SYSTEMATIC BIOLOGY ANIMALIA-ICON.gif (50692 bytes)
KINGDOM ANIMALIA
PHYLUM ONYCHOPHORA

INTRODUCTION TO THE ONYCHOPHORA

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 A).  Now they mainly inhabit terrestrial environments that are high in humidity.  However, their fossils (i.e. Hallucigenia of the Burgess shale fauna; Figure B) indicate that they once occupied marine benthic environments.  They 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. 

FIGURE A. Peripatus, a living onychophoran.

FIGURE B. Hallucigenia, an onychophoran from the mid-Cambrian Burgess Shale.

Images taken from:
A: http://www.museums.org.za/bio/onychophora/
B: http://www.nmnh.si.edu/paleo/shale/phallu.htm

 SYNOPTIC DESCRIPTION OF THE PHYLUM ONYCOPHORA

The following information came from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984), Brusca and Brusca (2003), Hickman (1973), Meglitsch and Schramm (1991), Ruppert and Barnes (1991),  Storer and Usinger (1965), and Tudge (2000).

 

I. SYNONYMS: velvet worms, onychophorans.

II. NUMBER: >80 species known.

III. PHYLUM CHARACTERISTICS:

A. Structure

Symmetry: Bilateral

Body Cavity: Coelomic pouches greatly reduced to gonoducts and excretory ducts. Most of the cavity made of haemocoel.

Body Covering: Covered by a thin cuticle with tubercles in rings or bands. No superficial segmentation of the cuticle.

Support: Hydrostatic skeleton made by sinuses of haemocoel..

Digestive System: Anteroventral mouth flanked by two oral papillae and surrounded by perioral lobes. Food tube simple with terminal anus. Capture food by secretion of sticky threads from oral papillae. Threads entangle prey.

Circulatory System: Open. With a long, segmented dorsal heart and a large haemocoel cavity.

Locomotion: Move by many pairs of unjointed, stubby legs, each terminates in a pair of claws.

Excretory System: Pared metanephridia opening by their own ducts at each internal "segment". Anterior nephridia modified to salivary glands; posterior nephridia modified to gonopores.

Nervous System: Brain attached to paired ventral nerve cords attached at ladder-like ganglionic swellings. Eyes, sensory antennae and other sensory swellings.

Endocrine System: None.

Respiratory System: Small tufts of minute tracheae each opening by a spiracle.

B. Reproduction:

Reproductive System: Dioecious. A few with external fertilization; most with internal fertilization. Oviparous, ovoviviparous, and viviparous (placenta attaches embryo to uterus wall).

Development: Large yolky eggs (in oviparous forms) cleave superficially. Direct development.

C. Ecology: Free-living terrestrial animals that inhabit leaf litter, stream banks and other areas of high humidity. Usually nocturnal predators.

HIERARCHICAL CLASSIFICATION THE ONYCHOPHORA

  The onychophorans  today are but a remnant of a former diverse group.  Thus, they like the priapulids, have a problematic taxonomy.  Meglitsch and Schram (1991) recognize taxa no higher than the family level.  Most other sources (e.g. Brusca and Brusca 2003; Ruppert et al. 2004) do not even attempt to present a taxonomy.  I have given a provisional taxonomy with a single class (Onycophorida) and a two orders: Paronynychophora (all extinct) and Euonychophora.

CLASS ONYCHOPHORIDA (2 ORDERS)

Onychodictyon

Cretoperipatus, Eoperipatus, Epiperipatus, Heteroperipatus, Macroperipatus, Mesoperipatus, Oroperipatus, Peripatus, Plicatoperipatus, Speleoperipatus, Typhloperipatus

Acanthokara, Aethrikos, Akthinothele, Anoplokaros, Austroperipatus, Baeothele, Centrorumis, Cephalofovea, Critolaus, Dactylothele, Dystactotylos, Euperipatoides, Florelliceps, Hylonomoipos, Konothele, Lathropatus, Leuropezos, Mantonipatus, Metaperipatus, Minyplanetes, Nodocapitus, Occiperipatoides, Ooperipatellus, Ooperipatus, Opisthopatus, Paraperipatus, Paropisthopatus, Paropisthopatus, Peripatoides, Peripatopsis, Phallocephale, Planipallipus, Regimitra, Ruhbergia, Sphenoparme, Symperipatus, Tasmania, Tasmanipatus, Tetrameraden, Vescerro, Wambalana


LITERATURE CITED

Barnes, R. D. 1980. Invertebrate Zoology. Saunders College/Holt, Rinehart and Wilson, Philadelphia.

Barnes. R. S. K. 1984a. Kingdom Animalia. IN: R. S. K. Barnes, ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc., Sunderland, MA. pp. 129-257.

Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass.

Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago. 

Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis

Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company.  New York.

Meglitsch, P. A. and F. R. Schramm. 1991. Invertebrate Zoology. Oxford University Press, New York, Oxford.

Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. 2nd Edition. Oxford University Press. Oxford. [L]

Ruppert, E. E., R. S. Fox, and R. D. Barnes. 2004. Invertebrate Zoology: A Functional Evolutionary Approach. Seventh Edition. Thomson, Brooks/Cole. New York. pp. 1-963. 

Storer, T. I. and R. L. Usinger. 1965. General Zoology. 4th Edition. McGraw-Hill Book Company. New York .

Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York.

 

By Jack R. Holt.  Last revised: 02/07/2010