Classification of Balanoglossus

• Phylum :- Hemichordata (Marine, enterocoelous coelomate. Notochord confined to head region)
• Class :- Enteropneusta (Several gill-slits, intestine straight, solitary.)
• Genus
  • Balanoglossus (Tongue worm)
  • Glossobalanus (Acorn worm)

Geographical distribution of Balanoglossus

World-wide or cosmopolitan. Habit and habitat : It is a marine animal, adapted for burrowing life in the sandy bottom. The animal lives inside the V-tubes (tubicolous). Most of them live in shallow water but a few go deeper upto approximately 15,000 feet. They burrow slowly by soft proboscis.

Characteristics of Balanoglossus

• Balanoglossus is commonly called as ‘Acorn worm.’ It lives in a U-shaped burrow and at one opening of burrow fecal castings may be seen. V-tube burrow at other end has opening for anterior end and also another additional opening.
• It measures 10 cm to 2.5 meters, depending on the species.
• It is bilaterally symmetrical, triploblastic deuterostome with worm-shaped body divisible into three regions-anterior proboscis or prosoma, middle collar or mesosoma and posterior trunk or metasoma.
• Proboscis is conical, collar is funnel like, while trunk is cylindrical.
• Proboscis contains heart vesicle, central sinus and buccal diverticulum. It has thick muscular body wall and its cavity or coelom opens to the exterior by the proboscis pore.
• Collar contains the mouth and collar coelom which opens by a pair of collar pores on dorsal surface.
• Trunk region contains most of the internal organs, such as pharynx, gonads or hepatic region.
• Pharynx and gonads constitute branchiogenital region. Posteriormost part is abdomen. Branchio-genital region is composed off
  • genital wing having gonad
  • branchial region containing paired gills and
  • hepatic region having hepatic caecae.
• Sexual dimorphism. Fertilization external and development includes tornaria larva. It possesses power of regeneration.
• Alimentation is complete and circulatory system usually contains contractile sac of heart.

Special features of Balanoglossus

Balanoglossus and allied forms have phylogenetic importance. Certain tissue in the form of anterior projection from the mouth cavity was formerly interpreted as a notochord and thus placing this group within the phylum Chordata. Recent studies show such structure mainly as buccal pouch and now the hemichordates are not included with the chordates but as separate phylum Hemichordata. It shows affinities with (i) Chordates (Bateson, 1885), (ii) Echinoderms (Metshinkoff, 1869), and (iii) Annelida (Spangel, 1893).

Gill-slits, nerve cord and the so-called notochord are identical with chordates. Hyman (1959) objected its so-called notochord. Blastopore, ciliated bands and enterocoelus coelom are common features both in Hemichordates and Echinoderms. It resembles with Annelids also in having segmented body. It may be concluded that invertebrate features out weigh the chordate features and hence hemichordates are included in invertebrates but under separate Phylum Hemichordata.

Identification of Balanoglossus

Since the specimen contains short proboscis and all above features hence, it is Balanoglossus.