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The hemichordates are so named because they have 3 characters that are otherwise thought to be chordate synapomorphies —gill slits, a dorsal nerve cord, and a diverticulum of the foregut that projects into the protocoelom and resembles a truncated notochord. Despite these similarities with chordates, the hemichordates are regarded as their own phylum and sister group to the phylum Echinodermata . Hemichordates and echinoderms have a dipleurula larva, a tricoelomate arrangement of the body plan, and a heart-kidney (echinoderm axial) complex. Found at all latitudes, hemichordates are not found on land or in fresh water; they are strictly marine animals. There are about 106 described hemichordate species from 3 extant classes: Enteropneusta, Pterobranchia, and Planktosphaeroidea . Class Enteropneusta Enteropneusts,oracornworms,arethemostspecioseclass of hemichordate, with approximately 76 described species . They are benthic, mobile worms that inhabitant sand or mud, live entangled in kelp holdfasts, or are associated with the underside of rocks. Most form mucus-lined burrows that smell strongly of haloindols or brominated phenols (Higa, Fujiyama, and Scheuer 1980) that are secreted from epithelial cells along with the mucous. In the deep sea there are several species that reside on the surface of the sediment, including one that clings to rocky outcroppings near hydrothermal vents. Most adult worms are less than 50 cm long, and the largest, Balanoglossus gigas, grows to lengths of 1.5 meters. The 4 families of enteropneust are Harrimaniidae, Saxipendiidae, Spengelidae, and Ptychoderidae. They are classified by the structure of the gills, gonads, liver sacs, and diverticula of the coelomic cavities. Development may either be direct or indirect via a tornaria larva. A taxonomic key to the enteropneust genera may be found in Cameron (2002). Class Pterobranchia Pterobranchs, or sea angels, are colonial, tube-dwelling hemichordates that resemble bryozoans. They are uncommon , and most of the 29 described species are collected by 1173 70 Hemichordata of the Gulf of Mexico Christopher B. Cameron  Hemichordata. After Pratt 1916. 1174 ~ Hemichordata Abbreviations Habitat codes used in the table include: ben = benthic; bur = burrower; sft = soft substrates (mud, sands, clays); and inf = infaunal. References 1. Bayer, F. M. 1962. A new species of Cephalodiscus (Hemichordata: Pterobranchia), the first record from the tropical western Atlantic. Bulletin of Marine Science of the Gulf and Caribbean. 12: 306–312. 2. Cameron, C. B. 2002. The anatomy, life habits and later development of a new species of enteropneust, Harrimania planktophilus (Hemichordata: Harrimaniidae) from Barkley Sound. Biological Bulletin 202: 182–191. 3. Hadfield, M. G., and R. E. Young. 1983. Planctosphaera (Hemichordata: Enteropneusta) in the Pacific Ocean. Marine Biology 73: 151–153. 4. Higa, T., T. Fujiyama, and P. J. Scheuer. 1980. Halogenated phenol and indole constituents of acorn worms. Comparative Biochemistry and Physiology (B) 65: 525–530. 5. Horst, C. J. van der. 1924. West-Indische Enteropneusten. Bijdragen tot de Dierkunde 23: 33. 6. Horst, C. J. van der. 1939. Hemichordata. In H. G. Bronns, ed. Klassen und Ordnungen des Tier-Reichs wissenschaftlich dargestellt in Wort und Bild. Akademische Verlagsgesellschaft, Leipzig. Band 4, Abt. 4, Buch 2, Tiel 2. 737 pp. 7. Spengel, J. W. 1893. Die Enteropneusten des Golfes von Neapel. Fauna und Flora des Golfes Neapel, Monograph. Herausgegeben von der Zoologischen Station zu Neapel. 18, Berlin. 757 pp. 8. Spengel, J. W. 1932. Planctosphaera. Scientific Results of the Michael Sars North Atlantic Deep-Sea Expedition 5: 590–600. 9. Willey, A. 1899. Remarks on some recent work on the Protochordata, with a condensed account of some fresh observations on the Enteropneusta. Quarterly Journal of Microscopical Science 42: 223–244. dredge in Antarctic waters, although some colonies are found in shallow tropical waters attached to the underside of rocks or coral rubble. The zooids that comprise the colony form by budding from a basal disk (Cephalodiscus) or from a common stolon (Rhabdopleura). Individual zooids are between 1 mm and 5 mm long, but the encrusting collagenous /proteinaceous tubes in which zooids reside may encrust a large area. Pterobranchs brood large, yolky eggs that develop into a planula-like larva, which eventually crawls from the protection of the tubes to establish a new colony. Although none are reported from the Gulf of Mexico, both genera are found in ankle-deep water in Bermuda, and Cephalodiscus atlanticus occurs 272 meters deep on Cay Sal Bank (Bayer 1962). Class Planktosphaeroidea Planctosphaera pelagica (Spengel, 1932) is a large, spherical larva that may exceed 25 mm diameter and is the sole member of this monotypic class (Horst 1939). Collected during deepwater trawls in the Atlantic Ocean and off...

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