Spineless Wonders of Evolution 203the lip of the shell are segmented gills, kidneys, hearts, gonads, and paired retractor muscles
to pull down the shell. You couldn’t ask for a more classic transitional form. It is almost
completely molluscan, yet it still has features of its segmented worm ancestry. Since 1952,
scientists have come to realize that two soft-bodied wormlike groups, the Caudofoveata and
the Aplacophora, are actually very primitive mollusks as well, even though they have no
shells and look more like worms in their external form. We now have a nice transition from
segmented worms to shell-less wormlike mollusks to mollusks with shells but with relicts of
segmentation, and finally to the great radiation of unsegmented mollusks, including clams,
snails, squids and octopuses, and all their extinct relatives.
How about transitional forms for the largest, most diverse phylum in the world, the
Arthropoda, or “jointed legged” animals, which includes insects, spiders, scorpions, crus-
taceans (crabs, shrimp, lobsters, and barnacles), horseshoe crabs, trilobites, centipedes, mil-
lipedes, and many other groups? The tree of life (fig. 5.7) shows that arthropods are more
closely related to nematodes (roundworms) and rotifers among living organisms. How
could we imagine a transition between a nematode “kind” and an arthropod “kind” such as
a millipede? It turns out that the transitional forms are still alive today; they are known as
“velvet worms” or phylum Onychophora (fig. 8.16), pronounced “on-ih-KOFF-o-ra.” There
are about 80 species of these creatures, living mostly in the tropical jungles of the world;
they were originally mistaken for “slugs” when they were first described in 1826. But more
careful observation (fig. 8.16) shows that although they may look superficially wormlike,
they have many features of arthropods as well. Unlike the unsegmented nematode worms,
onychophorans are segmented and have legs that resemble those of caterpillars. Their par-
tially segmented legs end in horny hooked “claws.” Onychophora have cuticles made of
the protein chitin, just like arthropods, and periodically have to molt in order to get larger
(a feature found elsewhere only in the arthropods). They also have antennae, compound
eyes, and mouthparts that are much like those found in arthropods. As outlined by Brusca
and Brusca (1990:683), there are many other features that unite onychophorans with arthro-
pods and make them outstanding transitional forms between the phylum Nematoda and
the phylum Arthropoda.
The clincher is that we also have them in the Cambrian as well. The Burgess Shale
in Canada and the Chengjiang Fauna of China produce amazing fossils of a variety of
marine onychophorans known as lobopods, including Aysheaia (fig. 8.17A) and Hallucigenia
(fig. 8.17B) from the Burgess Shale and Microdictyon from Chengjiang. Aysheaia is almost
indistinguishable from some modern onychophorans, but Hallucigenia and some of the other
Cambrian forms show an amazing array of spines and other features, giving the group much
more diversity than we would appreciate from seeing only their living relatives.
FIGURE 8.16. The living transitional form between
worms and arthropods, known as the “velvet
worms” or phylum Onychophora. Although they
have segmented wormlike bodies, they also have
jointed appendages and antennae and shed their
cuticle like arthropods do. (Photo from IMSI Master
Photo Collection)