Hemichordata (Stomochordata) A phylum first encountered in the Middle Cambrian Burgess Shale, British Columbia, Canada. The hemichordates are related to the chordates (Chordata), for although they lack a notochord, the gill slits are very similar to those of primitive vertebrates and the dorsal collar nerve cord of hemichordates, which is sometimes hollow, is similar to the dorsal, hollow nerve cord of chordates. Hemichordates are exclusively marine. The body and coelom are divided into three sections, the middle section bearing gill slits. Development is by a tornaria larva, indicating affinities with the Echinodermata; the link between echinoderms and chordates is supported by biochemical evidence, all three groups containing creatine phosphate in the muscles whereas in all other invertebrates the phosphate is a compound of arginine. The two classes, Enteropneusta and Pterobranchia, may not be descended from a common stock and nowadays are often ranked as separate phyla.

Hemichordata (acorn worms) Phylum first encountered in the Middle Cambrian Burgess Shale, British Columbia, Canada. The acorn worms are related to the chordates, for although they lack a notochord, the gill slits are very similar to those of primitive vertebrates.

Hemichordata A phylum of soft-bodied marine invertebrates that comprises two classes: the Enteropneusta (acorn worms), which are cylindrical burrowing animals; and the Pterobranchia, which are colonial forms with vase-shaped bodies. The coelom is divided into three regions and the body into three sections: proboscis, collar, and trunk; like chordates, hemichordates possess gill slits but they lack a notochord. Some acorn worms develop via a ciliated larva (tornaria), which has some similarities to an echinoderm larva.