Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth.

So far, three other extant species have been described, in two distinct classes: Uniplacotomia (Hoilungia hongkongensis in 2018 and Cladtertia collaboinventa in 2022[14]) and Polyplacotomia (Polyplacotoma mediterranea, the most basal, in 2019[15]).

The specific epithet adhaerens is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination.

[22] In 1907, Thilo Krumbach published a hypothesis that Trichoplax is not a distinct animal but that it is a form of the planula larva of the anemone-like hydrozoan Eleutheria krohni.

[17] The development of electron microscopy in the mid-20th century allowed in-depth observation of the cellular components of organisms, following which there was renewed interest in Trichoplax starting in 1966.

[32] The middle layer is the thickest made up of numerous fiber cells, which contain mitochondrial complexes, vacuoles and endosymbiotic bacteria in the endoplasmic reticulum.

The body consists of an outer layer of simple epithelium enclosing a loose sheet of stellate cells resembling the mesenchyme of some more complex animals.

[40] One of the at least 20 described species turned out to have two bacterial endosymbionts; Grellia which lives in the animal's endoplasmic reticulum and is assumed to play a role in the protein and membrane production.

[43] Located in the dorsal epithelium there are lipid granules called shiny spheres which release a cocktail of venoms and toxins as an anti-predator defense, and can induce paralysis or death in some predators.

[44][45] The Placozoa show substantial evolutionary radiation in regard to sodium channels, of which they have 5–7 different types, more than any other invertebrate species studied to date.

[47] Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies.

[48] In addition to fission, representatives of all species produced “swarmers” (a separate vegetative reproduction stage), which could also be formed from the lower epithelium with greater cell-type diversity.

[49] There is no convincing fossil record of the Placozoa, although the Ediacaran biota (Precambrian, 550 million years ago) organism Dickinsonia appears somewhat similar to placozoans.

If the absence of these features is an original trait of the Placozoa, it would mean that a nervous system and muscles evolved three times should placozoans and cnidarians be a sister group; once in the Ctenophora, once in the Cnidaria and once in the Bilateria.

Both the modern Placozoa and all other animals then descended from this multicellular beginning stage via two different processes:[53] While the probability of encountering food, potential sexual partners, or predators is the same in all directions for animals floating freely in the water, there is a clear difference on the seafloor between the functions useful on body sides facing toward and away from the substrate, leading their sensory, defensive, and food-gathering cells to differentiate and orient according to the vertical – the direction perpendicular to the substrate.

In the proposed functional-morphology model, the Placozoa, and possibly several similar organisms only known from the fossils, are descended from such a life form, which is now termed placuloid.

Although very successful in their ecological niche, due to the absence of extracellular matrix and basal lamina, the development potential of these animals was of course limited, which would explain the low rate of evolution of their phenotype (their outward form as adults) – referred to as bradytely.

[51] A concept based on purely morphological characteristics pictures the Placozoa as the nearest relative of the animals with true tissues (Eumetazoa).

The digestive gastrodermis in the Cnidaria or the gut epithelium in the bilaterally symmetrical animals (Bilateria) may have developed from endoderm, whereas ectoderm is the precursor to the external skin layer (epidermis), among other things.

The extreme degree of simplification that would have to be postulated for the Placozoa in this model, moreover, is only known for parasitic organisms, but would be difficult to explain functionally in a free-living species like Trichoplax adhaerens.