Hemichordate

Some sea animals look like worms.

These are called acorn worms. They live in the ocean. Some live in holes in the sand. They eat tiny bits of food from the sea.

Other kinds live in groups. They live in small tubes. These tubes help them stay together. These animals have been around for a very long time. They are very interesting to study.

Hemichordates are a group of sea animals. They have been around since a very long time ago. Most of them look like worms.

There are two main groups in this family. One group is called Enteropneusta. These are acorn worms. They are often alone. Many live in burrows in the sand. They eat bits of food from the ocean floor.

The second group is called Pterobranchia. These animals usually live in colonies. A colony is a group of animals living together. They live in tiny tubes called a coenecium. These tubes are made of a material called collagen.

Some pterobranchs use arms to catch food from the water.

Scientists study these animals to learn about history. Hemichordates are close relatives to chordates. Chordates are animals like us. By looking at these worms, we can learn how early animals grew. Some species grow in a way that lets them feed on tiny plankton first. Others grow directly into adults.

There are about 120 species living in the sea today.

Hemichordates are a special group of marine animals. They live in the ocean and have been around since the Cambrian period. This was a very long time ago in Earth's history. Scientists study them to understand how complex life began. They are the closest living relatives to chordates. Chordates are the group that includes humans and other animals with backbones.

These animals have a body divided into three main parts. The first part is the anterior prosome. The middle part is the mesosome. The last part is the posterior metasome, or trunk. In acorn worms, the front part is called a proboscis. This muscular part helps them move and find food. The mouth sits between the proboscis and the collar. The long trunk contains the gut and the gills.

There are two main classes of hemichordates. The first class is Enteropneusta, which are often called acorn worms. These animals are usually solitary and live alone. Many species live in burrows in the sand. They eat bits of food from the ocean floor. The second class is Pterobranchia. These animals mostly live in colonies. They build tiny tube networks called a coenecium.

Scientists have found many different types of these creatures. There are about 120 to 130 known species living today. Some species are still being discovered in the deep sea. One group called Graptolithina was once thought to be extinct. Now, scientists know it belongs with the pterobranchs. This group includes the living genus Rhabdopleura.

Learning about hemichordates is like looking at a map of life. By studying how they grow, we learn about our own history. Some species, like Ptychodera flava, have an indirect way of growing. This means they start as tiny larvae that eat plankton. Others, like Saccoglossus kowalevskii, grow directly into adults. These different ways of growing help scientists see how many paths life can take.

Hemichordates are a phylum of marine animals that provide a vital window into evolutionary history. These organisms are triploblastic, meaning they develop from three distinct germ layers. They are also eucoelomate, which means they possess a true body cavity. Most hemichordates exhibit bilateral symmetry, where the body can be divided into two identical halves. Scientists consider them the sister group to echinoderms, such as starfish. Together, these two groups form a larger lineage known as Ambulacraria. Because they are the closest extant phylogenetic relatives to chordates, they are essential for studying the origins of our own biological development.

The body plan of a hemichordate is organized along an anteroposterior axis. This axis is divided into three specific regions: the anterior prosome, the intermediate mesosome, and the posterior metasome. In the Enteropneusta class, commonly known as acorn worms, these parts have specific names. The prosome is a muscular, ciliated proboscis used for movement and food collection. The mouth is located between the proboscis and the intermediate collar. The longest part is the posterior trunk, or metasome. This trunk contains the pharynx, which is lined with gill slits, as well as the esophagus, intestine, and gonads.

Pterobranchs utilize a different anatomical specialization for their lifestyle. Their prosome is a muscular cephalic shield used for movement and secreting a protective structure. The mesosome in pterobranchs often develops into tentaculated arms used for filter feeding. In the genus Rhabdopleura, there is one pair of arms, while the genus Cephalodiscus has several. The metasome contains a looped digestive tract and gonads. It also extends into a contractile stalk. This stalk allows individuals to connect to others in a colony through asexual budding.

There are two primary classes of living hemichordates, though a third is proposed. The Enteropneusta, or acorn worms, are generally solitary organisms. They often live in burrows and function as deposit feeders. Some species act as pharyngeal filter feeders, while the family Torquaratoridae are free-living detritivores. The second class is Pterobranchia, which are mostly colonial. They live within a branching, tubular network of collagen called a coenecium. A third class, Planctosphaeroidea, is suggested by the larva of a single species, Planctosphaera pelagica.

Developmental strategies in hemichordates vary significantly between species. Some undergo direct development, while others use indirect development. Indirect developers, such as Ptychodera flava, begin life as a planktotrophic tornaria larva. This larval stage must feed on plankton before it can transform into an adult. In contrast, Saccoglossus kowalevskii is a direct developer. Scientists study the cleavage patterns of these embryos to understand cell fates. In P. flava, the fourth division creates a 16-cell embryo with specific macromeres and micromeres. These cells eventually form the ectoderm and internal tissues during gastrulation.

Research into the genetic makeup of hemichordates reveals deep connections to chordates. For example, Saccoglossus kowalevskii possesses dorsalizing factors like bmp2/4. This gene is homologous to the dpp gene found in Drosophila. During gastrulation, the expression of bmp2/4 narrows toward the dorsal midline. Hemichordates also utilize a bmp antagonist called chordin in their endoderm. They possess a nerve net and longitudinal nerves, but they lack a centralized brain. Some species even show a hollow neural tube during early life, a primitive trait shared with the common ancestor of chordates.

History and discovery continue to expand our knowledge of this phylum. The Graptolithina, a group of graptolites once thought to be extinct, are now classified within the pterobranchs. This group is represented today by the single living genus Rhabdopleura. While approximately 120 to 130 species are currently described, many more are likely to be found in the deep sea. The discovery of the stem group Gyaltsenglossus helped scientists understand how early hemichordates combined traits from different classes. These marine worms remain a cornerstone for understanding how complex life transitioned through the Cambrian period.