Hemichordate

Hemichordates are a group of interesting sea animals that live in the ocean. They are closely related to animals called echinoderms, which include sea stars and sea urchins. Scientists have found fossils of hemichordates in rocks from the Cambrian period, which was a very long time ago.

There are two main types of hemichordates. One type is called acorn worms. These worms usually live alone in small tubes they make in the sand or mud. They eat by gathering tiny bits of food from the water or the ground. The other type is called pterobranchs. These animals usually live together in groups and build tube-like homes for themselves.

Hemichordates are special because they help scientists understand how different animal groups are related. By studying these ocean creatures, we can learn more about the history of life on Earth.

Anatomy

The body of hemichordates has a special muscular structure. Acorn worms have a worm-like shape with three parts: a front part called the proboscis, a middle collar, and a long back trunk. The proboscis helps the worm move and catch food. The trunk holds parts like the pharynx with gill slits, and a long intestine.

Pterobranchs have a head shield that helps them move and make a tube-like home. They have arms for feeding and a looped digestive system. Some of these animals can stay attached to others in a group.

Hemichordates also have a simple nervous system without a central brain. Instead, they have nerve networks that help them sense light and chemicals. Their circulatory system is open, with blood moving through vessels and bathing the tissues directly. They breathe through gill slits in their trunk, drawing in water to get oxygen. Their digestive system is a tube that runs from mouth to anus, breaking down food with the help of cilia.

Development

Hemichordates are interesting sea worms that are closely related to both echinoderms and chordates. Scientists study them to learn more about how chordates, including animals like humans, develop.

There are two main types of hemichordates: enteropneusts and pterobranchs. Enteropneusts can develop in two ways. One way includes a special larval stage that feeds on tiny plants and animals in the water before becoming an adult worm. The pterobranchs are another group that has been studied a lot.

Scientists have looked closely at two species: Saccoglossus kowalevskii and Ptychodera flava. Saccoglossus kowalevskii develops directly into an adult, while Ptychodera flava goes through a larval stage first. Both species start with cells dividing in a pattern that helps form the body of the future worm.

Classification

See also: List of bilateral animal orders

Hemichordata are a group of animals found in the ocean. They have two main types: the Enteropneusta, also called acorn worms, and the Pterobranchia, which includes the graptolites. There is a third possible type, Planctosphaeroidea, but we only know about it from a single young specimen. There are about 120 living species of Hemichordata, and scientists keep finding more, especially deep in the ocean.

The family tree of these animals shows they are closely related to another group called Echinodermata. Some scientists think Pterobranchia might actually come from within the Enteropneusta group. There is also a chance that an ancient creature called Etacystis might have been part of this group.

Phylogeny

A phylogenetic tree shows where hemichordates fit in the bigger picture of animal family trees.