Phylum Echinodermata
Phylogeny
Highly derived clade of metazoans
7000 extant species, 13,000 species
known from rich fossil record dating
back to early Cambrian times
Extensive fossil record – Most Paleozoic echinoderms were sessile, most modern
species are mobile
Strictly marine animals, mostly benthic except for some pelagic sea cucumbers
(Rynkatropa pawsoni) that is commensal on deep-sea anglerfish
Diversified into number of life styles: predators, feed on detritus, filter-
feeders, scrape algae from rocks
Ecological Importance: as high-level predators or algal grazers – in some regions
of deep sea they compose 95% of biomass
Ambulacraria: Echinodermata + Hemichordata due to exclusive gene expression
patterns not found in chordata (Brown et al, 2008)
Deuterostomes?
- Supported by molecular data (only -5000 genes in common with protostomes
(D.melanogaster and C, elegans) – used Stronglylocentrotus purparaus (sea urchin)
genome and Reciprocal BLAST technique (source?)
- Development is stereotypical of deuterostomes: exceptions having a well-developed
coelom
- Pentaradial symmetry secondarily derived: evolutionarily and developmentally –
larvae are bilateral
Schematics
5 Well Defined Classes (obtained from 18S and 28S molecular data)
,Bauplan Features
Success of echinoderms lies partly in exploitation of radial symmetry upon a
relatively progressive coelomate architecture + mesodermally derived calcareous
endoskeleton
Tendency for radially symmetrical animals to be sessile/planktonic and to face
their environments on all sides as suspension feeders or passive predators
- Generalisation applies to those with primary radial symmetry and those secondarily
derived by way of a sessile lifestyle
- Echinoderms: unique in that they combine mobility with radial symmetry – display a
diverse array of feeding strategies + lifestyles
a) Calcareous Endoskeleton
- Calcitic skeleton composed of many ossicles
- Endoskeleton arising from mesodermal tissues and covered by epidermis
- Stereom: single calcite crystal (CaCO3 + 5% MgCO3)
- Pores are populated by dermal cells and fibres (stroma)
- 0.1% organic matrix protein
- Birefringent optical properties
- Embedded in soft tissues or fused together
- Calcium crystals deposited in a special way:
spicules refract light (plays a part in visual
system)
b) Pentaradial Body Organisation
- Central disk and 5 set of body parts
- Five-fold organisation of skeleton and most organ systems
- Body Orientation: Madreporite (opening of water vascular
system- breaks pentaradial symmetry and radii
- Pentaradial symmetry evolved from triradiate plan
(Helicoplacoids)
- Unique motility with radial symmetry
, c) Water Vascular System
- Functions: Locomotion, respiration, feeding
- Water-filled canals branching from a ring
canal
- The canals lead to podia (tube feet)
arranged along branches (ambulacra)
- Tube feet are sucker-like appendages
- Tube feet are extended and retracted by
hydraulic pressure
- Embryological origin from coelom (left mesocoel)
- Fluid similar to sea water and cells (coelomocytes) circulated by cilia
- Lack of cutaneous gas exchange methods and their lack of excretory-
osmoregulatory structures prevents terrestrial invasion
d) Hemal System/Excretory System
- Function: Circulation, Gas Exchange
- Internal transport accomplished largely by
perivisceral coeloms, augmented by various
degrees by the water vascular system and
hemal system , both derived from the coelom
- Hemal system = complex array of canals and
spaces, mostly enclosed within coelomic
channels (perihemal sinuses)
- Holothurians – bilaterally arranged and most well-
developed, Crinoids – channels form netlike plexi
- Fluid moved by cilia and muscle pumping
- Oral and aboral ring connected by axial sinus
- Axial gland produces some coelomocytes
- Extend to the gonads
- Gas exchange occurs in the dermal gills or papillae
- Excretion and ions exchange through podia and papillae
e) Nervous System and Sense Organs
- Nervous system is decentralised, somewhat diffuse and without cerebral ganglion –
three main neuronal networks:
ectoneural (oral) system, hyponeural
(deep oral) system, entoneural
(aboral) system interconnected by a
nerve net
Phylogeny
Highly derived clade of metazoans
7000 extant species, 13,000 species
known from rich fossil record dating
back to early Cambrian times
Extensive fossil record – Most Paleozoic echinoderms were sessile, most modern
species are mobile
Strictly marine animals, mostly benthic except for some pelagic sea cucumbers
(Rynkatropa pawsoni) that is commensal on deep-sea anglerfish
Diversified into number of life styles: predators, feed on detritus, filter-
feeders, scrape algae from rocks
Ecological Importance: as high-level predators or algal grazers – in some regions
of deep sea they compose 95% of biomass
Ambulacraria: Echinodermata + Hemichordata due to exclusive gene expression
patterns not found in chordata (Brown et al, 2008)
Deuterostomes?
- Supported by molecular data (only -5000 genes in common with protostomes
(D.melanogaster and C, elegans) – used Stronglylocentrotus purparaus (sea urchin)
genome and Reciprocal BLAST technique (source?)
- Development is stereotypical of deuterostomes: exceptions having a well-developed
coelom
- Pentaradial symmetry secondarily derived: evolutionarily and developmentally –
larvae are bilateral
Schematics
5 Well Defined Classes (obtained from 18S and 28S molecular data)
,Bauplan Features
Success of echinoderms lies partly in exploitation of radial symmetry upon a
relatively progressive coelomate architecture + mesodermally derived calcareous
endoskeleton
Tendency for radially symmetrical animals to be sessile/planktonic and to face
their environments on all sides as suspension feeders or passive predators
- Generalisation applies to those with primary radial symmetry and those secondarily
derived by way of a sessile lifestyle
- Echinoderms: unique in that they combine mobility with radial symmetry – display a
diverse array of feeding strategies + lifestyles
a) Calcareous Endoskeleton
- Calcitic skeleton composed of many ossicles
- Endoskeleton arising from mesodermal tissues and covered by epidermis
- Stereom: single calcite crystal (CaCO3 + 5% MgCO3)
- Pores are populated by dermal cells and fibres (stroma)
- 0.1% organic matrix protein
- Birefringent optical properties
- Embedded in soft tissues or fused together
- Calcium crystals deposited in a special way:
spicules refract light (plays a part in visual
system)
b) Pentaradial Body Organisation
- Central disk and 5 set of body parts
- Five-fold organisation of skeleton and most organ systems
- Body Orientation: Madreporite (opening of water vascular
system- breaks pentaradial symmetry and radii
- Pentaradial symmetry evolved from triradiate plan
(Helicoplacoids)
- Unique motility with radial symmetry
, c) Water Vascular System
- Functions: Locomotion, respiration, feeding
- Water-filled canals branching from a ring
canal
- The canals lead to podia (tube feet)
arranged along branches (ambulacra)
- Tube feet are sucker-like appendages
- Tube feet are extended and retracted by
hydraulic pressure
- Embryological origin from coelom (left mesocoel)
- Fluid similar to sea water and cells (coelomocytes) circulated by cilia
- Lack of cutaneous gas exchange methods and their lack of excretory-
osmoregulatory structures prevents terrestrial invasion
d) Hemal System/Excretory System
- Function: Circulation, Gas Exchange
- Internal transport accomplished largely by
perivisceral coeloms, augmented by various
degrees by the water vascular system and
hemal system , both derived from the coelom
- Hemal system = complex array of canals and
spaces, mostly enclosed within coelomic
channels (perihemal sinuses)
- Holothurians – bilaterally arranged and most well-
developed, Crinoids – channels form netlike plexi
- Fluid moved by cilia and muscle pumping
- Oral and aboral ring connected by axial sinus
- Axial gland produces some coelomocytes
- Extend to the gonads
- Gas exchange occurs in the dermal gills or papillae
- Excretion and ions exchange through podia and papillae
e) Nervous System and Sense Organs
- Nervous system is decentralised, somewhat diffuse and without cerebral ganglion –
three main neuronal networks:
ectoneural (oral) system, hyponeural
(deep oral) system, entoneural
(aboral) system interconnected by a
nerve net
