A phylum of bilaterally symmetrical 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; hemichordates possess gill slits, like chordates, but they lack a notochord. Some acorn worms develop via a ciliated larva (tornaria), which has some similarities to an echinoderm larva, and molecular studies confirm the hemichordates as the group most closely related to the echinoderms in evolutionary terms.

http://www.ucmp.berkeley.edu/chordata/hemichordata.html Introduction to the hemichordates, on the website of the University of California Museum of Paleontology