Protostome

Protostomia is a group of animals known for a special way their bodies develop very early in life. In these animals, a small opening in the embryo, called the blastopore, often becomes the mouth. This was an important discovery that helped scientists understand how different animals are related.

Protostomes include many familiar creatures like arthropods (such as insects and spiders), molluscs (like snails and clams), annelids (earthworms and leeches), flatworms, and nematodes (roundworms). Because of this wide variety, protostomes are very important in studying animal life.

Together with Deuterostomia and Xenacoelomorpha, protostomes make up the larger group called Bilateria. These animals all share certain features, like having a body shape that is the same on both sides (bilateral symmetry), a clear front and back end, and three main layers of cells that form their bodies. This helps scientists classify and understand the wide world of animals better.

Protostomy

See also: Embryological origins of the mouth and anus

In animals like earthworms, the first step in forming their digestive system starts when the embryo creates a dent called the blastopore. This dent deepens to become the tube they use to eat and digest food, known as the archenteron.

Protostomes were named this way because scientists once thought that in all these animals, the dent would become the mouth, and the anus would form later. But we now know that this is not always true and can vary a lot among different protostomes. Protostomes include well-known animals like arthropods, molluscs, annelids, flatworms, and nematodes.

Evolution

The common ancestor of protostomes and deuterostomes was a worm-like aquatic animal from long ago. The two groups separated over 600 million years ago. Today, protostomes have evolved into over a million different species, while deuterostomes have about 73,000 species.

Protostomes are divided into two main groups: Ecdysozoa, which includes arthropods and nematodes, and Spiralia, which includes molluscs, annelids, platyhelminths, and rotifers. A modern family tree showing how these groups are related is shown below.^{[excessive citations]} The times when these groups split into new groups are given in mya (millions of years ago).