Xenacoelomorpha

Xenacoelomorpha is a small group of animals that do not have a backbone or a shell. It includes two closely related groups called acoelomorphs and xenoturbellids. Scientists gave this group its name in February 2011 after studying both how these animals look and how their DNA is similar. These animals are interesting because they help scientists understand how different animal groups are related to each other. By looking at both physical features and DNA, researchers can learn more about the history and evolution of many animal species.

Phylogenetics

Before scientists used special tools to study tiny bits inside the cells of these animals, people thought xenacoelomorphs looked like flatworms. But with new science, they learned that xenacoelomorphs are very different from flatworms.

Studies show that xenacoelomorphs are closely related to all other animals. This makes them one of the simplest groups of animals. Scientists are still learning more about these animals and how they are related to other sea creatures.

Characteristics

The phylum Xenacoelomorpha includes small, flat, worm-like creatures that live in marine and sometimes brackish water environments on the sediments. Some species live freely, while others are parasitic or symbiotic. They can be found very deep in the water, almost 4 km down, and near special places called hydrothermal vents.

These creatures have both male and female parts and reproduce sexually without a special early stage called a larval stage. They have a simple body shape with a front and back and matching left and right sides. Their bodies do not have a true inner space, and they lack a system to remove waste. Their bodies are covered in tiny hair-like structures called cilia, and some have simple balance organs and very basic eyes.