Bilateria

Many animals have two sides.

They have a head and a tail. They also have a back and a belly. This helps them move in one direction.

Most animals look like this. They have a left side and a right side. This group is very big. They make up almost all animals we know.

Some of these animals are worms. They have a tube for food inside. This tube goes from a mouth to an end.

Other animals are very different. Some have a head with eyes and sense parts. This helps them find food.

These animals are very successful. They live all over our world.

Most animals have a body with two sides.

This means they have a left side and a right side. They also have a head and a tail. They have a back and a belly too. This body plan helps them move in one direction.

This group is called Bilateria. They are very successful. Over 98% of all known animal species are bilaterians.

This group includes many things like worms, snails, and even insects.

Most bilaterians have a head. This is called cephalization. It is when sense organs and nerves gather at the front. Having a head helps animals find food.

Most also have a gut that goes from a mouth to an anus. Some simple worms only have one opening for food.

Scientists study how these animals grow. They look at the first opening in an embryo. In one group, this opening becomes the mouth. In the other group, it becomes the anus. These two groups are called protostomes and deuterostomes.

One of the oldest fossils found is Ikaria wariootia.

Bilateria is a huge group of animals that share a special body plan. Most of these creatures have two sides that look the same. This is called bilateral symmetry. They have a front end called a head and a back end called a tail. They also have a top side called a back and a bottom side called a belly.

This shape helps them move in one direction through their world. Because they move forward, they often have a head with sense organs. This process is called cephalization, which means nerves and senses gather at the front.

Most bilaterians have a body that works in a specific way. They have three layers of cells when they are very young embryos. These layers are called the endoderm, mesoderm, and ectoderm. Most also have a complete digestive tract. This is a tube that goes from a mouth to an anus.

Some animals have a body cavity, which is a space inside them. Others do not have this space at all. Even though they look different, they all share this basic way of growing. This makes them one of the most successful groups of living things.

Scientists have been studying these animals for a long time. An Austrian scientist named Berthold Hatschek named the group in 1888. Later, in 1910, Karl Grobben helped organize them into two main paths. One path is called protostomes and the other is called deuterostomes. Scientists tell them apart by looking at how an embryo grows. In protostomes, the first opening becomes the mouth. In deuterostomes, that same opening becomes the anus.

We can see how old this group is through fossils. One of the oldest known bilaterians is named Ikaria wariootia. It lived between 571 and 539 million years ago. Another important fossil is called Kimberella.

There is also a fossil called Vernanimalcula, but scientists are not sure if it is a real animal. Today, bilaterians are everywhere. They make up over 98% of all known animal species on Earth. This includes everything from tiny worms to large animals.

It is fun to think about how these animals connect to us. Many animals you see every day are part of this group. You might see a snail in a garden or a fish in a tank. Even insects like weevils belong to the Bilateria.

They all share that same basic plan of having a left and a right side. By looking at a simple insect, you are seeing a very successful way to live. The world is full of these symmetrical creatures.

Bilateria is a massive group of animals defined by their unique body plans. These animals exhibit bilateral symmetry during their early embryonic development. This means their bodies are organized around a central longitudinal axis. They have a clear front end, known as the anterior, and a rear end, called the posterior. They also possess a top side, the dorsal surface, and a bottom side, the ventral surface. This structure creates distinct left and right sides. This specific organization is a major reason for their biological success. Bilaterians are incredibly dominant, making up over 98% of all known animal species on Earth.

The way these animals function is tied to their symmetrical shape. Because they typically move in one direction, their front end encounters the environment first. This leads to a process called cephalization. During cephalization, sense organs and central nerve ganglia concentrate at the anterior end. This allows the animal to sense food or danger more efficiently. Most bilaterians, known as nephrozoans, also possess a complete digestive tract. This is a "through gut" that runs from a mouth to an anus.

Some species may use a hydrostatic skeleton, which is a wormlike body supported by fluid pressure.

Bilaterian embryos are generally triploblastic, meaning they develop from three distinct germ layers. These layers are the endoderm, the mesoderm, and the ectoderm. These layers eventually form the different tissues and organs of the adult animal. Some bilaterians also possess a body cavity. This might be a primary cavity derived from the blastocoel or a secondary cavity called a coelom. However, some members of this group lack a body cavity entirely.

These internal structures vary widely across different species within the clade.

Scientists traditionally divide Bilateria into two main lineages: protostomes and deuterostomes. This classification is based on how the embryo develops. In protostomes, the first opening in the embryo, the blastopore, becomes the mouth. In deuterostomes, that same opening becomes the anus. The deuterostome group includes echinoderms, hemichordates, and chordates. The protostome group includes arthropods, annelids, molluscs, and flatworms.

Recent studies have added complexity to this view. Some taxonomists now recognize additional superphyla like Ecdysozoa and Spiralia among the protostomes.

There is an ongoing debate about the nature of the "urbilaterian." This is the hypothetical most recent common ancestor of all Bilateria. One theory, the planuloid–aceloid hypothesis, suggests the ancestor had a solid body. This view was supported by researchers like Ludwig von Graff and Elie Metchnikoff. An alternative is the Archicoelomata hypothesis, proposed by A. T. Masterman in 1899. This theory suggests the ancestor already had a coelom. Other scientists propose the ancestor might have looked like a bottom-dwelling worm.

These different models attempt to explain how complex body cavities evolved.

The fossil record provides clues about when these animals first appeared. Trace fossils in Ediacaran sediments offer some of the earliest evidence. One of the oldest identified bilaterians is Ikaria wariootia, which lived 571 to 539 million years ago.

Another important fossil is Kimberella. There is also a fossil called Vernanimalcula, though its status as a true bilaterian is controversial. Some ancient burrows in Uruguay were thought to be 585 million years old. However, newer evidence suggests these specific fossils are actually from the late Paleozoic era.

Modern research continues to challenge how we group these animals. The phylum Xenacoelomorpha was established in 2011. This group includes the xenoturbellids and presents a challenge to traditional taxonomy. Some evidence suggests they may not belong to either the protostome or deuterostome groups. This has led to the proposal of the Nephrozoa clade. This clade would include all bilaterians except for the Xenacoelomorpha.

As of 2024, scientists are still working to resolve these complex evolutionary relationships.