Three substantially different schemes for deuterostome relationships have been proposed. The 'traditional' view (e.g. Maisey, 1986; Peterson, 1995; Donoghue et al., 1998; illustration (a)) was to place the hemichordates as basal to chordates since they both share ciliated gill slits and giant nerve cells, as well as other features, which are not seen in echinoderms. Enteropneusts were sometimes said to be closer relatives of chordates since their gill slits are similar, they have a very short dorsal hollow nerve cord, and a number of other features of the gut not seen in pterobranchs (Peterson, 1995). Most authors regard amphioxus as the closest relative of the Vertebrata on the basis of 10-15 features that are not seen in tunicates.
The 'calcichordate' model (Jefferies, 1986, 1997; illustration (b)) places hemichordates basal to echinoderms and uro-chordates as sister group to vertebrates, based on evidence from embryology and fossils.
The third view (illustration (c)) is supported by morphological and molecular data and is now widely accepted (Smith et al., in press). The first molecular studies in which the 18S rRNA genes of echinoderms, hemichordates, and chordates were compared were inconclusive, but newer work (e.g. Bromham and Degnan, 1999; Cameron et al., 2000; Peterson and Eernisse, 2001; Furlong and Holland, 2002; Winchell et al., 2002) definitively pairs hemichordates with echinoderms, as the clade Am-bulacraria, and places cephalochordates closer to chordates than urochordates. See Box 3.1 for phylogeny of Vertebrata.
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D CHORDATA G DEXIOTHETICA
D CHORDATA G DEXIOTHETICA
I = AMBULACRARIA
Cladograms showing the relationships of the main deuterostome groups: (a) the 'traditional' model, (b) the 'calcichordate' model, and (c) the molecular model. Synapomorphies: A DEUTEROSTOMIA, blastopore becomes anus during development, bipartite mesocoel, mesocoelomic ducts; B, stomochord, paired gill slits; C, multiple pairs of gill slits, pharyngeal slits U-shaped, dorsal hollow nerve cord, preoral ciliary organ, mouth anterior and ventral and anus posterior and ventral or dorsal, multiciliated cells; D CHORDATA, notochord present and not attached to gut, dorsal hollow nerve cord with neural-plate stage in development, endostyle organ, a true tail used in swimming; E, digestive caecum, open capillary junctions, somites present, lateral-plate mesoderm, neural tube differentiated into grey and white matter, cerebral vesicle in brain; F, ciliated extensions of the mesocoel either absent or present as water vascular system (but not as lophophore), anus not anterior and dorsal; G DEXIOTHETICA, dexiothetism (rotation and partial loss of right side of precursor form), stone canal, calcite skeleton internalization of protostome; H, specialized olfactory areas in buccal cavity, hind-tail tripartite, dorsal longitudinal canal connected with notochord; I AMBULACRARIA, trimeric arrangement of the adult coelom, axial complex with hydropore, dipleureula larva with neotroch.
Formation, and still highly controversial. The group was named by Shu et al. (2001) on the basis of three genera, Vetulicola, Xidazoon and Didazoon (Figure 1.6(a, b)). These animals look like sausage balloons, knotted in the middle: the body is in two parts, with bulbous sections in front of, and behind, a flexible connection. There is a large mouth with a strengthened rim, and preserved internal structures include the guts and a possible endostyle. Both parts of the body appear to be crossed by transverse bands of tissue. On the
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