Trichoplax adhaerens.

The intriguing genome of the Trichoplax (Placozoa meaning flat animals):
Trichoplax Genome Sequenced: 'Rosetta Stone' For Understanding Evolution

Nice Pic


Place on the evolutionary tree

Currently there is only one named species in the phylum: Trichoplax adhaerens.

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It's a flattened blob, a few millimeters across and made up of a few thousand cells. It's main claim to fame is its remarkable simplicity: it is a multicellular animal that consists of only four apparent cell types, and the only obvious organization is into an upper and lower surface. The upper surface consists of a sheet of covering cells, while the lower surface contains two cell types: the gland cells that secrete digestive enzymes onto whatever the animal is sitting on, and the cylinder cells that absorb whatever nutrients are released. In between is a loose network of fiber cells that are responsible for the animal's movement.

Link
Nerves, sensory cells and muscle cells are absent.
Interestingly (from the link):

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One other strange thing: in culture, Trichoplax is consistently asexual and reproduces by fission, but older cultures at high density begin to produce small motile presumptive sperm cells, and as individual animals desintegrate, they spew out ova. The two have never been observed to come together, though, so there is no fertilization, and while the ova may divide a half dozen times, they all eventually die. It is possible that there is another stage in the life cycle that is not viable under laboratory conditions and has never been observed.

The genome of this critter is even more fascinating.
From the nature article:
The Trichoplax genome and the nature of placozoans

Table 1 | Developmental transcription factors in the Trichoplax genome
Homeobox (Hox genes)

• A) ANTP-class: Trox-2 (Hox/ParaHox-like), Not, Dlx, Mnx, Hmx, Hex, Dbx and seven others.
• B) PRD-class (paired box and homeobox): PaxB, Pitx, Otp, Gsc and five others
• C) POU-class(POU domain and homeobox): POU class 4 (Brn-3), one other
• D) LIM-class (LIM domain and homeobox): islet, apterous, Lhx1/5 and one other
• E) SIX-class (sine oculis homeobox): Six3/6 and one other
• F) TALE-class: Pbx/Exd, Irx, Meis
• G) HNF-class: Hnf

Going down the list, what are the functions of these Hox genes?
1) Trox-2 (Hox/ParaHox-like)
Hox/paraHox-like genes are involved in axial patterning in bilaterarian organisms. Basically, they control the formation of the anterior–posterior (AP) axis. Function of Trox-2?

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We speculate that Trox-2 functions within a hitherto unrecognized population of possibly multipotential peripheral stem cells that contribute to differentiated cells at the epithelial boundary of Trichoplax.

2) Not
In mice, Not controls the development of the caudal notochord. What is the notochord?
Wiki

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The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In lower vertebrates, it persists throughout life as the main axial support of the body, while in higher vertebrates it is replaced by the vertebral column. The notochord is found on the ventral surface of the neural tube.

What does it do in this flat, simple organism?

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The homeobox gene Not is highly conserved in Xenopus, chicken and zebrafish with an apparent role in notochord formation, which inspired the name of this distinct subfamily. Interestingly, Not genes are also well conserved in animals without notochord such as sea urchins, Drosophila or even Hydra, but appear to be highly derived in mammals. A search for homeobox genes in the placozoan Trichoplax adhaerens, one of the simplest organisms available today, revealed only two homeobox genes: a Not homologue and the previously described gene Trox-2, which is most similar to the Gsx subfamily of the Hox/ParaHox cluster genes. Not has a unique expression profile in Trichoplax. It is highly expressed in folds of intact animals and in the wounds of regenerating animals. The dynamic expression pattern of Trichoplax Not is discussed in comparison with the invariable expression pattern of Trox-2 and the putative secreted protein Secp1. The high sequence conservation of Not from Trichoplax to lower vertebrates, but not to mammals, represents a rare example of an apparent gene decay in the lineage leading to humans.

Interesting preadaptations. It gets better.

Next, a look at the other Hox genes in this organism and their functions in higher animals.
Dlx, Mnx, Hmx,Hex, Dbx etc...

Hox genes video
Nice overview of Hox genes.