555-545 million years ago
The ancestral hemichordates were wormlike animals which gave rise to the deuterostomes (one of the two groups of coelomate aminals which include echinoderms and chordates). They evolved several differences in their embryos (such as which embryonic opening forms the mouth and how embryonic cells divide) which would distinguish deuterostomes from other coelomates (the protostomes). After the first hemichordate lineages separated from that which would lead to the chordates, there was a modification of the body axis so that the heart was ventral and the brain was dorsal, unlike the condition in protostomes and primitive hemichordates (acorn worms).
Of the groups of animals which possess a coelom, there is a difference in whether the mouth or the anus develops as the first opening to the embryonic digestive tract. In protostomes (such as mollusks and arthropods), the embryonic pore becomes the mouth and the anus develops as a new opening while in deuterostomes (the chordates, hemichordates, and echinoderms), the anus develops first and the mouth develops as a new opening. This difference corresponds with several others and represents the major division in coelomate animal phyla. Protostome embryos undergo spiral cell arrangement after cleavage and cell fates (what the embryonic cells will form in the adult) are determined early. The opposite is true in deuterostomes. These 2 groups also differ in other characteristics such as the site of mesoderm formation and how the coelom forms. Thus, coelomates are divided into two groups: the protostomes (which comprise most animals on earth) and the deuterostomes (which include vertebrates like ourselves). The most primitive deuterostomes are the hemichordates.
Many of the hemichordates and primitive chordates are roughly worm-like
in body form. The echinoderms reorganized this general body plan and as
a result, their relationship to chordates is not evident in their external
appearance. A diversity of echinoderm groups evolved in the Cambrian and
Ordovician; many of which have become extinct. Hemichordates lack a chordate
feature known as the notochord although they have a rod in the same position,
which may be equivalent to the embryonic precursor of the notochord. They
have a pharynx with multiple openings, a dorsal nerve cord and a ventral
blood vessel. These are all very important characteristics of the chordates
(a group which includes the vertebrates). Acorn worms and colonial pterobranchs
still exist today. Graptolites were a group of hemichordates that appeared
in Cambrian. Although most died out in the Paleozoic, recently modern
representatives were identified (it had previously been thought that this
group went extinct) (Prothero, 1998).
This same analysis indicates that tunicates (urochordates) arose from the enteropneust worms (Cameron, 2000). As a result, it seems that worm-like hemichordates are the best model for the ancestors of the next group, the chordates, rather than other hemichordates and echinoderms which have modified their ancestral worm-like body plan. Modern enteropneust worms (acorn worms) are depicted in the following images:
Enteropneust worms are classified as hemichordates but are similar to protostomes in possessing a dorsal heart and protostome-like blood flow (Gerhart, 2000; Benito, form Harrison 1997, p. 20). This and other evidence suggests that the hemichordates separated from the chordate lineage at about the time the body axis rotated in ancestral deuterostomes (Benito, form Harrison 1997, p. 44). Later hemichordates and chordates possess a dorsal nerve cord and a ventral heart.