Balanoglossus

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Balanoglossus
Unidentified specimen from Réunion
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Hemichordata
Class:	Enteropneusta
Order:	Enteropneusta
Family:	Ptychoderidae
Genus:	Balanoglossus Delle Chiaje, 1829
Species
See text.

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".^[1] 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).