Protostome

Many animals belong to one big group.

This group has over a million kinds. Some look like worms. Others have hard shells.

Some are even tiny bugs. When they are very young, they start as a small dent. This dent helps make their gut. The dent can become a mouth. This is how they grow.

They are very different from other animals. It is fun to see them all!

Many animals belong to a large group called protostomes.

This group is very big. It has over one million species. You might know some of them. Insects and crabs are part of this group.

Snails and worms are also protostomes.

Scientists used to think all protostomes grew the same way. They looked at how embryos grow. An embryo is a very young animal. In many animals, the embryo starts with a small dent. This dent is called a blastopore.

In protostomes, this dent often becomes the mouth. This is why they have that name.

These animals also form body parts in a special way. They use a way called schizocoely. This happens when solid tissue splits apart to make a body cavity. This is different from other animals.

Long ago, protostomes and other animals had a common ancestor. This ancestor was a worm-like animal that lived in water. It lived over 600 million years ago. Today, protostomes are split into two main groups. One group is called Ecdysozoa. The other group is called Spiralia.

Protostomes are a huge group of animals. They belong to a larger group called Bilateria. These animals have a front and a back. They also have three layers of tissue.

This group is very successful on our planet. There are over one million species alive today. This is much more than the 73,000 species in their sister group.

You might see them in your own backyard. They are everywhere in nature.

Scientists study how these animals grow from an embryo. An embryo is a very young animal. In many animals, the embryo forms a small dent. This dent is called a blastopore.

The name protostome comes from Greek words meaning "first mouth." This is because the dent often becomes the mouth. However, scientists now know this is not always true. The way the dent develops can change a lot.

It is actually quite variable among different members.

These animals also make their body cavities in a special way. This cavity is called a coelom. In protostomes, this happens through schizocoely. This is a process where solid tissue splits apart. The tissue splits to create a space inside.

This is different from other animals. Other animals make their cavities through enterocoelic pouching. That happens when gut walls fold inward to make a pouch.

Protostomes have a very long history on Earth. They split from other animals over 600 million years ago.

Their common ancestor lived in the ocean. This ancestor was likely a worm-like aquatic animal. It lived during a time called the Ediacaran.

Today, scientists divide protostomes into two main groups. One group is called Ecdysozoa. The other group is called Spiralia.

Many familiar creatures belong to these groups. The Ecdysozoa include arthropods and nematodes.

You know arthropods like insects or crabs. The Spiralia include molluscs and annelids.

This group includes snails and earthworms. Flatworms are also part of this large family.

It is amazing to see how many different shapes they take. They are a major part of life on Earth.

Protostomia is a massive group of animals known as a clade. This group includes many creatures we see every day. You might recognize arthropods, molluscs, annelids, flatworms, or nematodes. These animals belong to a larger group called Bilateria. Bilateria are animals that have bilateral symmetry. This means they have a front and a back, called an anteroposterior axis. They also have three distinct germ layers in their bodies.

Scientists often study how these animals develop from an embryo. In complex animals, the first step of gut development is very specific. The embryo begins by forming a small dent on one side. This dent is called the blastopore. As the embryo grows, this dent deepens into a digestive tube called the archenteron. The name "protostome" comes from Greek words meaning "first mouth." This is because scientists once believed the blastopore always became the mouth.

However, modern science shows that development is actually very complex. The fate of the blastopore is extremely variable among different species. While the name "protostome" is sometimes disputed, the group is still a valid evolutionary category. It is often compared to a sister group called Deuterostomia. In deuterostomes, the process is usually the reverse. In those animals, the original blastopore becomes the anus instead of the mouth.

Protostomes also develop their internal body cavities in a unique way. This body cavity is called a coelom. Most protostomes are also called schizocoelomates because of how they form this space. They use a process called schizocoely to create the coelom. During schizocoely, a solid mass of embryonic tissue splits apart. This splitting creates the cavity inside the body. This is different from deuterostomes, which use enterocoelic pouching. In that process, the walls of the gut fold inward to make pouches.

The history of these animals stretches back hundreds of millions of years. Protostomes and deuterostomes shared a common ancestor long ago. This ancestor was likely a worm-like animal that lived in the water. It lived during a period called the Ediacaran. Scientists believe these two main groups diverged over 600 million years ago.

Today, the protostome group is incredibly diverse and successful. There are over one million different species of protostomes alive on Earth. This is a much larger number than the deuterostome group. There are only about 73,000 species of deuterostomes. Scientists divide the protostomes into two major groups. One group is the Ecdysozoa, which includes arthropods and nematodes.

The other group is the Spiralia.

The Spiralia group contains many different types of animals. It includes molluscs, annelids, rotifers, and platyhelminths. You can see these different forms in snails or earthworms.

Because they are so varied, they play many roles in our world. From tiny worms to complex insects, they are a major part of the tree of life.