What is "balanoglossus"

Balanoglossus \Bal`a*no*glos"sus\, n. [NL., fr. Gr. ba`lanos acorn + glw^ssa tongue.] (Zo["o]l) A peculiar marine worm. See Enteropneusta, and Tornaria.

Balanoglossus is an ocean-dwelling acorn worm (Enteropneusta) genus of great zoological interest because, being a Hemichordate, it is an "evolutionary link" between invertebrates and vertebrates. Balanoglossus is a deuterostome, and resembles the Ascidians or sea squirts, in that it possesses branchial openings, or " gill slits".It has notochord in the upper part of the body and has no nerve chord. It does have a stomochord, however, which is gut chord within the collar. Their heads may be as small as 2.5 mm (1/10 in) or as large as 5 mm (1/5 in).