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).
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