Balanoglossus

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 also have a stomochord, which is however 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 Carl 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