvet worms remain open all the time, causing considerable water loss in arid conditions.
The digestive tract sets off slightly behind the head, with the mouth lying on the underside, a little away from the anterior tip of the body. The different mouth parts help break the prey, assisted by two salivary glands that provide digestive, hydrolytic enzymes. It is followed by a chitin-lined pharynx and a muscular throat that comprises the first part of the front intestine. The rear part of the front intestine comprises the esophagus, which leads to the central intestine. This part of the intestine is composed of a single epithelial layer and helps coat the food particles with a mucus-based peritrophic membrane. This membrane protects the inner lining of the intestine from being damaged by jagged ends of the food particles.
The rear intestine opens into the rectum, where undigested food material arrives. Finally, it loops over and leads to the anus, located at the posterior end of the rectum.
Almost every segment of the body contains a pair of excretory organs called nephridia. Each nephridia contains a small pouch connected to an opening, the nephridiopore, through a flagellated duct called the nephridioduct. The pouch contains specialized cells called podocytes that help in ultrafiltration of the blood.
Nitrogenous waste is mostly expelled in the form of uric acid (uricotelic).
Both male and female velvet worms have a pair of gonads that open into a common genital opening, the gonopore, through the gonoduct.
In females, the ovaries are fused, but the anatomy of the gonoduct depends on whether the species is egg-laying (oviparous) or live-bearing (viviparous). In the latter, ovaries open into slender oviducts, followed by a uterus for the development of the embryo. In oviparous varieties, however, the gonopore lies at the tip of the long egg-laying apparatus (ovipositor). Some female velvet worms also have a special sperm-storage organ, the receptacle seminis, where sperm is stored temporarily.
In males, the testes are separate, followed by the corresponding sperm vesicle (the vesicula seminalis) and an exit channel, the vasa efferentia. The two exit channels drain into a common sperm duct, the vas deferens. Only males of the genus Paraperipatus have been found to possess a penis-like intromittent organ.
The phylum Onychophora gets its name from the Ancient Greek words onyches, meaning claws, and pherein, meaning carry. This name refers to the chitinous claws on the feet of these animals.
They are thought to be closely related to arthropods, owing to the similarities these two groups share. For instance, they are similar in the composition of their exoskeleton, reproductive and excretory organs, open circulatory systems with tubular hearts, tracheal respiration, and embryonic development. They also have two body appendages per segment. However, the antennae, mandibles, and oral papillae of the two groups are non-homologous and have developed independently. Moreover, these invertebrates are also closely related to water bears (phylum Tardigrada); however, the latter lack circulatory and respiratory structures.
The velvet worms, arthropods, and water bears together form a monophyletic taxon, the Panarthropoda.
According to the Articulata hypothesis by Georges Cuvier, velvet worms are considered closely related to the annelids. This consideration was based on similarities in body shape, layered musculature, flexible outer skin, paired excretory organs, and, most importantly, body segmentation. This hypothesis also presents velvet worms as an intermediate link between annelids and arthropods. It is believed that parapodia of annelids evolved into stub feet of velvet worms, which were later modified into appendages of arthropods.
Modern taxonomy, however, primarily focuses on cladistics, and though the Articulate hypothesis remains relevant, an alternative hypothesis, the Ecdysozoa Hypothesis, is most widely accepted today.
Currently, over 200 species of velvet worms fall under two broad families.
There are very few fossil specimens available due to their poor preservation potential.
It is believed that these animals switched from aquatic to terrestrial lifestyles around 490 to 430 million years ago, between the Ordovician and late Silurian Periods.
Although a few Cambrian fossils have velvet worm-like features, due to the lack of sufficient supporting evidence, their classification remains ambiguous. However, some, like the genera Hallucigenia and Collinsium, bear features unique to the phylum and are considered their undisputed members. Antennipatus, a fossil genus from France dating back to the Late Carboniferous Period, has many characteristics of a velvet worm and is probably the first terrestrial representative of its group.
In 2018, members of the genus Antennipatus were thought to be the oldest velvet worms, but some scientists debated this decision. Later, in 2021, reports from molecular dating revealed that the 300 million-year-old Antennipatus was a stem group member. Cretoperipatus from the Late Cretaceous Period (100 to 90 million years ago) is probably the oldest crown group representative of velvet worms.
Velvet worms are found in both tropical and temperate zones of the Southern Hemisphere. While peripatids live in equatorial and tropical regions, peripatopsids are restricted to the south of the Equator. Among all members of the animal kingdom, velvet worms are the only animals that are exclusively terrestrial.
They are abundant in Central and South America, the Caribbean islands, West and South Africa, New Guinea, Australia, New Zealand, and Asia. In Asia, they are mostly found in northeastern India, Thailand, Indonesia, and Malaysia. They have also been unintentionally introduced to Santa Cruz Island in the Galapagos.
Being photophobic (light-hating), they resort to dark and humid habitats, particularly in the rainforests and a few in grasslands, where they hide in crevices in the soil. Some prefer living among moss cushions and leaf litter, in termite tunnels and rotting wood, and under tree logs and stones. Sometimes, they are also spotted in cocoa and banana plantations in South America and the Caribbean islands.
They are carnivores feeding on small invertebrates like woodlice, termites, barklice, millipedes and centipedes, worms, spiders, and cockroaches. They also feed on various worms and snails, depending on their size.
While walking, these worms raise their trunks above the ground and move in non-overlapping steps. If the substrate is hard and rough, they use their claws for movement, whereas on soft surfaces, they utilize only the foot cushions at the base of these claws.
Rather than relying on their leg muscles for movement, these animals continuously alter their body length. As the annular muscles contract, the body’s cross-section narrows, and the corresponding segment elongates. Conversely, when the longitudinal muscles contract, the segment shortens. As speed increases during running, their bodies become increasingly longer and slimmer.
These animals are ambush predators that gently creep up on their prey at night and sense them using their antennae. They investigate the prey for over ten seconds and try to assess its size and nutritional value. If found suitable, they decide to feed on it. However, if these worms are too hungry, they instantly squirt their slime at the prey and consume it. In the case of small-sized prey, slime is not used at all.
They first identify a soft part of the prey’s body and then puncture it using the jaws, thus injecting their saliva. This digestive juice sets the process of chemical breakdown while the worms salivate on their slime, waiting for the prey to digest. As the feed is slowly digested, liquified tissue from the prey’s body is sucked in using the pharynx.
Velvet worms spend about 90% of their feeding time ingesting the prey. The rest of their time is spent locating, investigating, squirting, and injecting it.
Velvet worms typically live a solitary life; however, socialization is observed for activities like hunting and defense.
Members of the genus Euperipatoides, one of the most widely studied genera of velvet worms, form social groups of up to fifteen individuals. These groups hunt together, aggressively attacking individuals of other groups.
Dominance hierarchy is quickly established and maintained within these groups, with the superior individuals often chasing and biting the subordinate ones. A dominant female always gets the first taste of a kill, followed by other subordinate females, males, and the young.
These invertebrates may survive up to six years.
Velvet worms primarily reproduce sexually, with the exception of the parthenogenetic species Epiperipatus imthurni. They are gonochoric, existing in both male and female forms. In certain species, females tend to be larger than males (dimorphic) and often have more legs than them.
The mating process varies across the group, especially the ways in which the males transfer the spermatophore (sperm-filled capsules) to the female genital tract. In this process, most males produce pheromones from glands on the underside of their legs to attract potential mates. While some males deposit the spermatophore directly into the female body, others, especially some Australian species, place the capsule on the top of their heads and press it against the female genitals. Since sperm are mostly released inside the female body, fertilization is internal.
They exhibit all three forms of reproduction: oviparity (egg-bearing), viviparity (egg-live-bearing), and ovoviviparity (live-bearing). However, most species are ovoviviparous and produce medium-sized eggs that remain in the uterus. In contrast, members of the family Peripatopsidae are exclusively oviparous and lay yolk-rich eggs (1.3 to 2.0 mm in diameter) covered by a protective chitinous shell. Viviparous individuals are found in both families of velvet worms, which lay live young. In these individuals, the embryos develop within the uterus and are nourished by the placenta (mammotrophic).
After a gestation period of about 15 months, the offspring is born. In general, a female velvet worm gives birth to anywhere between 1 to 23 offspring. The development from a fertilized ovum to an adult usually takes around 6 to 17 months and lacks a larval stage.
They are preyed upon by spiders, centipedes, birds, and rodents. The clay-colored thrush (Turdus grayi) is a major predator in Central America, whereas, in South America, the orange-banded coral snake (Micrurus hemprichii) almost exclusively feeds on these animals.
A number of mites also parasitize on the skin of velvet worms.
Due to their naturally low population densities and nocturnal activity, it is challenging to assess the global conservation status of velvet worms accurately. Currently, only 11 species have been evaluated for the IUCN Red List and assigned under the following categories.
| Species | IUCN Category |
| Mesoperipatus tholloni | Data Deficient (DD) |
| Plicatoperipatus jamaicensis | Near Threatened (NT) |
| Peripatoides indigo, Peripatoides suteri, Peripatopsis alba, Peripatopsis clavigera | Vulnerable (VU) |
| Macroperipatus insularis, Leucopatus anophthalmus | Endangered (EN) |
| Opisthopatus roseus, Peripatopsis leonina, Speleoperipatus spelaeus | Critically Endangered (CR) |
The vulnerability of this group is attributed to habitat destruction and fragmentation due to rapid industrial growth, draining of wetlands, and unsuitable farming practices, like slash-and-burn agriculture.
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