DIVERSITY OF LIFE
DESCRIPTION OF THE PHYLUM ONYCHOPHORA (GRUBE 1853)
|Onychophora (o-ne-KO-fo-ra) is formed from two Greek roots that mean "claw bearers" (claw -nichi (νύχι); and bearer -phoros (φόρος). The reference is to the small claws at the ends of the telescoping legs.|
The velvet worms look like annelids with telescoping legs (Figure 1). The legs, which are hollow extensions of the body wall, have no joints, but they do terminate in claws. In the absence of a rigid exoskeleton, onychoporans employ a hydrostatic skeleton with an exterior cuticle. Thus they walk more like a polychaete with parapodia (similar to Nereis).
Onychophorans like Peripatus (Figure 1) have a differentiated head whose first segment bears two long, leg-like antennae at the base of which is a small ocellus. On the underside of the head is a pair of oral papillae and a round mouth that bears a pair of mandibles (Figure 2) on the second segment. The oral papillae expel a proteinaceous glue that they can shoot as a stream with great accuracy. It is with this glue that they entrap their prey (usually insects). Their saliva may act as a kind of proteolytic digestive enzyme allowing the velvet worms to remove the food from an insect carcass mainly by sucking the fluids.
Now they mainly inhabit terrestrial environments that are high in humidity. In general, the living members of the phylum inhabit the tropics and subtropics of South and Central America, Africa, Southeast Asia, New Zealand, and Australia (Monge-Najera 1995). This classic southern continental distribution suggests that the ancestral onychophorans inhabited Gondwanaland before it breakup. Indeed, terrestrial onychophorans have been found as fossils even back to the Pennsylvanian. This was on the North American plate which suggests that they must have had a global continental distribution (this was before the formation of Pangaea) and the Onychophora subsequently became extinct in Laurasia.
Fossils of animals interpreted as Onychophora are found in the Burgess Shale fauna (Lower Cambrian). Bergstrom and Hou (2001) do not consider these to be true Onycophora and refer to the Cambrian taxa as Xenusians. If so, they likey were sister to living onycophorans. The xenusians, unlike the living taxa were marine. One of them was so weird that it was given the name Hallucigenia (Figure 3). They looked like Onycophora, but they also resembled large tardigrades. Budd (2001) rejects the view that the Cambrian xenusians had affinities with the Onychophora. Dzik and Krumbiegel (1989) interpret some of the xenusians as priapulids with lobopods. Budd (1993) sees similarities between the legs of Xenusians and anomalocarids, and Budd (1998) places the xenusians between the onychophorans and anomalocarids and arthropods.
Onycophora are part of a larger natural group known as the panarthropods (Mandibulata, Chelicerata, Tardigrada, and Onychophora; Brusca and Brusca 2003, and Nielsen 2001). Brusca and Brusca (2003) summarize a large body of literature on the panarthropods and indicate that the onychophorans have generally been considered a sister group to the tardigrade-"arthropod" line. Nielsen (2001) suggests that the specializations of the living onychophorans (tracheae, specialized nephridia, and slime glands) show that the extant line is from a specialized offshoot of the Paleozoic marine fauna. Monge-Najera (1995), on the other hand, found a strong relationship with the Euarthropoda and a weak relationship with tardigrades.
Though there is strong support for the Panarthropoda as a clade within the Ecdysozoa (e.g. Patel et al. 1989, Garey 2001, and Gabriel and Goldstein 2007), the position of the onychophora remains questionable. Budd (2001) supposes that the tardigrades are sisters to the Arthropods (a zygotaxon that he calls Tactopoda) and interprets Cambrian arthropod-xenusian fossils according to similarities with tardigrades, not the onycophorans. Most analyses, however, have the Onycophora as the sister to the Arthropods (e.g. Mayer and Whitington 2009).
The question of the relationship between the Onychophora and Arthropoda is not a trivial one. If the Xenusians are ancestral onychophorans, then certain morphological characters can be polarized. For example, the eyes of onychophorans are of the arthropod type but the innervation and histology suggest that they are homologs of the simple median eyes of many arthropods (Mayer 2006). Furthermore, the fossil and living onychophorans suggest that if they are ancestral in form, the last common ancestor of the euarthropods had only a pair of ocelli and a single pair of legs per segment. Eirksson and Budd (2000) assumed that Onychophora was sister to the Arthropoda and interpreted their observations on cephalic nerves in velvet worms to be the basal arthropod condition. On the other hand, Strausfield et al. (2006) conclude that the onychophora emerge within the arthropods as sisters to the Chelicerata. If so, they had to have become secondarily simplified.
Waggoner (1996) accepts the Onychophora as a taxon which includes the terrestrial forms only. He places them within a larger taxon which he calls Polylobopoda, within the Lobopoda and equivalent to the Tardigrada. The Xenusia are equivalent to the Onychophora.
The system that we use here acknowledges the problems with monophyly if the xenusians are included. Therefore, we consider the Phylum Onychophora to be the monophyletic group in a narrow sense until a general consensus develops regarding the xenusian taxa (see Figure 4).
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By Jack R. Holt and Carlos A. Iudica. Last revised: 04/10/2013