1. Phylum Hemichordata (85 species)
Class Pterobranchia (Sea Angels)
Class Enteropneusta (acorn worms)
Small group of marine worms that show
similarities
to Echinoderms and Chordates
2. Tornaria of Acorn Worms Auricularia of
Sea Cucumbers
Has a deuterostome embryology and resembles in ciliation
feeding mechanism (opposed band) and general appearance an
auricularia larva of sea cucumbers
Also shares with echinoderms:
- Adult axial complex similarity (hemal –excretory system)
- Tripartite coelom
3. Adult Characteristics of Acorn Worms
3 partite coelom like echinoderms
Proboscis for digging (protocoel)
Collar with mouth (mesocoel)
Stomochord first considered a
notochord
Main body is the trunk (metacoel)
Up to 100 pair of pharyngeal slits
N.S. An epidermal net…. Like the
echinoderm nervous system but..
in some a dorsal hollow nerve cord
4. Ch Hemi Echino C H E
Conflicting views of Deuterostome phylogeny
Pharynx
DNC
larvae
Axial
complex
From Bromham and Degnan, 1999
Traditional Hemi + echino
Proto”D” Proto”D”
5. C E H
Conflicting views of Deuterostome phylogeny
From Bromham and Degnan, 1999
Proto”D”
Hemichordate
Echinoderms
Fossil
Stylophoran
(Calcichordate)
6. C H E C H E
Conflicting views of Deuterostome phylogeny
C E H
Pharynx
DNC
larvae
Axial
complex
Fossil
stylophoran
From Bromham and Degnan, 1999
Traditional Hemi + echino Calcichordate
-- Mitochondrial and 18s r-RNA sequence analysis strongly supports the
Hemichordate-echinoderm (lineage Ambulacraria) relationship to the
exclusion of the chordate clade.
Proto”D” Proto”D” Proto”D”
7. C H E
Conflicting views of Deuterostome phylogeny
larvae
Axial
complex
From Bromham and Degnan, 1999
Hemi + echino (Ambulacraria)
-- Hemichordates, Fossil
Stylophorans, and
Chordates all have
pharyngeal slits and a
dorasal nerve chord, but
Echinoderms do not, yet
Hemichordates are the
sister group of Echinoderms
and not of Chordates, and
Stylophorans are more
related to echinoderms
then they are to chordates.
Proto”D”
How is this possible??
