IS PERIPATUS A VALID EVOLUTIONARY INTERMEDIATE?

by Richard Peachey

Peripatus is an organism classified within the phylum Onychophora, a group known as "velvet worms" or "onychophorans" (meaning "claw-bearers"). These animals live mostly in humid forests, especially in the tropics. They hide by day under rotting logs and in leaf litter, and hunt for prey at night. The largest species is about 15 cm long. Velvet worms have between 14 and 43 pairs of stumpy, unjointed legs with claws at the tip (Starr and Taggart, p. 665; diagram below from Barnes et al., p. 176).

 

Peripatus drawing

 

Onychophorans are able to capture even quite active animals such as grasshoppers by spraying a mucus-like substance up to half a meter from adhesive glands near their mouth. The sprayed substance hardens almost immediately on exposure to air, forming an extremely sticky meshwork that entangles the prey (Barnes et al., p. 177).

As shown by the following quotations, Peripatus and onychophorans in general have often been called upon to assist the case for evolution. They are sometimes described as the "missing link" between annelids, the phylum to which earthworms and leeches belong, and arthropods, the group including joint-legged animals such as crabs, spiders, and insects (Ballard et al., p. 1345). (Note: Bold print in the quotations below indicates emphasis added.)

"Peripatus is a missing link between the annelids and the arthropods. It is obviously a segmented animal; its excretory, reproductive, and nervous systems are similar to those of the annelids, while its circulatory and respiratory systems are similar to those of the arthropods." (Mader, p. 501)

". . . there are all sorts of gaps [in the fossil record]: absence of gradationally intermediate transitional forms between species, but also between larger groups—between, say, families of carnivores, or the orders of mammals. In fact, the higher up the Linnaean hierarchy you look, the fewer transitional forms there seem to be. For example, Peripatus, a lobe-legged wormlike creature that haunts rotting logs in the Southern Hemisphere, appears intermediate in many respects between two of the major phyla on Earth today: the segmented worms and the arthropods. But few other phyla have such intermediates with other phyla, and when we scan the fossil record for them we find some, but basically little, help." (Eldredge, pp. 77f.)

"An animal that comes closer than any other to being the 'missing link' between any two phyla is the PERIPATUS, member of the small phylum Onychophora. . . ." (Buchsbaum, p. 258)

In contrast to such claims for Peripatus and the onychophorans, a recent college textbook on invertebrate biology, authored by evolutionists, argues that these animals cannot be ancestral to the arthropods (Barnes et al., p. 177):

"For many years, the onychophorans have been of scientific interest chiefly as living examples of a half-way stage between the worm and arthropod grades of organization. Like worms, they are soft-bodied and possess hydrostatic skeletons, ciliated excretory ducts and smooth muscle layers in the body wall; and, similarly to the arthropods, they bear legs, tracheae, a heart with ostia, longitudinally partitioned blood sinuses, and jaws derived from the appendages, in this case from the claws that terminate the walking legs. The precise structure of their arthropod-like features, however, strongly indicates that they have achieved them in parallel, and that whereas they may illustrate what the early uniramians, for example, may have looked like, the onychophorans cannot be ancestral to any known arthropod group. Their tracheae, for instance, are simple, mostly unbranched tubes issuing many at a time from the many (up to 75) spiracles that are scattered over each leg-bearing 'segment' (Fig. 8.9), and their jaws, which move in the anterior/posterior plane, act independently of each other and function as ripping organs by virtue of their pointed tips rather than as chewing appcndages. Other onychophoran peculiarities include the structure of the ventral nerve cords, with their numerous connectives but without 'segmental' ganglia."

In agreement with the above quotation, Ballard et al. conclude "that onychophorans are a highly specialized assemblage [and not] a primitive 'missing link' " (p. 1347). As for the overall evolutionary status of onychophorans, these authors are uncertain: "Parsimony analysis . . . suggests that onychophorans form a sister group to chelicerates (spiders and scorpions) and crustaceans plus hexapods [insects], but this relationship is not well supported by monophyly testing. These relationships conflict with [all four major] current hypotheses of evolutionary pathways within arthropods" (p. 1345).

Drawing on the results of Ballard et al., and other researchers, evolutionist Peter Price notes that while (from an evolutionary point of view) "the affinities among the arthropods and related groups are by no means clear," it is nevertheless clear enough that "velvet worms are not a missing link between the arthropods and the annelids" (pp. 130-131).


References

Ballard, J. William O., Gary J. Olsen, Daniel P. Faith, Wendy A. Odgers, David M. Rowell, and Peter W. Atkinson. 1992 (Nov 20). "Evidence from 12S Ribosomal RNA Sequences That Onychophorans Are Modified Arthropods." Science 258:1345-1348.

Barnes, R. S. K., P. Calow, and P. J. W. Olive. 1993. The Invertebrates: a new synthesis. 2nd edition. Oxford: Blackwell Scientific Publications.

Buchsbaum, Ralph. 1951. Animals Without Backbones. Harmondsworth, Middlesex, UK: Penguin Books.

Eldredge, Niles. 2000. The Triumph of Evolution and the Failure of Creationism. New York: W. H. Freeman and Company.

Mader, Sylvia S. 1979. Inquiry into Life. 2nd edition. Dubuque, Iowa: Wm. C. Brown Company Publishers.

Price, Peter W. 1996. Biological Evolution. Forth Worth: Saunders College Publishing.

Starr, Cecie, and Ralph Taggart. 1989. Biology: The Unity and Diversity of Life. (5th edition). Belmont, CA: Wadsworth Publishing.