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The phylum Echinodermata comprises a diverse group of marine invertebrates that are almost all benthic, living on the sea floor and adapted to a range of environments, from the inter-tidal zone to deep ocean trenches. Echinoderms are typically characterised by a mesodermal skeleton made of calcite plates (each with a specialised mesh or 'stereom' structure) and arranged in the adult with pentameral or five-rayed symmetry. A 'water-vascular system' of fluid-containing tubes transports nutrients and operates hydrostatic 'tube feet' for locomotion and respiration. Echinoderm larvae are bilaterally symmetrical and undergo complex transformations (including rotation and vertical stacking of coleom compartments) to produce the final adult form, with characteristic pentamery. Living echinoderm classes include the crinoids (sea lilies), asteroids (starfish), ophiuroids (brittlestars), echinoids (sea urchins) and holothurians (sea cucumbers). The rich fossil record of echinoderms begins in the Cambrian, with the main groups alive today clearly distinguishable by the Ordovician, along with extinct sub-phyla such as the stalked blastozoans and asymmetrical homalozoans or 'carpoids', which are evidently stem-group echinoderms.
Major echinoderm groups
Crinoids are considered to be the most primitive of living echinoderms and like the extinct blastozoans have a fixed or 'pelmatozoan' mode of life, attached to the sea floor by a stalk and occuring mostly in deep ocean waters. A cup or 'theca' of calcite plates arises from the stalk and branches into several arms or 'brachia' each bearing projecting pinnules to make an efficient filtration fan. The mouth and anus are located on the roof or 'tegmen' of the cup, surrounded by the arms. The comatulids or feather stars (e.g. Antedon and Oligometra) are an exception to the typical fixed form, as although juveniles are stalked, adults replace the stalk with short hooks or 'cirri' and can swim.
Asteroids are well known for having five arms with a mouth at the centre of its underside. Gonads extend into the arms, as does the gut, which is usually blind-ending. Slightly more advanced but also stellate in form are the ophiuroids, with five thin arms strengthened by a core of cylindrical ambulacral plates. Whereas starfish have a radial nerve located externally in the epithelium, in ophiuroids and more derived echinoderms the radial nerve and water-vascular vessels are covered by an epithelial flap, creating an 'epineural sinus'.
Echinoids and holothurians are the most derived echinoderms. Echinoids are often globular though may be flattened or heart-shaped (and bilaterally symmetrical) depending on the environment to which they are adapted. The stereom skeleton is composed of alternating areas of ambulacral and interambulacral plates, corresponding to the oral or 'axial' under-surface of asteroids and ophiuroids. The mouth region or 'peristome' bears a jaw apparatus ('Aristotle's lantern') with five calcite teeth supported by a perignathic girdle, and where interambulacral areas abut the peristome, feathered gills project. The apical disc or aboral region comprises a central anus or periproct surrounded by genital plates with pores for gamete release and a large 'madreporite' plate, directly connected to the water-vascular system. Holothurians are cylindrical or worm-like echinozoans with a tough collagenous skin and calcite plates generally reduced to microscopic spicules or platelets embedded in the skin. The mouth and anus are at opposite ends, and the mouth is surrounded by feeding tentacles (enlarged tube feet) and supported within by a 'perioral ring' of calcite ossicles.
Echinoderms in the tree of life
Echinoderms fall within a large group called the deuterostomes, and evidence from molecules and morphology indicates that they are the sister group of the hemichordates, forming a clade called the 'Ambulacraria'. The phylum Hemichordata includes enteropneusts (acorn worms), pterobranchs and the extinct graptolites, and although superficially very different these animals share several key features with echinoderms. Both have larvae with a 'trimeric' organisation of coelom types and adult metanephridia (or kidneys) composed of a homologous 'axial complex' with vascular fluid being pumped and filtered across podocytes before expulsion through a hydropore. It is also very likely that early Palaeozoic echinoderms shared the larval hemichordate feature of pharyngeal gill slits. Molecular support for the relationship comes from unique sialic acids, posterior Hox9-13 gene complex and phylogenies based on 18S rRNA, 28S rRNA sequences and mitochondrial gene order. Echinoderms have an intriguing place in the tree of life, as their association with hemichordates places them close to the urochordates (sea squirts), cephalochordates (lancelets) and vertebrates, to which we ourselves belong.
Echinoderms and convergent evolution
The excellent fossil record of echinoderms has assisted in revealing frequent convergence within the phylum, as forms or specific structures have re-evolved independently over evolutionary time. One case is the repeated appearance of paired pores in echinoids (sea urchins) and a group of blastozoans called diploporite cystoids, and another relates to elongated holes or 'lunulate perforations' in flattened echinoids called sand dollars. Lunules evolved several times in different Eocene sand dollars, possibly as an adaptation for stability in their typically turbulent sandy habitat. However, the evolution in echinoderms of features, from defensive spines to eye lenses, that converge on those of far more distantly related groups is even more striking.
Arrays of calcite micro-lenses that converge incredibly on those in the trilobite compound eye have been reported in the light-sensitive brittlestar Ophiocoma wendtii and some starfish and sea urchins. Ophiocoma lens arrays are located on the dorsal arm plates, and each lens is corrected for spherical aberration, underlain by bundles of nerves fibres and in daylight protected by chromatophore pigments. Although each lens array cannot function like a single compound eye due to calcite crystal orientation, combined information from all the arrays provides enough resolution for rapid movement towards shade and away from predators. Other brittlestars (e.g. Ophioderma brevispinum, Ophiura ophiura) appear able to detect damaging polarised light and the shade of passing predators or shelter, for example by using polarised calcite ossicles and photosensitive dermal neurons. Sea urchins provide an astonishing window into convergence on the compound eye. Experiments with species of Echinometra and Strongylocentrotus that seek safety from predators in shaded crevices have shown that their spines restrict received light to focussed areas of dermal photoreceptors. This results in limited spatial vision of up to 10 degrees acuity as the whole spiny body functions like a single compound eye.
In echinoids, different types of larvae occur, from primitively free-swimming or 'pelagic' feeders (planktotrophic), to pelagic larvae that feed on stored reserves (lecithotrophic) and finally brooding, where the adults protect larvae that undergo direct development. The developmental change from planktotrophic to lecithotrophic larvae has occurred in many lineages and interestingly, brood protection has evolved independently in at least seven echinoid orders and about 20% of species. Outside the echinoderms, parental care in the form of brood protection in a cavity or pouch is known in certain fish, amphibians, snails, brachiopods and bryozoans among others, highlighting the strong adaptive value of this convergent reproductive strategy.
Echinoderm tube feet are soft structures lined with longitudinal muscle and controlled hydrostatically from an inflation sac or 'ampulla' beneath the pore pair from which each arises. The tip of each tube forms a prehensile disc, useful in locomotion and reminiscent of the smooth adhesive pads of certain insects and frogs.