Since their discovery in the 19th-century, some of the biggest questions in hemichordate
evolution have focused on the group's origins and the relationship between its two main branches: the enteropneusts and the pterobranchs, including graptolites.
Sequences from seventeen species were used to infer phylogeny resulting in five clusters, a cluster of ten Mammalian species, a single isolated cluster of Red Jungle Fowl, a single cluster of Amphibians, a cluster consisting of Fish and a cluster with a species of hemichordate
and Atlantic salmon (Figure 3).
But Shu and others contested that interpretation, arguing in the April 4 NATURE that Yunnanozoon was a hemichordate
, a separate phylum that includes the modern acorn worm.
flava has recently become one of the major hemichordate
model species for evolutionary developmental biology research (review in Rottinger and Lowe, 2012), and studies of its genome have provided new insights into the evolution of deuterostomes (Freeman et al, 2012; Ikuta et al, 2013).
At present, 135 hemichordate
species are described: 121 are solitary Enteropneusta, or acorn worms; and 24 are colonial, tube-dwelling Ptero-branchia (Appeltans et al.
A brown organic layer of about 5mm thickness was on top of the dark gray reducing sediments, and the surface was dotted by tubes of polychaete onuphid worms, crustacean holes, hemichordate
fecal mounds, and feeding tracks of the predatory snail, Natica unifasciata.
The proboscis skeleton of a hemichordate
is a localized, conspicuous thickening of the extracellular matrix (= basal lamina) that delimits epithelia throughout the body.
Beyond the echinoderms, a similar pattern is observed in the obligately planktotrophic larva of Ptychodera flava, a hemichordate
(Henry et al.
Development and neural organization of the tornaria larva of the Hawaiian hemichordate
Since chordates develop directly, we chose a direct-developing hemichordate
, Saccoglossus kowalevskii (Agassiz), to facilitate comparisons of adult development and anatomy.
Examples are about 300 days (mostly during metamorphic competency) for tornariae of the hemichordate
Ptychodera flava, inferred from spawning dates and plankton samples (Hadfield, 1978); up to 316 days for veligers of the gastropod Aplysia Juliana in culture (Kempf, 1981); 14 months (at least 12 during competency) for non-feeding larvae of the seastar Mediaster aequalis in culture (Birke-land et al.
1997), and axons are associated with the ciliary band of plutei, other echinoderm larvae, and hemichordate
larvae (Burke, 1983; Dautov and Nezlin, 1992; Chee and Byrne, 1999; Beer et al.