Onychophora

Abstract

This page presents a brief overview of the Onychophora, from Cambrian to Recent.

Keywords: Onychophora, velvet worm, Peripatus, Peripatoides, Aysheaia, fossil history, systematics

Introduction

Onychophorans, of which there are several endemic species in New Zealand, share a number of characteristics with both annelids (segmented worms) and arthropods, although they are more closely related to the latter and are sometimes, as here, regarded as a class within the Arthropoda. Other authors (e.g. Nielsen 2001) regard the Onychophora as a phylum in its own right.

Currently there are around 10 genera and 110 species recognised within two extant families: the Peripatidae (known from the circumtropical regions of Mexico, Central and northern South America, equatorial West Africa, and South East Asia) and the Peripatopsidae (found in Chile, South Africa, Australia including Tasmania, and New Zealand).

Anatomy

[Some of the following is not my writing. Unfortunately, I cannot remember where it came from. My apologies to the unknown author whom I have failed to cite. I’ll rewrite it as soon as I can.]

The body is caterpillar-like, encased in a thin chitinous cuticle. The anterior end is indicated by the antennae and by the ventrally directed mouth, while the posterior end, projecting behind the last pair of walking legs, bears the terminal anus. There are many pairs of legs attached latero-ventrally, these being the only external signs of segmentation. The body is ringed by annuli on which are tubercles set in rows. Each tubercle ends in a tiny chitinous spine.

The body itself is not segemented except for the head, which is divided into three segments. The first contains the two large antennae with an eye at the base (Jamaica and South Africa have cave dwelling species which do not have eyes). Some males also have other appendages believed to be involved in sperm transfer.

The first pair of appendages are the antennae, these are tactile and the chief sense organs. A small eye is situated dorsally behind the base of each antenna and has a spherical lens. The mouth is directed ventrally and surrounded by ridged lips. In the sides of the mouth cavity are a pair of jaws, the appendages of segment 2.

The second segment contains the jaw like mouth which is used for rasping into prey and then sucking out the nutrients. The third segement holds the first pair of parapodia-like legs.

Each jaw is a low papilla with a pair of chitinous teeth. The roof of the anterior end of the mouth cavity is thickened to form the tongue which has a row of small chitinous teeth on its surface. Oral papillae, located lateral to the mouth, are presumed to have a sensory funtion. Defensive slime-glands open through the ends of the oral papillae.

Each of the legs bears a pair of chitinous claws for gripping, although on smooth substrate they walk on walking pads. Variously, the body between the limb insertion points, and the limbs themselves, may be finely annulated.

The legs are conical in shape and terminate distally in a plantigrade foot. The foot is retractable and is usually raised if the going is easy, being brought into play on slippery surfaces. The foot terminates distally in a pair of retractable claws and bears 3 prominent tubercles. Ventrally on the distal ridges of the leg proper are spinous pads; the shape and number of pads are helpful in recognising some species. Basally on the leg a groove runs along the ventral surface of the leg at right angles to the body axis. On all legs except 4 and 5 the excretory pore may be found at the basal end of this groove. If crural glands are present they will be found distal to the end of the groove.

Contemporary Onycophorans are ceolomates and have haemocoel, which means they have a lined body cavity filled with blood, rather than a vascular system. They have a muscular tubular heart which pumps the colorless blood around the body cavity. Locomotion is essentially annelid-like, with the body cavity functioning as a hydrostatic skeleton. The parapodia-like legs are also filled with blood and a valve at the base keeps them firm and muscular coordination can extend them or retract them and make them move forward or make them move backward.

Onycophorans have a cuticle with a-chitin but lacking collagen, which is periodically shed to permit growth (ecdysis). New cuticle is secreted underneath the old one by the ectodermal cells which develop microvilli that are subsequently withdrawn. Ecdysteroids have been found in various tissues but their function remains unknown (Hoffmann 1997; Nielsen 2001, p. 198). Unlike insect dermis, the cuticle of modern representatives is non-articulated, thin and soft and covered in hundreds of papillae and sensory hairs giving them a velvety texture, hence the common name ‘velvet worm.’ However, a characteristic feature of several fossil species is the paired internal sclerotic plates above the limb insertions, which may be variously developed into (presumeably defensive) spikes.

Like insects the Onycophora breathe through spiracles. Spiracles open out to the enviroment and oxygen enters through a system of tubules (trachae) and is absorbed into the tissues across the moist surfaces. However, unlike the insects, onycophorans have no control on the spiracles and they are always open, making the animal extremely vulnerable to dessication, so high levels of humidity are required.

Contemporary onycophorans are able to predate organisms several times larger than themselves by immobilising it with a gluey secretion from glands in its head, projected up to 30cm. The secretion holds the prey while the animal approaches it, bites through the cuticle, and injects a toxic, digestive saliva into the wound.

The animal preys on small arthropods by squirting and entrapping them in a glue fired from openings beside the mouth. It then injects saliva into the prey, dissolving the inner contents, and thus enabling the Peripatus to suck them out.

Onychophorans themselves have few predators, except perhaps insect carnivores such as centipedes, birds and rodents.

Phylogeny and Evolution

“Onychophorans are thought to be the sister taxon of [eu]arthropods and are segmented. However, onychophorans lack engrailed expression in their dermis. Instead, expression is observed in the posterior half of the developing limb and in a segmental pattern in the lateral mesoderm. The limb staining suggests shared ancestry of the onychophoran and arthropod limbs. However, given the close relationship of Arthropoda and Onychophora, and their segmented body plans, the lack of segmental ectodermal expression in Onychophora suggests that the ancestral role of engrailed was not segmentation; this absence may be a consequence of evolutionary loss of skeletons. Onychophoran dermis lacks a chitinous cuticle; thus Onychophora lack an exoskeleton. Furthermore, Cambrian fossils thought to be stem group onychophorans, such as Microdictyon, Hallucinogenia, and Xenusion, bear skeletal elements above the limb on each segment. Therefore, the absence of engrailed transcription in the ectoderm of modern Onychophora could well be a consequence of evolutionary loss of exoskeletal elements...” (Jacobs et al. 2000, p. 343-345).

Fossil History

A number of fossils from the Cambrian have been described which look more or less like onychophorans. The Cambrian forms are marine, however, and are incompletely understood. It is possible they are not really related closely at all. Some, such as the Middle Cambrian form Aysheaia are rather similar to living forms (but see Whittington [find REF] for a contrary view). Others were armored with various plates and spines which, disarticulated, contribute substantially to the “small shelly fauna.” All of these Cambrian forms differed from living onychophorans in being marine.

Hallucigenia is most widely known from the Middle Cambrian, Burgess Shale form, Hallucigenia sparsa, famously misinterpreted by Conway Morris (and later commentators) upside-down and back-to-front. Subsequently, another species, Hallucigenia fortis, has been described from the Lower Cambrian Chengjiang fauna.

Maas & Waloszek 2001, reports an undescribed “lobopodian” from the Upper Cambrian ‘Orsten’ beds of Sweden. Although only about a tenth the size of the better-known Cambrian onychophorans, the Swedish “’Orsten’ lobopodian shares with the Lower to Middle Cambrian lobopodians not only the annulated segmental limbs but also the segmental paired dorsal outgrowths on the finely annulated tubular body, which has a diameter of about 100 to 120 mm. ... The body and limbs are virtually cylindrical, and the limbs were apparently stretched virtually laterally due to a thicker bridge linking right and left legs. [The cuticle] shows a cell-like surface microstructure that resembles the onychophoran condition” (Maas & Waloszek 2001, p. 457).

Another Cambrian fossil organism which might belong within this clade is Kerygmachela, known from the Lower Cambrian Sirius Passet locality.

The earliest (and the only) fossil terrestrial onychophoran, Helenodora, was found in the Pennsylvanian deposits of Mazon Creek, near Chicago, a locality that has yielded a great many fossils of soft-bodied organisms. The fossil is not very different from living onychophorans (Thompson & Jones 1980).

Systematics

The systematics adopted here mostly follows Hou & Bergström 1995, though with some minor changes as noted.

Phylum Arthropoda Siebold & Stannius 1895

Supersubphylum Protarthropoda Lankester 1904

Class Onychophora Grube 1853

Discussion: The class Onychophora was used by Hou & Bergström 1995 to include the terrestrial forms: the modern onychophorans and the single known fossil terrestrial species, Helenodora. These authors also noted their belief that the fossil marine form, Onychodictyon, “is closer to modern onychophorans than any of the other Cambrian lobopodians” (p. 11). Their cladogram (Hou & Bergström 1995, fig. 7) depicts Onychodictyon, Helenodora, and modern onychophorans together comprising a well-formed clade. Yet in their systematic section (p. 17) Onychodictyon is left outside Onychophora, in the class Xenusia, on morphologic grounds. In the spirit of a more cladistic taxonomy, the class Onychophora is here broadened to include the marine Onychodictyidae (Onychodictyon).

Order Euonychophora Hutchinson 1930

Family Peripatidae Evans 1901

Type: Peripatus Guilding 1825

Discussion: The members of this family are generally coloured red-brown and possess 22 to 43 pairs of legs.

Genus Peripatus Guilding 1825

...

Family Peripatopsidae Bouvier 1907

This family contains organisms which are generally coloured blue-green and possess 14 to 25 pairs of legs.

Genus Peripatoides Pocock 1894

Peripatoides novaezealandiae

Description: Peripatoides novaezealandiae has 15 pairs of legs with hooks at the end, two robust feelers, and is velvety in appearance and comes in colours of blue, green, grey and brown; and may reach 80 mm in length.

Occurence: Never abundant but “not uncommon” at Titirangi, near Auckland (Gill 1998).

Habit: Peripatus live in damp areas such as under moss, in rotting logs, behind the bark of trees, and in leaf litter.

Family incertae sedis

Genus Helenodora Thompson & Jones 1980

Helenodora inopinata Thompson & Jones 1980

Order Paronychophora Hou & Bergström 1995

Family Onychodictyidae Hou & Bergström 1995

Genus Onychodictyon Hou, Ramsköld & Bergström 1991

Type Species: Onychodictyon ferox Hou, Ramsköld & Bergström 1991

Class Xenusia Dzik & Krumbiegel 1989

Order Protonychophora Hutchinson 1930

Family Aysheaiidae Walcott 1911

Genus Aysheaia Walcott 1911

Aysheaia pedunculata Walcott 1911

This problematical fossil was first described from the Burgess Shale by Walcott in 1911.

Family Xenusiidae Dzik & Krumbiegel 1989

Description: [VERIFY] >20 leg-bearing segments; paired, rounded sclerites on each segment; spiny legs.

Genus Xenusion Pompeckj 1927

Xenusion auerswaldae Pompeckj 1927

Xenusion, from early Cambrian sandstones of eastern Europe. This form was also armed with spines, although they were shorter than those of Hallucigenia. Only two specimens have been found so far.

Order Scleronychophora Hou & Bergström 1995

Family Eoconchariidae Hou & Shu 1987

Genus Microdictyon Bengtson et al. 1981 emend. Chen, Hou & Lu 1989

Species: Microdictyon effusum (type?), M. rhomboidale, M. robisoni, M. sinicum (Chen, Hou & Lu 1989), M. sphaeroides, M. tenuiporatum

Fig. 1: Microdictyon sp. specimen from Chengjiang region. Image courtesy of The Natural Canvas.

Family Hallucigeniidae Conway Morris 1977

Genus Hallucigenia Conway Morris 1977

Type Species: Hallucigenia sparsa Conway Morris 1977

Other Species: Hallucigenia fortis Hou & Bergström 1995

Family Cardiodictyidae Hou & Bergström 1995

Genus Cardiodictyon Hou, Ramsköld & Bergström 1991

Type Species: Cardiodictyon catenulum Hou, Ramsköld & Bergström 1991

Class incertae sedis

Hou & Bergström 1995 retain Archonychophora within the Xenusia. However, they also note their belief that the single member, Luoishania, is a sister taxon to all other onychophorans (e.g. their fig. 7). If correct, then a cladistic approach to onychophoran taxonomy suggests removing the Archonychophora to a new class.

Order Archonychophora Hou & Bergström 1995

Family Luolishaniidae Hou & Bergström 1995

Genus Luolishania Hou & Chen 1989

Type Species: Luolishania longicruris Hou & Chen 1989

Conclusion