Introduction to major clades within the echinoderms - sea urchins, brittle stars, sea lillies, sea stars. Also covers 6 major common features of the echinoderms in detail. Extra reading with sources cited.
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
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