Balanoglossus and Tornaria Larva Notes

Balanoglossus - Morphology, Coelom, Tornaria Larva

General Classification

  • Phylum: Chordata

  • Group: Acraniata

  • Subphylum: Hemichordata

  • Class: Enteropneusta

  • Type: Balanoglossus

Balanoglossus Overview

  • Balanoglossus is also known as the acorn or tongue worm.

  • Its body is:

    • Worm-like, brittle, and soft

    • Enterocoelous (body cavity)

    • Marine and cosmopolitan

    • Tubicolous, living in U-shaped burrows in sandy bottoms

  • It acts as a connecting link between chordates and non-chordates by exhibiting features of both lineages:

    • Presence of notochord and pharyngeal gill slits (features of chordates)

    • A dorsal heart (similar to non-chordates)

External Features

  • Body Structure:

    • Soft, elongated, cylindrical

    • Brightly colored but emits an unpleasant odor

    • Length varies from 9 cm to 45 cm

  • Body divided into three regions:

    1. Anterior: Proboscis

    2. Middle: Collar

    3. Posterior: Trunk

Morphological Details

1. Proboscis

  • Shape: Conical, hollow structure with thick muscular walls

  • Contains a cavity known as proboscis coelom that opens via a mid-dorsal pore at its base

  • Attaches to the collar via a proboscis stalk; mouth located ventrally on the stalk

2. Collar

  • Structure: Hollow with thick muscular walls

  • Contains collar coelom divided into right and left halves by mesentery

  • Externally opens via pores at the posterior end, leading to gill pouches

  • Overlaps the proboscis stalk anteriorly and begins the trunk posteriorly

3. Trunk

  • Largest bodily region, divided into three parts:

    1. Branchiogenital region: Contains gill sacs, dorsal surface has two rows of gill pores

    2. Hepatic region

    3. Abdominal region

  • Genital wings are extensions that accommodate the gonads

Coelomic Structure

  • Balanoglossus possesses a true coelom:

    • Comprises five separate cavities:

    1. One in the proboscis

    2. Two in the collar

    3. Two in the trunk

    • Cavities do not communicate, separated by mesenteries

Types of Coelomic Cavities

  1. Protocoel:

    • Located in the posterior proboscis; opens via a mid dorsal pore

    • Filled with connective tissue and muscles

  2. Mesocoels:

    • Paired cavities in the collar opening through paired dorsal pores

    • Also contain connective tissue and muscle fibers

  3. Metacoels:

    • Paired cavities in the trunk that do not open externally; contain radial and longitudinal muscle

Significance of Coelom

  • Allows the animal to become turgid and burrow effectively by taking in seawater

  • Coelomic fluids play roles in excretion and house coelomocytes, which aid in waste removal

Tornaria Larva

  • Appearance: Minute, transparent, ovoid larva, 1-9 mm in length

  • Anterior end features:

    • Ectodermal thickening, apical plate with sensory cilia and eyespots

  • Contains three ciliated bands:

    • Preoral band: Collects food

    • Post oral band (telotroch): Aids locomotion

    • Circum-anal ring: Encircles the anus

Internal Structure of Tornaria

  • Mouth leads to oesophagus, stomach, and intestine, opening at the posterior end via anus

  • Contains a sac-like cavity for future proboscis coelom; a heart vesicle develops into the adult's cardiac sac

Significance of Tornaria

  • Shows similarities to larval forms of annelids (trochophore larvae) and echinoderms (Bipinnaria, Auricularia)

  • Evidence of evolutionary connections

Comparative Features with Other Phyla

Hemichordates vs Chordates Similarities

  1. Buccal diverticulum homologous to the notochord

  2. Blood supply of gill slits resembles lower chordates

  3. Presence of a dorsal hollow nerve cord

Differences

  1. Buccal diverticulum is simply an extension of the gut

  2. Gill slits are dorsal in hemichordates, lateral in chordates

  3. Hollow nerve cord limited to the collar region

  4. Hemichordates lack defined features like cephalization, exoskeleton, endoskeleton, metamerism, etc.