The Phylum Hemichordata comprises the worm-like Class Enteropneusta, and the tentaculate Class Pterobranchia. Maybe the monotypic Planctosphaeroidea should be included as a hemichordate class also, but the position of this group is uncertain, as the only species known is described as Planctosphaera pelagica (Spengel, 1932) from a hemichordate-like pelagic larval stage that could not yet be linked to any adult hemichordate. P. pelagica is known from the Pacific Ocean and the eastern North Atlantic Ocean, and has not been reported from the North Sea.
Hemichordata have ciliated gill slits and are therefore linked with the Chordata, in particular because they also have a dorsal and sometimes tubular nerve cord in the collar (the body part between the proboscis and the trunk). However, hemichordates do not have a notochord and are therefore placed in a separate phylum from the chordates.
The hemichordate adult body is either vermiform, as in the enteropneusts (also known as acorn worms), or saccate, as in the pterobranchs, and basically three-parted with prosome (proboscis), mesosome (collar) and metasome (trunk). Hemichordata are either solitary (Enteropneusta) or colonial (most Pterobranchia), strictly marine and benthic (i.e. sessile or sedentary) in the adult phase. At least one enteropneust species has been reported swarming at the surface of shallow water, feeding on phytoplankton (Brusca and Brusca, 1990) — note that this species, Glandiceps hacksi (Marion, 1885) does not occur in the North Sea.
Asexual reproduction occurs in some enteropneusts and in most pterobranchs. Sexual reproduction may involve direct development or indirect development. The Enteropneusta that produce non-yolky eggs (for instance Balanoglossus) are known for their characteristic tornaria larva with ciliary bands resembling those of certain larvae of Phylum Echinodermata.
[After Hayward and Ryland, 1995; Brusca and Brusca, 1990]