Hemichordata is a unique phylum of marine invertebrates found in a wide range of depths and habitats. Two distinctly different body plans exist in this phylum: the large, solitary acorn worms (Class Enteropneusta), and the filter-feeding colonies of the class Pterobranchia. The hemichordate body is divided into three parts: proboscis (protostome), collar (mesosome) and trunk (metasome) - reflecting an underlying tricoelomate organization (see below). Hemichordates express all the attributes of Deuterostomia and at least one unique structure, the stomochord, a cartilaginous element that projects into the protocoel and supports the heart/kidney complex. The principal nervous system occurs dorsally in the collar region, and there is also an extensive epidermal nerve net. They have a pharynx in the trunk region, which may contain one or more pairs of gill slits. They have several chordate traits that make the hemichordates an important target of research for understanding deuterostome and chordate evolution (Brown et al. 2008).
There are approximately 90 described solitary species in the class Enteropneusta (‘gut-breather’), with many more species not yet described. These worms are benthic inhabitants of mud and sand from shallow waters to the deep sea (Cannon et al. 2009, Deland et al. 2010). A keen observer can often spot the coiled ropelike castings of these animals on exposed tidal flats. They have numerous gill slits, a straight digestive tract and a terminal anus, as adults. They lack tentaculated arms, reproduce sexually by releasing eggs or sperm into the surrounding seawater, and grow from moderate to considerable lengths (2 cm to 2.5 m in the case of Balanoglossus gigas). As exceptionally fragile animals, they have a striking ability of regeneration (Rychel and Swalla 2008). Acorn worms may be deposit feeders (the majority of the burrowing species), suspension feeders (nonburrowing species), or both. Food particles are trapped in mucus on the proboscis and transported to the mouth.
In contrast to the relatively common acorn worms, members of the class Pterobranchia (‘gill-wing) are minute colonial animals that superficially resemble bryozoans and hydroids. Pterobranchs are suspension feeders using their hollow, ciliated arms and tentacles to capture small particles from the water. They have a U-shaped digestive tract, may or may not possess gill slits, and exhibit one to ten pairs of ciliated feeding arms. However, pterobranch organ systems are similar to the enteropneusts. They may be found on the surfaces of shells and rocks, but easily overlooked. Only 27 pterobranch species are currently described, assigned to three genera. Pterobranch colonies are hermaphroditic, but the zooids themselves are usually either male, female or immature.
Current hemichordate taxonomy has received some, but limited attention (Woodwick and Sensenbaugh 1985, Woodwick 1996, Giray and King 1996, King et al. 1994, 1995, Smith et al. 2003, Cameron et al. 2010, Deland et al. 2010). Most notable was the recent description of the deep sea Torquartoridae with an exceptionally wide collar (Holland et al. 2005). However, phylogenetic analyses suggest that these are part of the Ptychoderidae (Cannon et al. 2009). Only two comprehensive, but dated, monographs (Spengel 1893, Van Der horst 1939) exist for enteropneusts and none for pterobranchs. Currently there are 22 recognized hemichordate genera and about 117 recognized species, although many of the genera are made up of a single specimen and are questionable.
Hemichordates have been pivotal for understanding when chordate-like morphological and developmental features evolved. Recent results (Cannon et al. 2009) indicate that colonial pterobranchs evolved from a solitary acorn worm-like hemichordate ancestor and are sister group to the Harrimaniid enteropenusts, although phylogenomic studies are showing a monophyletic enteropneust group. In addition, recent developmental genes expression data suggest that the chordate and deuterostomes ancestor was a worm-like solitary organism, similarly to enteropneust hemichordates (Brown et al. 2008).