Hemichordata: Acorn Worms Between Echinoderms, Chordates

Hemichordata is a small phylum of marine worm-like animals that includes acorn worms and the tiny colonial pterobranchs. The name means "half-chordate" because these animals were once placed inside Chordata, but scientists now keep them as a separate phylum. They live burrowed in sand and mud on the ocean floor.

They matter because they sit at an important junction in animal evolution. Their tornaria larva looks strikingly similar to the bipinnaria larva of echinoderms (starfish), yet adults share gill slits with chordates. This makes hemichordates a living bridge for studying how chordates evolved from simpler ancestors.

Key Concepts

• Three body regions: an anterior proboscis, a middle collar and a long posterior trunk.
• Stomochord: a short hollow outgrowth of the gut in the proboscis region; it resembles a notochord but is not a true one.
• Pharyngeal gill slits: paired openings in the trunk used for filter feeding and respiration, like chordates.
• Nervous system: a dorsal nerve cord exists only in the collar region and is not hollow along the whole body; elsewhere there is a nerve net.
• Open circulation: blood flows through open sinuses, with a heart-like vesicle near the proboscis.
• Reproduction: sexes are usually separate with external fertilisation; some species can also reproduce asexually by fragmentation.

How It Works

1. The muscular proboscis digs and pushes through sand, extending and contracting like an earthworm.
2. The animal secretes mucus over the proboscis, which traps tiny food particles from the sediment.
3. Cilia sweep the food-laden mucus along a groove into the mouth, located between proboscis and collar.
4. Water entering the mouth exits through the gill slits, while food passes into the straight gut of the trunk.
5. Waste is expelled through the anus at the rear, leaving the familiar coiled castings on tidal flats.

Worked Example

A student measures an acorn worm: proboscis 1 cm, collar 0.5 cm, trunk 8.5 cm. What fraction of the body is the trunk?

1. Total length = 1 + 0.5 + 8.5 = 10 cm
2. Trunk fraction = 8.5 / 10 = 0.85 = 85%
3. Proboscis + collar together = 1.5 / 10 = 15%

The trunk dominates the body, which is why the feeding proboscis and collar look like a small "acorn" at the front of a long worm.

Common Misconceptions

• "The stomochord is a notochord." It is only a short gut outgrowth, not the flexible rod that defines true chordates.
• "Hemichordates are chordates." Modern classification places them in their own phylum, closest to echinoderms.
• "They have a brain." There is no brain; coordination comes from a dorsal cord in the collar plus a diffuse nerve net.

Real-World Applications

• Evolutionary biologists study them as a model for chordate origins, comparing their genes with sea urchins and lancelets.
• The tornaria larva is used in developmental biology to trace how larval forms evolved.
• Their burrowing activity helps aerate marine sediments, supporting seafloor ecosystems.

Hemichordates are marine worms with a proboscis, collar and trunk, gill slits and a stomochord. Once thought to be chordates, they are now recognised as the sister group of echinoderms and a key witness to early chordate evolution.