Balanoglossus is a genus of ocean-dwelling acorn worms (Enteropneusta). It has zoological importance because, being hemichordates, they are an "evolutionary link" between invertebrates and vertebrates. Balanoglossus is a deuterostome, and resembles the sea squirts (Ascidiacea) in that it possesses branchial openings, or "gill slits". It has a notochord in the upper part of the body and has no nerve chord. It does have a stomochord, however, which is a gut chord within the collar. Their heads may be as small as per 2.5 mm (1/10 in) or as large as 5 mm (1/5 in). Discovery The discovery of gill-slits in this animal by Alexander Kovalevsky (1865) led to the creation of the class Enteropneusta by Karl Gegenbaur (1870). Classification William Bateson (1885) originally included them in phylum Chordata. Hyman (1959), however, placed them near Echinodermata and gave Hemichordata a status of an independent phylum. Habitat Balanoglossus is a tuberculos (burrowing) and exclusively marine animal. It is found in shallow waters between tide marks along the coast of warm and temperate oceans. Species The World Register of Marine Species lists the following species: Balanoglossus apertus Spengel, 1893 Balanoglossus aurantiaca Girard, 1853 Balanoglossus australiensis Hill, 1894 Balanoglossus biminiensis Willey, 1899 Balanoglossus borealis Willey, 1899 Balanoglossus capensis Gilchrist, 1908 Balanoglossus carnosus Willey, 1899 Balanoglossus clavigerus Delle Chiaje, 1829 Balanoglossus gigas Müller in Spengel, 1893 Balanoglossus hydrocephalus van der Horst, 1940 Balanoglossus jamaicensis Willey, 1899 Balanoglossus misakiensis Kuwano, 1902 Balanoglossus natalensis Gilchrist, 1908 Balanoglossus numeensis Maser, 1913 Balanoglossus occidentalis Ritter, 1902 Balanoglossus parvulus Punnett, 1903 Balanoglossus proterogonius Belichov, 1928 Balanoglossus robinii Giard, 1882 Balanoglossus salmoneus Belichov, 1928 Balanoglossus simodensis Miyamoto & Saito, 2007 Balanoglossus stephensoni van der Horst, 1937 Balanoglossus studiosorum van der Horst, 1940 References Enteropneusta Hemichordate genera