Etymology:- From the Greek Plakos for flat and Zoon for animal,
hence Placozoa is a flat animal.
Characteristics of Placozoa:-
1)Has no symmetry or constant shape.
2)Has no distinct tissues or organs.
3)Has no nervous system.
4)Has no body cavity or digestive cavity.
5)Body is shaped like a flat disc.
6)Body consists of a two layers of single cells.
7)Lives in marine environments.
8)One species only known Trichoplax adhaerens.
First discovered in 1883 in a marine aquarium in Austria they were until recently considered to be planuloid larvae of some kind. Then in the 1960s it was discovered that they could reproduce sexually and they were then given their own phylum.
T. adhaerens is a small flat grayish animal growing to a maximum of 3mm in diametre, its body is made up of a few thousand cells at most. Its body though basically disc shaped changes shape in like an amoeba as it moves around. It has no front or back and moves in any direction, gliding along on its cilliated under surface. It has only two layers of cells and 4 different types of cells making it the simplest of all multicellular organisms. The two cell layers, its top and bottom surface enclose a fluid filled space containing a network of stellate fibre cells. It has a surprisingly small amount of DNA in its cells, similar to species of bacteria, rather than other metazoans.
T. adhaerens is a marine animal, but it is very seldomly recognised or identified and its known distribution reflects the distribution of marine biology research stations rather than its true geographic distribution.
It feeds on protozoans which it covers with its body and then secretes digestive enzymes onto, the resulting nutrients are absorbed by the same cells that secrete the enzymes. Reproduction can be asexual by either binary fission or budding and accidental cutting of the body into several parts results in each part becoming a new smaller placozoan. Eggs and sperm are known to be produced but nothing else is currently known about its sexual reproduction.