The Reversal of Hallucigenia

Scientists have recently uncovered the impressive pedigree of a lowly, misunderstood creature

by Stephen Jay Gould

You can generate a lot of mischief just by strolling. When God asked Satan what he'd been doing, the foremost of the fallen angels responded: " . . . going to and fro in the earth and . . . walking up and down in it (Job 1:7). But you can also do a lot of good. Aristotle preferred to teach while ambling along the covered walk, or peripatos, of his Lyceum in Athens. His followers were therefore called peripatetics. In Greek, a patos is a path, and peri means "about." The name for Aristotle's philosophical school therefore reflects the master's favorite activity.

The same etymology lies behind my all-time favorite technical name for an animal--the genus Peripatus. I just love the sound, especially when pronounced by my Scottish friends who really know how to roll their r's. I can hardly ever bring myself to write about the animal without expressing delight in its name. The only reference in my book Wonderful Life speaks of the "genus with the lovely name Peripatus."

Peripatus is an elongated invertebrate with many pairs of stout, fleshy legs--hence the chosen name for this obligate walker. The Reverend Lansdown Guilding--quite a name itself, especially given the old stereotype of English clergymen as amateur natural historians--discovered and designated Peripatus in 1826. He falsely placed his new creature into the mollusk phylum (with clams, snails, and squids) because he mistook the antennae of Peripatus for the tentacles of a slug. Since true mollusks don't have legs, Guilding named his new beast for a supposed peculiarity.

Peripatus is the most prominent member of a small group known as Onychophora. Modern onychophorans are terrestrial invertebrates of the Southern Hemisphere (with limited extension into a few regions of the Northern Hemisphere tropics)--hence little known, and never observed in natural settings by residents of northern temperate zones.

About eighty species of living onychophorans have been described. They live exclusively in moist habitats, usually amid wet leaves or rotting wood. Most species are one to three inches in length, although the size champion from Trinidad, appropriately named Macroperipatus, reaches half a foot. They resemble caterpillars in outward appearance (although not in close evolutionary relationship). They are elongated, soft bodied, and unsegmented (the ringlike "annulations" on antennae, legs, and sometimes on the trunk are superficial and do not indicate the presence of segments, or true divisions of the body). The onychophoran head bears three paired appendages: antennae, jaws, and just adjacent to the jaws, the so-called slime papillae. Onychophorans are carnivores and can shoot a sticky substance from these papillae, thus ensnaring their prey or their enemies. Behind the head, and all along the body, onychophorans carry fourteen to forty-three pairs (depending on the species) of simple walking legs, called lobopods. The legs terminate in a claw with several spines--the source of their name, for Onychophora means "talon bearer."

The Onychophora present the primary case for a classical dilemma in taxonomy: how do we classify small groups of odd anatomy. (Oddness, remember, is largely a function of rarity. If the world contained a million species of onychophorans and only fifty of beetles, we would consider the insects as bizarre.) The chief fault and foible of classical taxonomy lies in its passion for clean order--an imposition bound to distort a messy world of continuity and complexity. A small group of distinctive anatomy sticks out like the proverbial sore thumb, and taxonomists yearn to heal the conceptual challenge by enforcing an alliance with something more familiar. Two related traditions have generally been followed in this attempt, both misleading and restrictive: the shoehorn ("cram 'em in") and the straightening rod ("push 'em between").

The shoehorn works by cramming odd groups into large and well-established categories, usually by forced and fanciful comparison of one or two features with characteristic forms of the larger group. For example, the Onychophora have sometimes been allied with the Uniramia, the dominant arthropod group that includes insects and myriapods (millipedes and centipedes), because both have single-branched legs (never mind that arthropod legs are truly segmented and that onychophoran lobopods are constructed on an entirely different pattern).

The straightening rod tries to push a jutting thumb of oddness back into a linear array by designating the small and peculiar group as intermediary between two large and conventional categories. The Onychophora owe whatever small recognition they possess to this strategy--for they have most commonly been interpreted as living relicts of the evolutionary transition between two great phyla: the Annelida (segmented worms, including leeches and the common garden earthworm) and the Arthropoda (about 80 percent of animal species, including insects, spiders, and crustaceans). In this argument, Peripatus is a superworm for its legs and a diddly fly for building these legs without true segments.

A third possibility obviously exists and