Hemichordata is a small, unusual phylum of worm-like creatures closely related to the phylums Chordata and Echinodermata. They vary widely in size, from the 8 ft (2.5 m) lengths of the acorn worms to the 1/25 in (1 mm) lengths of the pterobranchs. The two living classes also vary in appearance and habits. Acorn worms are large individuals that burrow through sediment like an earthworm, digesting any organic material in the soil, or feeding on suspended particles in the water. Pterobranchs, on the other hand, form large colonies in which each individual is connected to another by stems. These creatures create their own homes, a series of tubes composed of collagen secreted by glands on their bodies.
Hemichordates do have several features that, despite the other differences, link the classes together. First off, all hemichordates have a 3-fold division of the body, with the preoral lobe, the collar, and the trunk. All hemichordates have pharyngeal slits, or gill slits, that open into the pharynx. Acorn worms have up to 200 of these; pterobranchs have only 1.
An interesting feature in the hemichordates is the stomochord located in the collar. This stomochord closely resembles the notochord of the chordates. Another feature that resembles that of chordates are the presence of the two nerve chords, a dorsal nerve chord and a smaller ventral nerve chord. Due to these similarities to the phylum Chordata, Hemichordata was once placed in this phylum, but DNA studies have shown that hemichordates are closer to the echinoderms than the chordates, and so they were placed in their own phylum. There are three recognized classes, one of which is extinct:
Enteropneusta (acorn worms)
Graptolithina (graptolites) extinct
Pterobranchia (pterobranchs) 20 spp
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