The phylum Hemichordata includes around 90 described species of enteropneusts (acorn worms) and around 30 described pterobranchs. Like chordates, hemichordates are deuterostomes with pharyngeal gill slits and most have a dorsal (and sometimes hollow) nerve cord. However, they lack a notochord. As adults, all or nearly all hemichordates are benthic (bottom-dwelling) marine animals.
Acorn worms range in size from a few centimeters to over 2 meters. They typically live buried in soft sand or mud, among algal holdfasts, or under rocks. Although they are largely intertidal, a few deep sea species are known. Most pterobranchs live in small colonies of individual zooids within a secreted tube on the ocean floor at depths of 5 to 5000 m. In some species the zooids are connected to each other by tissue extensions called stolons. A long contractile stalk allows the animal to retreat into its tube. The individual zooids are small, rarely exceeding 1 cm in length, but colonies may measure 10 cm or more across. (Brusca and Brusca 2003; Cannon et al. 2009; Margulis and Chapman 2010)
Hemichordates are dioecious (i.e., there are separate males and females, although they cannot be distinguished externally) with external fertilization and, in general, indirect development (i.e., there is a distinct larval form). Asexual reproduction occurs in at least some acorn worms and in most pterobranchs. Acorn worms fragment small pieces from the trunk, each of which can then grow into a new individual. Pterobranch colonies are derived by budding of a single sexually produced individual. Sexual reproduction in pterobranchs produces non-feeding larvae that are brooded in the colony. (Brusca and Brusca 2003; Cannon et al. 2009 and references therein)
Hemichordates are sessile (attached to the substrate) or capable of only limited movement. Feeding habits of hemichordates vary. Acorn worms that burrow in soft sediments are largely direct deposit feeders, digesting organic material ingested with the substrate as they burrow. Other acorn worms are filter feeders, selectively trapping suspended organic particles from the water with their mucus-covered proboscis. Pterobranchs use small dorsally located tentacle-bearing "arms" to capture suspended organic particles. The tentacles on adjacent arms cross to form a lattice across which a mucus net is secreted. Food is trapped in the mucus and moved to the mouth by cilia on the tentacles and arms (and, in at least some species, on the entire body). (Brusca and Brusca 2003)
The evolutionary relationships among the deuterostome phyla (Echinodermata, Hemichordata, Chordata, and possibly Xenoturbellida) remain somewhat controversial (see General Description).
The phylum Hemichordata includes around 90 described species of enteropneusts (acorn worms) and around 30 described pterobranchs (see the online Hemichordata World Database). Like chordates, hemichordates are deuterostomes with pharyngeal gill slits and most have a dorsal (and sometimes hollow) nerve cord. However, they lack a notochord. As adults, all or nearly all hemichordates are benthic (bottom-dwelling) marine animals.
Acorn worms range in size from a few centimeters to over 2 meters. They typically live buried in soft sand or mud, among algal holdfasts, or under rocks. Although they are largely intertidal, a few deep sea species are known. Most pterobranchs live in small colonies of individual zooids within a secreted tube on the ocean floor at depths of 5 to 5000 m. In some species the zooids are connected to each other by tissue extensions called stolons. A long contractile stalk allows the animal to retreat into its tube. The individual zooids are small, rarely exceeding 1 cm in length, but colonies may measure 10 cm or more across. (Brusca and Brusca 2003; Cannon et al. 2009; Margulis and Chapman 2010) Although the Hemichordata are traditionally divided into two classes--the solitary, free-living Enteropneusta (acorn worms) and the colonial, tube-dwelling Pterobranchia--recent phylogenetic studies suggest that the pterobranch clade may actually be nested within the acorn worms (Cannon et al. 2009 and references therein).
The basic body plan of an acorn worm includes a short, often conical, proboscis; a collar, which bears the ventral mouth at its anterior end; and a long trunk with the anus situated at its posterior end (although harrimaniid enteropneusts have a post-anal tail as juveniles). In most species, there is a clear region of the trunk that is chacterized by the presence of numerous gill pores. In contrast, pterobranchs are colonial, usually pear-shaped or globular tiny animals living in secreted tubes. The pterobranch gut is U-shaped, and individual zooids suspension-feed with ciliated tentacles. Members of the genus Cephalodiscus possess a single pair of gill slits, whereas Rhabdopleura have no gill slits. (Brusca and Brusca 2003; Cannon et al. 2009 and references therein; Margulis and Chapman 2010)
Hemichordates are dioecious (i.e., there are separate males and females, although they cannot be distinguished externally) with external fertilization and, in general, indirect development (i.e., there is a distinct larval form). Asexual reproduction occurs in at least some acorn worms and in most pterobranchs. Acorn worms fragment small pieces from the trunk, each of which can then grow into a new individual. Pterobranch colonies are derived by budding of a single sexually produced individual. Sexual reproduction in pterobranchs produces non-feeding larvae that are brooded in the colony. (Brusca and Brusca 2003; Cannon et al. 2009 and references therein)
Hemichordates are sessile (attached to the substrate) or capable of only limited movement. Feeding habits of hemichordates vary. Acorn worms that burrow in soft sediments are largely direct deposit feeders, digesting organic material ingested with the substrate as they burrow. Other acorn worms are filter feeders, selectively trapping suspended organic particles from the water with the proboscis. These particles may then be trapped in mucus secreted over the surface of the proboscis and moved posteriorly by ciliary currents. Pterobranchs use small dorsally located tentacle-bearing "arms" to capture suspended organic particles. The tentacles on adjacent arms cross to form a lattice across which a mucus net is secreted. Food is trapped in the mucus and moved to the mouth by cilia on the tentacles and arms (and, in at least some species, on the entire body). (Brusca and Brusca 2003)
The Hemichordata share some characters, such as pharyngeal gill slits or pores, that have been shown to be homologous with those of chordates. However, a range of molecular, developmental, and morphological data indicate that hemichordates and echinoderms, not chordates, are sister groups, together comprising the Ambulacraria. Given that hemichordates are more closely related to echinoderms than to chordates, any truly homologous features shared between hemichordates and chordates must have been present in the most recent common ancestor of deuterostomes. (Swalla and Smith 2008 and references therein; Cannon et al. 2009 and references therein)
The deuterostome phyla include Echinodermata, Hemichordata, and Chordata (and possibly Xenoturbellida, a clade that is apparently sister to the Ambulacraria, but alternatively may be sister to all other deuterostomes [Swalla and Smith 2008] or even all other Bilateria [Hejnol et al., 2009; Nielsen 2010]). Chordata has been considered to be composed of three subphyla: Vertebrata, Cephalochordata (Branchiostoma and Epigonichthyes), and Urochordata (Tunicata). Cameron et al. (2000) investigated phylogenetic relationships within the deuterostomes using an 18S ribosomal DNA data set. They concluded that the deuterostomes are composed of two major clades: chordates and (echinoderms + hemichordates). Their analysis strongly supported the monophyly of each of four major deuterostome taxa: (Vertebrata + Cephalochordata), Urochordata, Hemichordata, and Echinodermata. Based on their analysis, in combination with morphological and life history data, the authors concluded that Urochordata should be treated as a phylum, rather than a subphylum within Chordata, that is sister to the Chordata (Vertebrata + Cephalochordata). This grouping of vertebrates with cephalochordates is consistent with the traditional view of deuterostome relationships, a view that has been challenged by a number of more recent studies, which recover vertebrates and tunicates as sister taxa (e.g., see review by Stach 2008 and references therein; Swalla and Smith (2008) note that mitochondrial and ribosomal evidence place cephalochordates as sister group to the vertebrates, whereas genomic evidence places tunicates as the sister group to the vertebrates). Their analysis also suggested that the pterobranchs are neither sister to the enteropneusts nor are they basal deuterostomes, as had been suggested in the past, but rather that the pterobranchs are nested within the enteropneusts. (Cameron et al. 2000)
Lambert (2005a,b) reviewed the history of research on hemichordates, cephalochordates, and tunicates as well as diverse aspects of their biology.
