Phylum Hemichordata — Revision Notes

⚡ 30-Second Revision

Habitat: — Exclusively marine.
Body Plan: — Tripartite (Proboscis, Collar, Trunk).
Skeletal Support: — Stomochord (buccal diverticulum, endodermal, NOT notochord).
Respiration/Feeding: — Pharyngeal gill slits (filter feeding).
Circulation: — Open type, dorsal heart.
Excretion: — Glomerulus (proboscis gland).
Nervous System: — Dorsal nerve cord (hollow in collar), ventral nerve cord.
Development: — Indirect, via Tornaria larva (resembles echinoderm larva).
Examples: — *Balanoglossus* (Enteropneusta), *Rhabdopleura* (Pterobranchia).
Phylogeny: — Deuterostomes, link between Echinodermata and Chordata.

2-Minute Revision

Phylum Hemichordata, meaning 'half-chordates,' comprises exclusively marine, worm-like animals. Their body is distinctly divided into three regions: an anterior proboscis, a short collar, and a long trunk.

A key feature is the stomochord, an endodermal outgrowth of the buccal cavity, which provides some support but is fundamentally different from the mesodermal notochord of true chordates. They possess pharyngeal gill slits for filter feeding and respiration, a characteristic shared with chordates.

The circulatory system is open, and excretion is carried out by a glomerulus located in the proboscis. Their nervous system includes both dorsal (hollow in collar) and ventral nerve cords. Reproduction is sexual, with external fertilization, and development often involves a free-swimming tornaria larva.

This larva's striking resemblance to echinoderm larvae highlights a close evolutionary relationship between Hemichordates and Echinoderms, placing both in the Deuterostomia clade. Examples include *Balanoglossus* (acorn worm) and *Rhabdopleura*.

5-Minute Revision

Phylum Hemichordata represents a small but evolutionarily significant group of exclusively marine invertebrates, often referred to as 'acorn worms' due to the shape of their proboscis in some species. Their body exhibits a unique tripartite division into a muscular proboscis, a short collar, and a long trunk. This distinct body plan is a primary identifier.

A crucial structure is the stomochord, which is a hollow diverticulum (outgrowth) from the roof of the buccal cavity, extending into the proboscis. It provides some support but is endodermal in origin and NOT homologous to the mesodermal notochord of Chordates.

This distinction is vital for NEET. Hemichordates also possess pharyngeal gill slits in the trunk region, which are used for filter feeding and gas exchange, a feature that initially led to their erroneous classification under Chordata.

Their circulatory system is open, featuring a dorsal heart. Excretion is performed by a specialized glomerulus (proboscis gland) located in the proboscis. The nervous system consists of a subepidermal nerve plexus, with a dorsal nerve cord (hollow only in the collar) and a ventral nerve cord.

Sexes are separate, and fertilization is external. Development is indirect, involving a free-swimming tornaria larva. This larva is remarkably similar to the bipinnaria larva of echinoderms, providing strong evidence for a shared evolutionary ancestry between Hemichordata and Echinodermata, both belonging to the Deuterostomia clade.

Key Examples:

Class Enteropneusta: — Solitary, burrowing forms like *Balanoglossus* (acorn worm).
Class Pterobranchia: — Colonial, sessile forms living in tubes, such as *Rhabdopleura*.

NEET Focus: Remember the stomochord vs. notochord difference, the tripartite body, pharyngeal gill slits, tornaria larva, and the examples. These are frequently tested points.

Prelims Revision Notes

Phylum Hemichordata: NEET Revision Notes

1. General Characteristics:

* Habitat: Exclusively marine. * Symmetry: Bilateral. * Organization: Organ-system level. * Germ Layers: Triploblastic. * Coelom: True coelom (eucoelomate), enterocoelous origin. * Body Plan: Soft, unsegmented, worm-like, distinctly tripartite.

2. Tripartite Body Division:

* Proboscis: Anterior-most, muscular, used for burrowing and food collection. Contains glomerulus. * Collar: Short, cylindrical, bears mouth ventrally. Dorsal nerve cord is hollow here. * Trunk: Longest, posterior part, contains most organs, including pharyngeal gill slits.

3. Key Structures & Functions:

* Stomochord: Hollow outgrowth from buccal cavity roof into proboscis. Endodermal origin. Provides support. Crucially, NOT homologous to notochord. * Pharyngeal Gill Slits: Paired openings in pharynx, for filter feeding and respiration.

Present in trunk. * Circulatory System: Open type. Includes dorsal heart and two main longitudinal vessels. * Excretory System: Single glomerulus (proboscis gland) in the proboscis, filters blood.

* Nervous System: Subepidermal nerve plexus. Dorsal nerve cord (hollow in collar), ventral nerve cord (solid). * Digestive System: Complete, U-shaped in Pterobranchia, straight in Enteropneusta.

4. Reproduction & Development:

* Sexes: Usually separate (dioecious). * Fertilization: External. * Development: Indirect, involving a free-swimming tornaria larva. * Tornaria Larva Significance: Resembles bipinnaria/dipleurula larvae of Echinodermata, indicating close evolutionary link.

5. Classification & Examples:

* Class Enteropneusta (Acorn Worms): Solitary, burrowing. E.g., *Balanoglossus*, *Saccoglossus*. * Class Pterobranchia: Colonial, sessile, live in tubes, U-shaped gut, ciliated tentacles. E.g., *Rhabdopleura*, *Cephalodiscus*.

6. Evolutionary Significance:

* Deuterostomes (like Echinoderms and Chordates). * Considered a 'connecting link' between non-chordates and chordates due to shared features (gill slits, dorsal nerve cord) but distinct from true chordates (stomochord vs. notochord).

7. Common Misconceptions to Avoid:

* Do NOT confuse stomochord with notochord. * Hemichordates are a separate phylum, not a subphylum of Chordata. * Circulatory system is open, not closed.

Vyyuha Quick Recall

To remember Hemichordata's key features, think: Half-Chordate Triple Story Gill Open Tornado.

Half-Chordate: Reminds you of the phylum name and its historical context.
Triple: For the Tripartite body plan (Proboscis, Collar, Trunk).
Story: For Stomochord (a 'story' of a notochord, but not the real one!).
Gill: For Gill slits (pharyngeal).
Open: For Open circulatory system.
Tornado: For Tornaria larva (looks like a tiny tornado, and links to Echinodermata's larval similarity).