Placozoa

These are tiny sea animals.

They look like little blobs. They do not have mouths or backs.

They eat by swallowing small bits of food.

These animals live in the ocean. They move by using tiny hairs. These hairs help them creep along the seafloor.

They are very simple. They do not have organs. They are made of many small cells.

Some of them can make more of themselves. They can grow small parts that pop off. These parts become new animals.

They are some of the simplest animals on Earth. Can you find them in the water?

Placozoans are some of the simplest animals on Earth.

They live in the ocean. They look like tiny, flat blobs. Most are only about 0.55 mm wide.

They do not have mouths, backs, or even nerves.

These animals are made of many cells. These cells form three layers. The top layer has tiny hairs called cilia. The animal uses these cilia to creep along the seafloor. The middle layer is the thickest part. The bottom layer helps the animal eat. It engulfs small bits of food from the sea.

Placozoans can make new animals in a few ways. They can use fission, which means splitting apart. They can also use budding. This is when a small part grows and pops off to become a new animal.

Scientists first found the first species in 1883. Its name is Trichoplax adhaerens. For a long time, we only knew of this one kind. New species were added in 2018. Now, we know of a few different types. They help us learn how the first animals lived.

Placozoans are some of the simplest animals living on Earth.

They live in the ocean as free-living creatures. This means they do not live as parasites on other animals. Most of these animals look like tiny, flat blobs. They do not have a regular shape or a body plan.

They lack mouths, backs, and even nerve cells. An individual animal might only be 0.55 mm wide. They are very small, but they are still true animals.

These animals work in a very simple way. Their bodies are made of many different cells. These cells form three main layers inside the body. The top layer is thin and has tiny hairs called cilia. The animal uses these cilia to creep along the seafloor. The middle layer is the thickest part of the body. The bottom layer helps the animal eat by engulfing small bits of food.

Some cells even contain tiny bacteria that help them live.

Scientists have been studying these creatures for a long time. A German zoologist named Franz Eilhard Schulze found the first species in 1883.

He found it in a seawater aquarium in Graz, Austria. He named the first species Trichoplax adhaerens. For over one hundred years, scientists thought only one kind existed. This changed in 2018 when new species were finally discovered. Karl Gottlieb Grell later created the group name Placozoa in 1971.

There are now a few different types of placozoans known to science. Most belong to a group called Uniplacotomia. This group includes Hoilungia hongkongensis and Cladtertia collaboinventa. Another group is called Polyplacotomia. This group includes Polyplacotoma mediterranea, which was described in 2019. Scientists use DNA to tell these species apart. This is because they look so much alike. Even their fossils, like Maculicorpus microbialis, are hard to find.

Placozoans help us understand how life began. They sit in a special place between sponges and more complex animals.

By studying them, we learn about the start of muscles and nerves. They can also make new animals through fission or budding. Fission is when an animal splits apart to make a copy. Budding is when a small part grows and pops off. These simple ways of living show us how early life worked.

Placozoans are a phylum of marine invertebrates that live freely in the ocean.

They are often described as the simplest animals on Earth. These creatures are non-parasitic, meaning they do not live off other animals. Instead, they live independently in seawater environments. Their bodies are essentially collections of cells that form a blob-like shape.

Because they lack complex organs, they are considered among the most basal animals. This means they represent a very primitive branch of the animal kingdom.

The anatomy of a placozoan is remarkably minimal. They lack a well-defined body plan, a mouth, a back, or even traditional nerve cells. An individual body typically measures about 0.55 mm in diameter. The structure consists of three distinct tissue layers called epithelia. The upper epithelium is the thinnest layer and contains flat cells. Each of these cells has a cilium, which is a tiny hair-like structure. These cilia allow the animal to move by creeping along the seafloor.

Below the top layer lies the middle layer, or intermediate epithelium. This is the thickest part of the animal's body. It is composed of numerous fiber cells that contain mitochondrial complexes and vacuoles. Some of these cells even host endosymbiotic bacteria within their endoplasmic reticulum. The third layer is the lower epithelium. This layer contains monociliated cylinder cells and various gland cells. These cells help the animal feed by engulfing organic detritus from the seafloor. This process of eating by engulfment is a key part of their survival.

Despite lacking a brain, placozoans possess a primitive analogue of a nervous system. They use specialized peptideric cells that act much like neurons. These cells can release vesicles and neuropeptides to communicate. While they do not have the projections found in more complex neurons, they can still produce electrical impulses. They also possess a high variety of sodium channels. In fact, they have five to seven different types of these channels. This is more than any other invertebrate species studied to date.

The history of placozoan discovery is a story of scientific debate. The first species, *Trichoplax adhaerens*, was discovered in 1883 by Franz Eilhard Schulze. He found the specimen in a seawater aquarium in Graz, Austria. The name *adherens* refers to how the animal sticks to glass slides. For many years, other scientists disagreed about what these animals actually were. Some argued they were merely flatworms or even larvae of other animals. It was not until the mid-20th century that electron microscopy allowed for better study. In 1971, Karl Gottlieb Grell officially established the phylum Placozoa.

For over a century, Placozoa was considered a monotypic phylum, meaning it had only one species. This changed in 2018 when researchers identified new species. Today, the phylum is divided into two distinct classes. One class is Uniplacotomia, which includes *Hoilungia hongkongensis* and *Cladtertia collaboinventa*. The other class is Polyplacotomia, which includes *Polyplacotoma mediterranea*. Because these animals look so similar, scientists must use mitochondrial DNA to tell them apart. This genetic sequencing reveals that each species has a unique set of genes.

Understanding placozoans helps scientists map the evolution of all animal life. They occupy a critical position on the phylogenetic tree between sponges and more complex animals.

By studying their simple structure, researchers can investigate the origins of muscles and nerves. If placozoans branched off very early, it suggests that complex systems like the nervous system evolved multiple times. Their unique biology, including their ability to reproduce through fission or budding, provides a window into the ancient past. Even their fossil record, such as the Middle Triassic *Maculicorpus microbialis*, offers clues about their long history.