Balanoglossus is a genus of ocean-dwelling acorn worms. It has zoological importance because, being hemichordates, they are an "evolutionary link" between invertebrates and vertebrates. Balanoglossus specimens are deuterostomes, and resemble the sea squirts in that they possess branchial openings, or "gill slits". They have notochords and stomochords but no dorsal nerve cord. Their heads are between 2.5 mm (1/10 in) and 5 mm (1/5 in) wide.

Discovery

Johann Friedrich von Eschscholtz discovered Balanoglossus in 1825 on Mashail Island, and described it as a worm-like holothurian. The discovery of gill slits in these animals 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 are burrowing, exclusively marine animals. They are 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: