  Results from molecular systematics and comparative developmental genetics
changed the picture of metazoan and especially bilaterian radiation. According
to this new animal phylogeny (introduced by Adoutte et al. 1999/2000), Grobbens
(1908) widely favoured protostome-deuterostome division of the Bilateria can be
upheld, but only with major rearrangements within these superphyla. On the
cladogenetic level, the Protostomia are split into two unexpected subgroups,
the Lophotrochozoa and Ecdysozoa. The deuterostomes are split into the
subgroups Chordata and Ambulacraria, which is not novel since Grobben (1908)
introduced the Deuterostomia in this way (together with the Chaetognatha as a
third line). However, many details of the new deuterostome phylogeny do not fit
traditional, morphology-based reconstructions. As a consequence, three
relatively unexpected proposals for early deuterostome evolution are favoured
today: An ambulacraria-scenario, a xenoturbellid-scenario, and a
chordate-scenario. The first two proposals are often discussed in the
literature, while the chordate-scenario is almost completely neglected.
Therefore, the paper presented focuses on the chordate scenario, i.e. the
hypothesis of an acrania-like ur-deuterostomian. It is argued that the
acrania-hypothesis is clearly preferable when biomechanic options of a
polysegmented, hydroskeletal body plan are taken into account. The so called
hydroskeleton hypothesis, rooted in the work of W. F. Gutmann, is the most
detailed anagenetic scenario which depicts an acrania-like ur-deuterostome.
