Trichoplax is a genus placed in the phylum Placozoa. The species Trichoplax adhaerens is one of four Placozoa species, alongside Hoilungia hongkongensis, Polyplacotoma mediterranea and Cladtertia collaboinventa. Placozoa is a basal group of multicellular animals, possible relatives of Cnidaria. Trichoplax are very flat organisms commonly less than 4 mm in diameter, lacking any organs or internal structures. They have two cellular layers: the top epitheloid layer is made of ciliated "cover cells" flattened toward the outside of the organism, and the bottom layer is made up of cylinder cells that possess cilia used in locomotion, and gland cells that lack cilia. Between these layers is the fibre syncytium, a liquid-filled cavity strutted open by star-like fibres. Trichoplax feed by absorbing food particles—mainly microbes—with their underside. They generally reproduce asexually by dividing or budding, but can also reproduce sexually. Though Trichoplax has a small genome in comparison to other animals, nearly 87% of its 11,514 predicted protein-coding genes are identifiably similar to known genes in other animals.
Discovery Trichoplax was discovered in 1883 by the German zoologist Franz Eilhard Schulze, in a seawater aquarium at the Zoological Institute in Graz, Austria. The generic name is derived from the classical Greek θρίξ (thrix), "hair", and πλάξ (plax), "plate". The specific epithet adhaerens is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination. Although from the very beginning most researchers who studied Trichoplax in any detail realized that it had no close relationship to other animal phyla, the zoologist Thilo Krumbach published a hypothesis that Trichoplax is a form of the planula larva of the anemone-like hydrozoan Eleutheria krohni in 1907. Although this was refuted in print by Schulze and others, Krumbach's analysis became the standard textbook explanation, and nothing was printed in zoological journals about Trichoplax until the 1960s. In the 1960s and 1970s a new interest among researchers led to acceptance of Placozoa as a new animal phylum. Among the new discoveries was study of the early phases of the animals' embryonic development and evidence that the animals that people had been studying are adults, not larvae. This newfound interest also included study of the organism in nature (as opposed to aquariums).
Morphology
Trichoplax generally has a thinly flattened, plate-like body in cross-section around half a millimetre, occasionally up to two or three millimetres. The body is usually only about 25 μm thick. Because they are so thin and fragile, and because the cilia which they use for locomotion are only loosely coordinated, they are constantly being split into two or three separate clones when their cilia moves in opposite directions, causing microfractures in the animal's epithelium. One hypothesis is that the larger a motile animal lacking a nervous system is, the less coordinated its locomotion becomes, placing an upper limit on their possible size. These colorlessly gray organisms are so thin they are transparent when illuminated from behind, and in most cases are barely visible to the naked eye. Like the single-celled amoebae, which they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form. These may facilitate movement to new habitats. Trichoplax lacks tissues and organs; there is also no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers: dorsal epithelia cells and ventral epithelia cells, each with a single cilium ("monociliate"), ventral gland cells, syncytial fiber cells, lipophils, and crystal cells (each containing a birefringent crystal, arrayed around the rim). Lacking sensory and muscle cells, it moves using cilia on its external surface. The collective movements of the cilia are completely coordinated by mechanical interactions.
Signal processing There are no neurons present, but in the absence of a nervous system the animal uses short chains of amino acids known as peptides for cell communication, in a manner resembling the way animals with neurons use neuropeptides for the same purpose. These specialized cells are called peptidergic cells. Unlike neurons, the cells do not use electrical impulses and their messaging is restricted to sending signals to other nearby cells, as they are unable to both send and receive signals. Individual cells contain and secrete a variety of small peptides, made up of between four and 20 amino acids, which are detected by neighbouring cells. Each peptide can be used individually to send a signal to other cells, but also sequentially or together in different combinations, creating a huge number a different types of signals. This allows for a relatively complex behavioural repertoire, including behaviours such as "crinkling", turning, flattening, and internal "churning". The genome of Trichoplax codes for eighty-five neurotransmitter receptors, more than in any other sequenced animal.
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