The newly advanced theory for the origin of the eumetazoans (Nielsen, 2008b) proposes that the ancestral eumetazoan which Haeckel (1874) called gastraea did not evolve directly from a blastaea, but from sexually mature larvae of a homoscleromorph-like sponge with a pelago-benthic life cycle.
Haeckel (1874) suggested that the eumetazoan ancestor was a gastraea with ectoderm, endoderm, and archenteron with blastopore, essentially having the same structure as that of larval and adult cnidarians and of ontogenetic stages of many bilaterians.
On the other hand, some more recent papers, such as Sly et al., (2003) and Raff (2008), propose that the bilaterian ancestor was a benthic, acoel-like organism, and this is much more in accord with modern versions of the gastraea theory.
The new version of the gastraea theory, which proposes that the eumetazoan ancestor was a gastraea evolved from a "neotenic" larva of a homoscleromorph-like ancestor (Nielsen, 2008b), explains how a gastraea-like "homoscle-romorph" larva could have evolved into the eumetazoan ancestor with sealed epithelia and a gut with extracellular digestion.
The eumetazoan ancestor, gastraea, has usually been described as radially symmetrical.
proposes that the protostomian ancestor, trochaea, was a gastraea with a periblastoporal ring of compound cilia functioning as a downstream-collecting system using the catch-up principle (as seen today in Symbion, which has a ring-shaped downstream-collecting system around the mouth; see Riisgard et al., 2000).
I find it impossible to imagine a nonfeeding ancestor, such as a compact planula larva, so the ancestor must have been a feeding organism of the general structure of a gastraea. There is no problem with "transfer of reproductive capacity from the larva to the adult" as suggested by Degnan and Degnan (2006); it is simply the adult holopelagic ancestor that becomes sessile.