Fish Tales 211With these basic parts of the chordate body plan outlined, we can now look at the steps
that produced a vertebrate from a nonvertebrate deuterostome (fig. 9.3). The most primi-
tive living relatives of the chordates belong to a closely related phylum, the Hemichordata
(“half chordates”). Today these include the acorn worms (fig. 9.3) and a group of plankton
feeders known as pterobranchs that bear little resemblance to vertebrates. Acorn worms are
known from about 80 species that live in U-shaped burrows in the sand and use their mus-
cular proboscis to dig and the collar behind it to trap food particles as they burrow. Ptero-
branchs, on the other hand, are tiny colonial animals that live on long ringed tubes, and the
animal consists largely of a U-shaped digestive tract with a fanlike filter-feeding device at
one end. To the casual person walking among the tide pools or the beach sand, neither of
these creatures resembles a fish, let alone a human. Yet careful examination of these animals
reveals important clues. Although hemichordates do not yet have a notochord, they have
the embryonic precursor of the notochord. In addition, both groups have a true pharynx,
which occurs in no other group but chordates and their relatives. Finally, they have nerve
cords along the back, and the digestive tract along the belly, a configuration that occurs
elsewhere only in chordates.
There is also a lot of embryological evidence that hemichordates are our closest relatives.
Their distinctive tornaria larva is nearly identical to the larvae of primitive chordates and
also very similar to some echinoderm larvae. All the recent molecular analyses consistently
show hemichordates as our closest relative other than echinoderms, or slightly closer to
echinoderms but also clustered with chordates. Finally, acorn worms don’t fossilize, but the
extinct relatives of the pterobranchs, known as graptolites, are extremely common in early
Paleozoic rocks. Once again, we have convergence of evidence from anatomy, embryology,
paleontology, and molecular biology that points to one conclusion. Although we may not
like to think of ourselves as having evolved from a creature like the acorn worm or ptero-
branch, that’s where the evidence leads.
How do we get to the next stage? According to one hypothesis (fig. 9.4), the ancestral
larvae that we chordates share with echinoderms developed into the filter-feeding ptero-
branchs (much like the primitive filter-feeding echinoderms). By retaining the embryonic
stages of pterobranchs, the filtering arms were lost, and an acorn worm develops from the
embryo instead. The next step is known as the “sea squirts” or the tunicates (fig. 9.5), which
today are represented by over 2,000 species in the ocean, although they are so tiny and trans-
lucent that most people never see one. These delicate little blobs of jelly hardly resemble us,
or even a fish for that matter. As adults, they are shaped like a little sac, with an opening at
the top through which water is sucked in, then filtered through a basketlike pharynx, and
finally out the little “chimney” on the side of their body. The adult sea squirt doesn’t suggest
much about chordates at all, although the pharynx is a clue. But the best evidence comes
from their larvae (fig. 9.5A, left diagram), which look nothing like the adult but instead a lot
like a fish or a tadpole. The sea squirt larva has a well-developed notochord, a muscular tail
with paired myomere muscles, a nerve cord on the back, and a digestive tract along the belly.
This peculiar larva swims around looking for a good rocky surface on which to land. Using
the adhesive pad on its snout, it attaches and within 5 minutes the tail begins to degenerate.
About 18 hours later the metamorphosis into the adult sea squirt is complete.
The adult sea squirt, of course, is too specialized to have had much to do with our
ancestry, but the larva is a different matter. Through a mechanism like neoteny (discussed
in chapter 3), the next stage of evolution of chordates would come not from the adults but