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., 2012; Porter et al., 2012; Ramirez et al., 2016).
Regarding NK homeobox genes, six or seven NK genes probably existed in the last common ancestor of sponges and eumetazoans
. The NK cluster in the last common ancestor to protostomes and deuterostomes probably contained a cluster of nine NK genes: Msx, NK1/slouch, NK3/bagpipe, NK4/tinman, Tlx/t-15, NK7, NK6/hgtx, NK5/Hmx, and Lbx j ladybird (Holland 2001, Garcia-Fernandez 2005, Larroux et al.
(25) 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. Since the number of cell types of Trichoplax is considerably lower (four) than that of higher metazoans (e.g., more than 200 in human), it appears probable that placozoans possess cryptic subtypes of cells (14).
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.
It would therefore be advantageous to reserve the term gastrulation for the process that separates primarily locomotory ectodermal cells from primarily digestive endodermal cells (in eumetazoans) (Ereskovsky and Dondua, 2006).
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., 2006, 2007, 2008; Simionato et al., 2007; Gauthier and Degnan, 2008).
Despite the qualitative conservation of the metazoan developmental "regulome" between sponges and eumetazoans, there must be inherent differences in the genomes of the ancestors that gave rise to these lineages.
The Cnidaria share with bilaterian animals several derived characters that distinguish eumetazoans
from sponges, including the possession of epithelio-muscle cells and nerve cells.