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).
The discovery of gill-slits in this animal by Alexander Kovalevsky (1865) led to the creation of the class Enteropneusta by Karl Gegenbaur (1870).
William Bateson (1885) originally included them in phylum Chordata.[1] Hyman (1959), however, placed them near Echinodermata and gave Hemichordata a status of an independent phylum.
Balanoglossus is a tuberculos[spelling?] (burrowing) and exclusively marine animal. It is found in shallow waters between tide marks along the coast of warm and temperate oceans.