CHARACTERS AND AFFINITIES OF HEMICHORDATES.

                   

 

 Definition and Introduction

Hemichordates is an independent phylum of marine invertebrates closely related to Echinodermata. They were previously classified as a subphylum of Chordata.

Members exhibit the following key features: 

- Enterocoelous coelom: Coelom develops through enterocoely in three compartments (protocoel, mesocoel, metacoel). 

- Pharyngeal gill slits: Used for filter feeding. 

- Stomochord: A buccal diverticulum, initially mistaken for a notochord. 

- Body division: Vermiform structure with three regions – proboscis, collar, trunk. 

 

Examples include Balanoglossus, Saccoglossus, and Glossobalanus, classified under Class Enteropneusta.

Affinities with Chordata

Similarities to chordates include: 

- Pharyngeal gill slits

- Dorsal hollow nerve cord in some species 

 

Objections to Inclusion in Chordata: 

Stomochord vs Notochord: Hemichordate stomochord is short, hollow, and lacks support functions. 

 Nervous System: Intraepidermal with a ventral nerve cord and circumcentric nerve ring, unlike chordates. 

Gill Slits: Numerous and dorsal, whereas chordates have fewer, lateral gill slits. 

Affinities with Echinodermata

Hemichordates share similarities with echinoderms: 

- Larval resemblance: Pelagic, transparent larvae with similar coelom development. 

- Enterocoelous coelom: Three-part coelom used for hydraulic function. 

- Shared structures: Heart vesicle and glomerulus in hemichordates are homologous to the dorsal sac and axial gland in echinoderms. 

- Regeneration ability: and similar ecological habits. 

 

Differences: Echinoderm larvae lack sensory hair plates, apical plates, and telotroch structures. 

Systemic Position and Current Understanding

- Hemichordates are grouped with Echinodermata in the clade Ambulacraria based on molecular phylogeny. 

- Gene expression studies support shared ancestry with chordates via pharyngeal gill slits. 

- Hemichordates play a pivotal role in understanding the evolutionary origin of chordates and relationships among deuterostome phyla (Chordata,Echinodermata, Hemichordata).