Phylum Onychophora – Characteristics, Classification

Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
Superphylum:	Ecdysozoa
(unranked):	Panarthropoda
Phylum:	Onychophora Grube, 1853
Class:	Udeonychophora

• The phylum Onychophora, also known as velvet worms or peripatus, is a fascinating group of organisms that belongs to the Animalia kingdom. The name Onychophora is derived from the Ancient Greek words “onyches” meaning “claws” and “pherein” meaning “to carry.” This name is fitting, as these creatures possess claw-like appendages and are known for their unique characteristics.
• Velvet worms have an elongated and soft-bodied structure with numerous legs, giving them a distinctive appearance. They have been compared to worms with legs, caterpillars, and slugs due to their velvety texture and worm-like shape. With their unique physique, they have captivated the attention of scientists and nature enthusiasts alike.
• One of the intriguing aspects of velvet worms is their predatory nature. They feed on other invertebrates and employ a unique hunting strategy. When hunting, they eject a sticky adhesive slime that ensnares their prey, allowing them to capture it effectively. This predatory behavior showcases their remarkable adaptation for survival in their environments.
• The phylum Onychophora consists of approximately 200 described species, although the actual number is likely greater. These species are divided into two families: Peripatidae and Peripatopsidae. The distribution of velvet worms is quite peculiar, with the peripatids being predominantly found in equatorial and tropical regions, while the peripatopsids are exclusively located south of the equator. This distribution adds to the unique nature of this phylum.
• One notable aspect of Onychophora is their exclusive adaptation to terrestrial environments. They are the only phylum within the Animalia kingdom that is entirely endemic to land. This characteristic sets them apart from other phyla, as they have adapted and thrived in various terrestrial habitats.
• In terms of evolutionary relationships, velvet worms are considered close relatives of two other phyla: Arthropoda and Tardigrada. Together, these three phyla form the proposed taxon known as Panarthropoda. The study of velvet worms is of great interest to paleontologists as they provide valuable insights into the ancestral arthropod, helping to reconstruct the evolutionary history of these diverse creatures.
• Besides their evolutionary significance, velvet worms are also renowned for their intriguing mating behaviors. They exhibit unique courtship rituals, and some species even bear live young. These reproductive characteristics contribute to their reputation as remarkable organisms in the field of zoology.
• In conclusion, the phylum Onychophora, or velvet worms, are intriguing creatures with their soft-bodied structure, numerous legs, and velvety texture. They are skilled predators, capturing their prey using adhesive slime. With their peculiar distribution and adaptation to terrestrial environments, they stand out among other phyla. Their close relationship to Arthropoda and Tardigrada and their notable reproductive behaviors further enhance their scientific interest. The study of velvet worms continues to provide valuable insights into the evolution and diversity of the animal kingdom.

General Characteristics of Phylum Onychophora

Phylum Onychophora, commonly known as velvet worms, is a unique group of invertebrates that share characteristics of both arthropods and annelids. Here are some general characteristics of the phylum Onychophora:

1. Body Structure: Velvet worms have elongated and cylindrical bodies that are divided into distinct segments. They exhibit a soft and flexible cuticle, which gives them a velvety appearance.
2. Size: Velvet worms vary in size, with most species measuring between a few centimeters to several inches in length. However, some giant species can reach lengths of up to 20 centimeters.
3. Legs: They have numerous pairs of short, fleshy, and unjointed legs called oncopods. The number of legs varies among species, but it can range from 13 to over 43 pairs. The legs are equipped with claws for gripping and crawling.
4. Antennae: Velvet worms possess a pair of prominent, segmented antennae on their head. These antennae serve as sensory organs, helping them detect their environment and prey.
5. Mouthparts: They have a well-developed mouth located on the ventral side of the head. The mouth is equipped with a pair of jaw-like appendages called mandibles, which are used for feeding and manipulating prey.
6. Respiration: Onychophorans have a simple respiratory system that involves the diffusion of gases through their body surface. They lack specialized respiratory organs such as gills or lungs.
7. Sensory Organs: Velvet worms have simple eyes known as ocelli, which can detect light and dark but do not provide detailed vision. They also possess tactile sensory receptors distributed throughout their body.
8. Reproduction: Most onychophorans reproduce sexually, although parthenogenesis (asexual reproduction) has been observed in some species. Sexual reproduction involves the transfer of sperm from males to females, and fertilization usually occurs internally. They exhibit various modes of reproduction, including ovipary (egg-laying), ovovivipary (egg-live-bearing), and vivipary (live-bearing).
9. Habitat: Velvet worms are primarily found in moist terrestrial habitats, such as tropical rainforests, leaf litter, and damp soil. They prefer environments with high humidity to prevent desiccation.
10. Feeding Behavior: Onychophorans are carnivorous predators. They feed on a variety of small invertebrates, including insects, spiders, worms, and other arthropods. They use a combination of adhesive secretions and toxic saliva to capture and subdue their prey.

These are some of the general characteristics of the phylum Onychophora. Velvet worms represent a fascinating group of organisms with a unique combination of traits, occupying an evolutionary position between arthropods and annelids.

Classification of the Phylum

The phylum Onychophora, or velvet worms, includes approximately 200 species that are classified into two families: Peripatopsidae and Peripatidae. Due to the conservative body features of the genus Peripatus, defined categories above the family level are absent.

Fossil records indicate the presence of Onychophora since the Mid-Cambrian period, around 520 million years ago. The genus Aysheaia is one of the most primitive marine fossil representatives of Onychophora. Fossils have also been discovered in Baltic and Myanmar ambers. Onychophorans transitioned to land before the Late Ordovician period, and the two living families, Peripatopsidae and Peripatidae, became distinct by the Late Triassic. Other fossil genera include Hallucigena and Tertiapatus.

Here are the characteristics of the two living families:

I. Peripatopsidae:

• Number of legs varies from 14 to 19 pairs.
• Legs have three complete spinous pads.
• Absence of a diastema on the inner side of the jaws.
• Primary dermal papillae without a constriction.
• Nephridial openings between the third spinous pads on the 4th and 5th pairs of legs.
• Genital opening located between or behind the last pair of legs.
• Oviparous or ovoviviparous.

Peripatopsidae is found in Chile, South Africa, Australasia, New Britain, and New Guinea. The family comprises 39 genera, and ongoing revision work is being carried out by C. Brockmon, A. Reid, R. Gleeson, and H. Ruhberg.

II. Family Peripatidae:

• Number of legs varies from 19 to 43 pairs.
• Legs have four to six complete spinous pads.
• Presence of a diastema on the inner blade of the jaws.
• Primary dermal papillae with a constriction.
• Nephridial openings between the third spinous pad on the 4th and 5th pairs of legs.
• Genital opening located between the legs of the penultimate pair.
• Skin pigment is brownish and can be extracted by alcohol.
• Ovoviviparous or viviparous.

Peripatidae is distributed in Mexico, Central America, Northern South America, Galapagos Islands, West Indies, West equatorial Africa, and Southeast Asia. The family comprises 10 genera, including Eoperipatus, Epiperipatus, Heteroperipatus, Macroperipatus, Mesoperipatus, Oroperipatus, Peripatus, Plicatoperipatus, Speleoperipatus, and Typhloperipatus.

The genus Peripatus is further divided into four sections by Clark (1913): Plicatoperipatus, Macroperipatus, Peripatus, and Epiperipatus.

These are the classifications of the phylum Onychophora, providing insights into the families and genera within this unique group of organisms.

Anatomy of Onychophora

• The anatomy of onychophora, commonly known as velvet worms, is characterized by several unique features. Velvet worms have a flattened cylindrical body with a cross-sectional shape. They range in size from 0.5 cm to 20 cm, with an average length of about 5 cm. Their body is segmented, and each segment bears a pair of unstructured appendages called oncopods or lobopods. The number of leg pairs varies among species, ranging from 13 to 43.
• The skin of velvet worms is covered with numerous fine transverse rings and can exhibit various colors such as orange, red, brown, green, blue, gold, or white. Segmentation in onychophorans is not easily visible externally but can be identified by the regular spacing of leg pairs, skin pores, excretion organs, and nerve cell concentrations. The development of segmentation occurs during embryonic development, with each segment’s rear border and the growth zone of the oncopods activating the segmentation gene.
• The oncopods, or stub feet, are conical appendages that lack joints and are internally hollow. They provide rigidity through hydrostatic pressure from their fluid contents and allow movement through stretching and contraction of the entire body. Each leg can also be shortened and bent by internal muscles. The feet contain crural glands at the shoulder, which secrete pheromones, and coxal vesicles on the belly side, which likely serve in water absorption.
• A pair of retractable chitin claws is present on each foot, giving velvet worms their scientific name “Onychophora,” which means “claw-bearing.” The claws are used for gaining a firm foothold on uneven terrain and are composed of three stacked elements. The outermost element is shed during molting, exposing the fully formed next element. In addition to the legs, velvet worms have three other body appendages on the head, including slender antennae, mouthparts with jaws (mandibles), and oral papillae housing slime glands.
• The skin of velvet worms acts as a hydrostatic skeleton, and their body cavity is filled with a blood-like fluid called haemocoel. Oxygen exchange occurs through the body surface and fine unbranched tubes called tracheae, which draw oxygen deep into the organs. The digestive system of velvet worms includes a mouth, salivary glands, throat, oesophagus, and central intestine. Digestion is aided by mucus and hydrolytic enzymes produced by salivary glands.
• Overall, the anatomy of onychophora exhibits unique adaptations that allow them to thrive in their habitats. Their segmented body, stub feet, chitin claws, and specialized organs contribute to their survival and ecological roles.

Locomotion of Onychophora

• Onychophora, commonly known as velvet worms, exhibit a unique and fascinating mode of locomotion. They move in a slow and deliberate manner, making it difficult for their prey to detect their presence. When it comes to getting from one place to another, velvet worms crawl forward using their legs, but with a distinct walking pattern where both legs of a pair move simultaneously, without overlapping each other.
• To navigate their environment, velvet worms raise their trunk relatively high above the ground. They rely on the coordination of various muscles and body segments to achieve locomotion. Interestingly, the actual movement of the velvet worm is accomplished through local changes in body length, rather than exertion of leg muscles alone. These changes in body length are controlled by the annular and longitudinal muscles.
• When the annular muscles contract, the body cross-section is reduced, causing the corresponding segment to lengthen. This mechanism is similar to how other worms with hydrostatic skeletons operate. As the segment elongates, the legs of that segment are lifted and swung forward. Following this, the longitudinal muscles contract, shortening the segment and moving the legs, which are now in contact with the ground, to the rear. This leg stroke is the primary driver of forward movement.
• The coordination of stretching and contracting segments is facilitated by the nervous system. Contraction waves propagate along the length of the body, with each pair of legs swinging forward and then downward and rearward in a coordinated succession. This rhythmic pattern allows velvet worms to propel themselves forward. While their speed can vary, some species of velvet worms, like Macroperipatus, can reach speeds of up to four centimeters per second. However, a more typical pace for these creatures is around six body-lengths per minute.
• Interestingly, as velvet worms increase their speed, their bodies undergo changes. They become longer and narrower, adapting to the increased momentum. Additionally, the length of each leg varies during each stride, providing further agility in their locomotion.
• In summary, the locomotion of onychophorans, or velvet worms, is a slow and deliberate process that relies on the coordination of annular and longitudinal muscles. By controlling the length of their body segments, velvet worms are able to generate forward movement. This unique mode of locomotion showcases the remarkable adaptability and complexity of these ancient and intriguing creatures.

Feeding of Onychophora

Feeding in onychophorans, or velvet worms, is an intriguing process that showcases their predatory nature and unique adaptations. Velvet worms are nocturnal ambush predators, primarily hunting at night. They have the ability to capture prey that is at least their own size, although capturing a large prey item may require a significant amount of their mucus-secreting capacity.

These fascinating creatures have a diverse diet, feeding on a wide range of small invertebrates. Their prey includes woodlice, termites, crickets, book/bark lice, cockroaches, millipedes, centipedes, spiders, various worms, and even large snails. Depending on their size, velvet worms typically eat every one to four weeks. They are considered ecologically similar to centipedes in terms of their feeding habits.

The process of feeding for velvet worms involves several distinct phases. When encountering potential prey, they employ their smooth and gradual movements to approach stealthily, avoiding detection. Once they reach their prey, velvet worms softly touch it with their antennae to assess its size and nutritional value. After each gentle poke, the antennae are quickly retracted to prevent alerting the prey.

This investigation phase can last for more than ten seconds as the velvet worm determines whether to attack the prey or if the prey becomes alarmed and flees. Hungry velvet worms spend less time investigating and are more prompt in applying their slime, a crucial aspect of their feeding strategy. Once slime is secreted, velvet worms are determined to pursue and consume their prey to recover their energy investment. They may spend up to ten minutes searching for prey that has been removed and return to their slime to consume it. In the case of smaller prey, they may choose not to use slime at all.

After deciding to attack, the velvet worm identifies a vulnerable spot, such as a joint membrane in arthropod prey, and punctures it with its jaws. It injects saliva into the prey, rapidly killing it and initiating the process of digestion. While waiting for the prey to digest, the velvet worm salivates on its slime and begins to consume it, along with anything attached to it. It then tugs and slices at the initial perforation, allowing access to the liquefied interior of the prey.

The jaws of the velvet worm move backward and forward along the axis of the body, utilizing a combination of musculature and hydrostatic pressure, as opposed to the side-to-side clipping motion seen in arthropods. The pharynx of the velvet worm is specially adapted for sucking, enabling the extraction of liquefied tissue. The arrangement of the jaws around the tongue and lip papillae ensures a tight seal and the establishment of suction.

In social groups of velvet worms, the dominant female is the first to feed, preventing competitors from accessing the prey item for the initial hour of feeding. Subordinate individuals begin feeding once the dominant female allows them to do so. The number of males in the group peaks after the females start to leave the prey item. After feeding, individuals clean their antennae and mouth parts before rejoining the rest of the group.

This predatory feeding behavior in onychophorans is likely a result of their need to remain moist and the limited time available for finding food due to the risk of desiccation. Their feeding strategy demonstrates a strong selection for a low cost-benefit ratio, which cannot be achieved with a herbivorous diet.

In summary, the feeding habits of onychophorans involve stealthy approaches, careful assessment of prey, the application of slime, precise puncturing and injecting of saliva, and subsequent consumption and digestion of the prey. These unique feeding adaptations highlight the remarkable predatory nature and specialized mechanisms of velvet worms.

Reproduction and life-cycle of Onychophora

The reproduction and life cycle of onychophorans, or velvet worms, exhibit interesting variations among different species. While almost all species reproduce sexually, there is one exception called Epiperipatus imthurni, where reproduction occurs through parthenogenesis without the involvement of males.

In most species, there is a noticeable sexual dimorphism, with females being larger than males. Female velvet worms are usually fertilized only once during their lifetime, and copulation can occur before their reproductive organs are fully developed. In some cases, sperm cells are stored in a specialized reservoir within the female’s body, allowing them to remain viable for extended periods. Fertilization takes place internally, although the method of sperm transmission varies widely among species.

In many species, including the genus Peripatus, a package of sperm cells called a spermatophore is placed into the female’s genital opening. The precise process of spermatophore transfer is not fully understood, and a true penis has been observed only in species of the genus Paraperipatus. Some Australian species possess dimples or specialized dagger- or axe-shaped structures on their heads. For example, the male of Florelliceps stutchburyae presses a long spine against the female’s genital opening, likely positioning the spermatophore in this manner. During this process, the female supports the male by clasping him with the claws of her last pair of legs.

The mating behavior of certain Peripatopsis species is particularly intriguing. The male places small spermatophores on the female’s back or sides. Amoebocytes from the female’s blood collect at the deposition site, and enzymes secreted by the female decompose both the spermatophore’s casing and the body wall on which it rests. This releases the sperm cells, which then move freely through the haemocoel (the primary body cavity), penetrate the external wall of the ovaries, and fertilize the eggs. The mechanism behind this self-inflicted skin injury and its resistance to bacterial infections is not yet fully understood.

Velvet worms exhibit various modes of reproduction, including ovipary (egg-laying), ovovivipary (egg-live-bearing), and vivipary (live-bearing). Ovipary is limited to the family Peripatopsidae and is often found in regions with unpredictable food supply or unsettled climates. In these cases, the yolk-rich eggs are coated with a protective chitinous shell. Maternal care is unknown in oviparous species.

The majority of velvet worm species are ovoviviparous, where the medium-sized eggs, protected by a double membrane, remain in the uterus. The embryos do not receive direct nourishment from the mother but rely on the moderate quantity of yolk present in the eggs (known as lecithotrophic development). The young emerge from the eggs shortly before birth. This mode of reproduction is believed to be the ancestral form, and both oviparous and viviparous species likely evolved from ovoviviparous ancestors.

True live-bearing species exist in both families, particularly in tropical regions with stable climates and year-round food availability. In these species, embryos develop from tiny eggs and are nourished by their mother in the uterus, a process known as matrotrophic development. Nutrients are supplied either through direct secretion from the mother into the uterus or via a placental connection between the uterine epithelium and the developing embryo. The former method is found outside the Americas, while the latter is more common in the Americas and the Caribbean, and less frequently in the Old World. The gestation period can last up to 15 months, and the offspring are born in an advanced stage of development. Within a single female, embryos can be at different stages of development and descended from different males. Some species release young only at specific times of the year.

A female velvet worm can have between 1 and 23 offspring per year. The development from fertilized egg to adult takes between 6 and 17 months and does not involve a larval stage. This is likely the original mode of development for velvet worms. Some velvet worms have been known to live for up to six years.

Diagnostic Features of Phylum Onychophora

• The phylum Onychophora, also known as velvet worms, is comprised of terrestrial, carnivorous, and free-living animals. They possess several diagnostic features that distinguish them from other phyla.
• One notable characteristic of Onychophora is their elongated, cylindrical body, which exhibits bilateral symmetry. Unlike many other organisms, Onychophorans do not have a clearly differentiated head. However, they do possess a pair of annulated antennae at the anterior end, along with a ventral mouth. The mouth is flanked by a pair of mandibles, each equipped with two claw-like blades, as well as a pair of short, conical, oral papillae.
• The body surface of Onychophorans is covered by a thin, flexible, and permeable chitinous cuticle. Unlike some other arthropods, this cuticle is not divided into articulating plates. It is periodically molted to accommodate growth and regeneration.
• Another distinguishing feature of Onychophorans is the presence of numerous pairs of short, unjointed fleshy legs along the body. These legs are hollow evaginations of the body and are equipped with a terminal pad, pairs of claws, and intrinsic muscles. The number of leg pairs can vary among species, ranging from 14 to 43.
• The body cavity of Onychophorans is well-developed and haemocoelic, serving as a hydrostatic skeleton. They possess a tubular heart that is open at both ends and features a pair of lateral ostia in each segment. The blood of Onychophorans is colorless.
• Respiration in Onychophorans is facilitated by tracheas, which are simple, tubular structures that form tufts arising from numerous small spiracles. These gas exchange organs ensure efficient respiration in terrestrial environments.
• Onychophorans possess paired segmental nephridia as excretory organs. These sac-like structures feature a ciliated funnel and nephrostome, with the nephridiopore located on the inner base of each leg.
• The nervous system of Onychophorans consists of a large bilobed brain and a pair of widely separated ventral nerve cords. These nerve cords are joined by commissures but lack distinct ganglia.
• Sense organs in Onychophorans include a pair of annular antennae, each housing a small, simple eye at its base. These sensory structures help Onychophorans navigate their environment and detect stimuli.
• Onychophorans are gonochoristic, meaning they have separate male and female individuals. They possess paired gonads, and fertilization occurs internally through spermatophores. Cleavage in Onychophorans is superficial.
• Reproduction in Onychophorans can vary among species. While some are oviparous, laying eggs externally, many species internally brood their eggs and give birth to live young.
• These diagnostic features collectively characterize the phylum Onychophora and distinguish them from other animal groups. Their unique anatomical and physiological traits enable them to thrive in terrestrial habitats as carnivorous, free-living organisms.

FAQ

References

• https://keys.lucidcentral.org/keys/v3/TFI/start%20key/key/Starting%20key/Media/HTML/Onychophora.html
• https://www.notesonzoology.com/phylum-onychophora/phylum-onychophora-classification-and-features-anthropods/1771
• https://www.biologydiscussion.com/invertebrate-zoology/onychophora/onychophora-anatomical-peculiarities-affinities-and-classification/33657#Classification_of_Onychophora