Balanoglossus is an ocean-dwelling acorn worm

Acorn worm

The Acorn worms or Enteropneusta are a hemichordate class of invertebrates, closely related to the echinoderms. There are about 90 species of acorn worm in the world, the main species for research being Saccoglossus kowaleskii....

 (Enteropneusta) genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of great zoological

Zoology

Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...

 interest because, being a Hemichordate

Hemichordata

Hemichordata is a phylum of marine deuterostome animals, generally considered the sister group of the echinoderms. They date back to the Lower or Middle Cambrian and include two main classes: Enteropneusta , and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a...

, it is an "evolutionary link" between invertebrate

Invertebrate

An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s and vertebrate

Vertebrate

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s. Balanoglossus is a deuterostome

Deuterostome

Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...

, and resembles the Ascidians

Ascidiacea

Ascidiacea is a class in the Tunicata subphylum of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide tunicin, as compared to other tunicates which are less rigid.Ascidians are found all over the world, usually in shallow...

 or sea squirts, in that it possesses branchial openings, or "gill slits", but differs in having no notochord

Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...

 and no nerve cord

Nerve cord

Nerve cord may refer to the following structures:* in invertebrates, it refers to the ventral nerve cord, whereas* in chordates, it stands for the dorsal nerve cord....

. It does have a stomochord

Stomochord

The stomochord is a flexible, hollow tube found in hemichordates. Stomochords were initially considered a variant of a primitive notochord, but are now recognized to not share histological composition to that of the notochord found in chordates....

, 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).

Species

The World Register of Marine Species lists the following species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

:

• Balanoglossus apertus (Spengel, 1893)
• Balanoglossus aurantiacus (Girard, 1853)
• Balanoglossus australiensis (Hill, 1894)
• Balanoglossus biminiensis (Willey, 1899)
• Balanoglossus capensis (Gilchrist, 1908)
• Balanoglossus carnosus (Willey, 1899)
• Balanoglossus clavigerus (Delle Chiaje, 1829)
• Balanoglossus gigas (Fr. Müller in Spengel, 1893)
• Balanoglossus jamaicensis (Willey, 1899)
• Balanoglossus misakiensis (Kuwano, 1902)
• Balanoglossus natalensis (Gilchrist, 1908)
• Balanoglossus numeensis (Maser, 1913)
• Balanoglossus occidentalis (Ritter, 1902)
• Balanoglossus proterogonius (Belichov, 1928)
• Balanoglossus salmoneus (Belichov, 1928)
• Balanoglossus stephensoni (van der Horst, 1937)