This is a reasonable starting point because, despite the morphological diversity and nonhomology of eyes, it is well documented that the majority of eumetazoans
use an opsin-based phototransduction system (Feuda et al.
Regarding NK homeobox genes, six or seven NK genes probably existed in the last common ancestor of sponges and eumetazoans
Erwin and Valentine, near the conclusion of The Cambrian Explosion, remark that "the pathway from sponges to eumetazoans
is the most enigmatic [my italics] of any evolutionary transition in metazoans" (p.
2014), these junctions manifested features typical of adherens junctions in eumetazoans
, so that here they will be referred to as adherens junctions.
Adults of the smallest free-living, nonparasitic multicellular animals such as rotifers and nematodes have a minimum of several hundred cells (1), and even the placozoan Trichoplax adhaerens, with the simplest body organization of all free-living, nonparasitic eumetazoans, is composed of more than 300 cells (1), (3).
Despite its very simple bodyplan, the model placozoan Trichoplax adhaerens has a genome that encodes for a plethora of transcription factors and signaling pathways associated with cell-type specification and developmental processes in much more complex eumetazoans.
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.
Most morphology-based studies and some studies on molecular phylogeny place the cnidarians as the sister group of the remaining eumetazoans, the bilaterians (see, for example, Sperling et al.
A true gastrula with sealed epihelia and an archenteron is seen in all eumetazoans, both in the larval and adult organization of enidarians and ctenophores (and acoelomorphs) and as an ontogenetic stage in eubilaterians.
Amphimedon possesses a large majority of transcription factor gene classes that have previously been found only in eumetazoans, including homeobox genes belonging to ANTP, prd-like, Pax, POU, LIM-HD, Six, and TALE classes, as well as basic helix-loop helix (bHLH), Rel, nuclear receptor, Mef2, Ets, Sox, T-box, and Fox genes (Larroux et al.
One obvious difference is that representative eumetazoans have a much larger number of developmental genes compared to Amphimedon, with individual transcription factor and signaling pathway gene classes and families having expanded differentially early in the eumetazoan lineage (Kusserow et al.
The Cnidaria share with bilaterian animals several derived characters that distinguish eumetazoans
from sponges, including the possession of epithelio-muscle cells and nerve cells.