Phylum Hemichordata – The Almost-Chordates
Imagine you are walking along a sandy beach at low tide and you notice a strange, coiled, worm-like cast on the sand. If you dig carefully, you might find the animal that made it — a soft, sluggish creature with a body divided into three distinct parts: a short, pointed snout (the proboscis), a fleshy ring behind it (the collar), and a long, wrinkled trunk. That animal is Balanoglossus, the most famous member of Phylum Hemichordata.
The name "Hemichordata" means "half-chordate." For a long time, biologists thought these animals were the missing link between non-chordates (like annelid worms) and true chordates (like fish, birds, and us). Why? Because they have a structure in the collar called the stomochord — a short, hollow rod that was once mistaken for a notochord. But the stomochord is not homologous to the notochord of chordates; it is a diverticulum of the gut. So hemichordates are not true chordates, but they share some key features with them.
The Precise Statement
Phylum Hemichordata consists of marine, worm-like animals with a tripartite body plan: proboscis, collar, and trunk. They possess a stomochord (a gut-derived rod in the collar) and pharyngeal gill slits (openings from the pharynx to the outside). They are bilaterally symmetrical, triploblastic, and coelomate. The body is soft and unsegmented, and they live in burrows or under rocks in shallow coastal waters.
The key diagnostic features of Hemichordata are:
- Tripartite body: proboscis, collar, trunk
- Pharyngeal gill slits (present at some stage)
- Stomochord (a short, hollow rod in the collar, not a true notochord)
- Open circulatory system with a dorsal heart
- Complete digestive tract (mouth and anus)
- Excretory organ: a proboscis gland (glomerulus)
Why "Half-Chordates"?
The link to chordates comes from two features:
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Pharyngeal gill slits — These are openings that connect the pharynx to the outside. In hemichordates, they are used for filter-feeding and respiration. This is a chordate hallmark, though in hemichordates the slits are not supported by gill arches.
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Dorsal nerve cord — In the collar region, the nerve cord is hollow and dorsal, similar to the chordate nerve cord. However, in the trunk it becomes a solid, ventral cord.
But the stomochord is the critical difference. In true chordates, the notochord is a flexible rod that runs the length of the body and is derived from mesoderm. The stomochord is a short, blind tube from the gut (endodermal origin) and does not function as a skeletal rod. So hemichordates are not chordates — they are a separate phylum that shares a common ancestor with chordates.
Do not confuse the stomochord with the notochord. The stomochord is not homologous to the notochord. Hemichordates are not chordates; they are a separate phylum that shows some chordate-like features.
Body Plan in Detail
| Region | Function | Key Structures |
|---|
| Proboscis | Burrowing, sensory, feeding | Proboscis gland (excretion), coelom |
| Collar | Support, nerve centre | Stomochord, dorsal nerve cord, mouth opens here |
| Trunk | Digestion, reproduction, respiration | Pharyngeal gill slits, digestive tract, gonads |
The proboscis is muscular and used to dig into sand. The collar contains the mouth and the stomochord. The trunk is long and bears the gill slits (often hundreds of pairs) and the anus at the posterior end.
Example: Balanoglossus (Acorn Worm) …