Hemichordata (/ˌhɛmɪkɔːrˈdeɪtə/ HEM-ih-kor-DAY-tə) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms.

They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms), and Pterobranchia.

The class Graptolithina, formerly considered extinct,[1] is now placed within the pterobranchs, represented by a single living genus Rhabdopleura.

The anteroposterior axis is divided into three parts: the anterior prosome, the intermediate mesosome, and the posterior metasome.

The body of acorn worms is worm-shaped and divided into an anterior proboscis, an intermediate collar, and a posterior trunk.

The proboscis is a muscular and ciliated organ used in locomotion and in the collection and transport of food particles.

[6] The prosome of pterobranchs is specialized into a muscular and ciliated cephalic shield used in locomotion and in secreting the coenecium.

The metasome, or trunk, contains a looped digestive tract, gonads, and extends into a contractile stalk that connects individuals to the other members of the colony, produced by asexual budding.

In the genus Rhabdopleura, zooids are permanently connected to the rest of the colony via a common stolon system.

They have a diverticulum of the foregut called a stomochord, previously thought to be related to the chordate notochord, but this is most likely the result of convergent evolution rather than a homology.

A hollow neural tube exists among some species (at least in early life), probably a primitive trait that they share with the common ancestor of chordata and the rest of the deuterostomes.

[13] Hemichordates are a phylum composed of two classes, the enteropneusts and the pterobranchs, both being forms of marine worm.

[15][16][17][18] The following details the development of two popularly studied species of the hemichordata phylum Saccoglossus kowalevskii and Ptychodera flava.

The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromeres and four smaller micromeres.

Hemichordata are divided into two classes: the Enteropneusta,[23] commonly called acorn worms, and the Pterobranchia, which includes the graptolites.