Phylum Hemichordata — Core Principles

Core Principles

Phylum Hemichordata comprises marine, worm-like animals characterized by a unique tripartite body plan: proboscis, collar, and trunk. They are bilaterally symmetrical and possess an organ-system level of organization.

A key feature is the presence of a stomochord, an anterior outgrowth of the buccal cavity, which provides some support but is not homologous to the true notochord of chordates. They also exhibit pharyngeal gill slits, crucial for filter feeding and respiration, and a dorsal nerve cord (hollow in the collar region).

The circulatory system is open, and excretion occurs via a glomerulus in the proboscis. Reproduction is sexual, often with external fertilization and development through a free-swimming tornaria larva, which shares striking similarities with echinoderm larvae, highlighting a close evolutionary link.

Hemichordates are deuterostomes, bridging the evolutionary gap between invertebrates and chordates, and are exemplified by acorn worms like *Balanoglossus*.

Important Differences

vs Phylum Chordata

Aspect	This Topic	Phylum Chordata
Notochord/Stomochord	Possess a stomochord (buccal diverticulum, endodermal, non-homologous to notochord).	Possess a true notochord (mesodermal rod, primary axial support).
Nerve Cord	Dorsal nerve cord is hollow only in collar region; ventral nerve cord is solid. Subepidermal nerve plexus.	Possess a single, dorsal, hollow nerve cord throughout body length.
Post-anal Tail	Absent.	Present at some stage of life cycle.
Body Plan	Tripartite body (proboscis, collar, trunk).	Usually bipartite (head, trunk) or tripartite (head, trunk, tail) but not with distinct proboscis/collar.
Circulatory System	Open type.	Closed type (except for some primitive chordates like tunicates).
Coelom	Enterocoelous, tripartite.	Enterocoelous (in primitive forms) or schizocoelous (in advanced forms).

Hemichordates differ from Chordates primarily in the nature of their axial support structure (stomochord vs. notochord), the complete absence of a post-anal tail, and differences in their nerve cord structure and circulatory system. While both share pharyngeal gill slits, the fundamental distinctions in these key chordate features necessitate their classification into separate phyla. Hemichordates represent an evolutionary branch that shares a common deuterostome ancestor with chordates but diverged before the full suite of chordate characteristics evolved.