In this article we will discuss about the classification of phylum echinodermata.
Classification of Phylum Echinodermata:
1. The echinoderms are non-colonial, coelomate, marine animals, the larval bilateral symmetry of which is replaced by radial symmetry in the adult.
2. The surface is covered with an exoskeleton of calcareous plates or ossicles, which usually support a system of movable or immovable calcareous spines.
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3. A large body cavity or coelom, and a part of the coelom (hydrocoel) is converted into a system of vessels, containing water. The hydrocoel is a multipurpose organ- system, capable of meeting the needs of respiration, locomotion, nutrition, sensory perception, or a combination of these. The alimentary, nervous and vascular systems are well-developed.
4. A characteristic system of vessels, the ambulacral system is connected with locomotion and other functions.
5. In most cases, locomotors organs are tube feet.
6. Almost all the systems exhibit radial symmetry.
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7. Abundant amoebocytes are present in the coelom, haemal vessels and water vessels.
8. Reproduction sexual; free-swimming, larvae are bipinnaria, brachiolaria, pluteus, auricularia and doliolaria.
The phylum Echinodermata has been divided into four subphyla:
Subphylum I. Echinozoa:
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1. Globoid echinoderms that never develop arms.
2. Mouth and anus at opposite poles of the body; in some, due to secondary adaptation for feeding these organs may be displaced.
3. Hydrocoel forms a ring around the mouth, giving rise to meridional water vessels traversing the body wall in the direction of anus.
4. Skeleton, nervous system, reproductive organs and muscular system tend to differentiate in meridional pattern.
The subphylum comprises five classes of which only two are represented by surviving members:
Class 1. Helicoplacoidea (extinct):
Lower Cambrian of California
Example: Helicoplacus.
Class 2. Holothuroidea:
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1. Free-living, elongated in the adoralaboral axis, through which passes a dorsoventral plane of bilateral symmetry containing hydro pores and gonoduct; one end (anterior) is thicker containing the mouth opening and the opposite end with anal opening.
2. The body is usually five-sided and along each side there extends a double row of suctorial tube feet.
3. No conspicuous external calcareous plates, but the integument is tolerably hard due to the presence of innumerable microscopic calcareous spicules.
4. Surrounding the mouth a circlet of tree-like tentacles present.
5. Madreporite situated in the interior of coelom.
6. A pair of tubular respiratory trees open into the cloaca; besides serving respiration and excretion of waste materials, these organs also have a hydrostatic function.
7. The sexes are separate; ovaries and testes are similar and consist of bunches of tubular follicles which opens on the dorsal surface, at a short distance behind the oral end.
About 500 living species; Ordovician to recent.
Subclass i. Dendrochirotacea:
1. Pharyngeal retractor muscles connecting the anterior part of the body wall with calcareous ring and capable of retracting it within the oral aperture.
2. Tentacles usually 10 (up to 30), with tree-like branches, (dendroid).
3. Tube feet many.
4. Respiratory trees present.
Examples: Cucumaria, Psolus, Thy one, etc.
Subclass ii. Aspidochirotacea:
1. Pharyngeal retractor muscles absent.
2. The oral tentacles usually 20 (15 to 30), each branched from a shield-shaped central stalk.
3. Locomotors tube feet normally present on the ventral side and those on the dorsal side usually converted into sensory organs.
4. Respiratory trees present.
Examples: Holothuria, Stichopus, Elpidia, Bathyplotes, etc.
Subclass iii. Apodacea:
1. Pharyngeal retractor muscle absent.
2. Test vestigeal, reduced to spicules scattered in the dermis.
3. Tentacles digitate or simple.
4. Tube feet reduced or lacking altogether.
Examples: Molpadia, Leptosynapta, Synapta, Chiridota, etc.
Class 3. Edrieasteroidea (Extinct):
Evolved in mid-Cambrian period and became extinct before the close of the Carboniferous.
Example: Isorophus.
Class 4. Echinoidea:
1. Free-living echinozoans with well-developed test, composed of meridionally arranged columns of plates.
2. Ambulacral meridians are correlated with the water vascular system (ambulacral plates), and intervening segments comprise inter-ambulacral plates.
3. Tube feet meridionally arranged, primitively suctorial and ampullate.
4. Outer surface of shell plate bears tubercles with which articulate movable spines of varying thickness and length.
5. Oral surface bears the central mouth surrounded by a membranous peristome.
6. Aboral surface bears a well-developed system of apical plates, with anus surrounded by a membranous periproct.
7. Madreporite lies close to anus; pedicellariae are stalked and 3-jawed.
The class includes about 850 living and 5,000 fossil species.
1. Subclass i. Perischoechinoidea:
1. Echinoids usually with a flexible test, showing marked instability in the number of columns of plates comprising the ambulacrals and the inter-ambulacrals.
2. Well-defined ambulacral system, test plates present.
Examples: Eotliuria, Melon echinus, Archaeocidaris, Goniocidaris, Ogmocidaris, etc.
Subclass ii. Enechinoidea:
1. Enormous variety of body forms, spherical, oval, disc-like, or heart-shaped, bilaterally symmetrical.
2. Lantern of Aristotle modified differently.
Examples: Echinus, Laganum, Echinosigra, Echinocarllum, Spatangus, etc.
Class 5. Gphiocistioidea:
Extinct. Ordvician to Devonian period.
Example: Volchovia.
Subphylum II. Homalozoa:
Extinct. Mid-Cambrian to Devonian.
Example: Enoploura.
Subphylum III. Grinozoa:
1. Fundamentally globoid echinoderms with a partial meridional symmetry tending to produce an aboral cup-shaped body (calyx) on which the ambulacra are restricted.
2. Usually fixed with a stalk composed of a row or rows of ossicles.
3. The mouth is on the free surface, near or in the centre, and the anus usually on the oral surface.
4. A system of narrow ciliated ambulacral grooves, functioning as food grooves, extends out radially from the mouth on the oral surface.
5. Tube feet, when present, are smaller, ciliated and without suckers.
Class 1. Crinoidea:
1. Calyx more or less globular, cap-like, strongly pentamerous, with oral surface directed upwards and aboral surface downward.
2. Mouth usually lies near the centre; anus also present orally, being situated on an anal tube.
3. The five arms—each usually subdivide into one or more forks, producing from ten to thirty branches or, in extreme cases, up to 200; in existing crinoids. The arms usually bears lateral appendages, the pinnules.
4. The stem is attached to the aboral pole of the calyx; it may be long and slender with root-like organs attaching it to the sea floor. Feather-stars discard the stem early in life, assuming a free-living mode of life.
5. The sexes are separate but seldom differ in external features. Gonads usually lie in the dilated bases of pinnules; development with metamorphosis; developing into a barrel-shaped, free-swimming doliolaria larva.
6. Madreporite, spines and pedicellariae absent.
7. Crinoidea includes about 630 existing and 5,000 extinct species.
Subclass i. Inadunata:
Extinct. Ranging from upper Cambrian to Permian.
Examples: Dendrocrinus, Anartiocrinus, etc.
Subclass ii. Camerata:
Extinct. Palaeozoic stalked genera, ranging from Ordovician to Permian period.
Example: Xenocrinus.
Subclass iii. Flexibilia:
Extinct. Palaeozoic stalked genera, ranging from Ordovician to Permian times.
Example: Forbesiocrinus.
Subclass iv. Articulata:
1. Calyx pentamerous, flexible, fundamentally dicyclic.
2. The uniserial arms bear pinnules and are usually branched; the arm retains its movable articulation with the radial plate, despite the incorporation of the lower brachial ossicles into the calyx.
3. In some forms stalk exist with nodal ring of cirri (Metacrinus) and some discard the stem while young and thereafter remain free-swimming (feather stars).
4. Mouth and ambulacral grooves remain exposed.
Examples: Antedon, Metacrinus, Comatula, Rhizocrinus, etc.
Subphylum IV. Asterozoa:
1. Radially symmetrical, free-living echinoderm with a star-shaped body, without stem.
2. The arms (usually five) develop after metamorphosis by the outward growth of the radial water vessels in the horizontal plane, carrying the perivisceral coelom and body wall with them. The gut sends caeca into the arms.
3. The nervous system is external, in the floor of the radial grooves, carried on the adorsal surface of the arms.
4. Respiration and excretion by dermal gills called papulae.
5. Pedicellariae possess movable, pincer- like spines.
6. From peristome or mouth, an open, narrow and median ambulacral groove runs along the oral surface of each arm to its tip.
7. Sexes usually separate; testes and ovaries are alike and radially arranged. The larva is either bipinnaria or a brachiolaria.
Class i. Stelleroidea:
1. Open ambulacral groove, carried in long slender arms arising from a small central disc.
Subclass i. Somasteroidea:
Exhibit characters intermediate between crinoids and other asterozoans.
1. Body flattened; the arm skeleton exhibits the pinnate structure of a biserial crinoid; the arms are petaloid, contracted at the bases and the disc is small.
2. The gut is blind, no anus; madreporite marginal; tube feet small, without suckers.
Examples: Chinianaster, Ampullaster, etc.
Subclass ii. Asteroidea:
1. Body strongly flattened, star-shaped, pentagonal, with 5-40 simple un-branched long or short triangular arms not sharply demarcated from the central disc.
2. Mouth opens in the centre of the oral surface surrounded by a membranous peristome and guarded by spines.
3. A small eccentric anus is situated on the aboral surface and adjacent to it, in an inter-radius. The madreporite or sieve plate is present.
4. Arms are hollow, each containing a prolongation of coelom and viscera; running along the radial midline in the ambulacral furrow, is the radial ambulacral vessel, from which short side branches open into the bases of the tube feet.
5. About 1,700 living and 300 fossil species; the largest starfish, Linckia, is one metre in diameter.
Examples: Asterias, Luidea, Solaster, Linckia, Astropecten, Ctenodiscus, Pentagonaster, Astrostole, etc.
Subclass iii. Ophiuroidea:
1. Body stellate and flattened with distinct oral-aboral surfaces.
2. Arms usually five, sharply demarcated from the body disc; slender, jointed, solid, cylindrical, muscular, flexible, rarely branched, and useful for clambering and wriggling; autotomy of arms is frequent and regeneration occurs rapidly, so those are called brittle star.
3. Ambulacral grooves are not covered with ossicles, forming epineural canals.
4. Visceral organs do not extend into the arms. Arms contain muscle skeleton, branches of circulatory canals, nerves and a narrow prolongation of coelom.
5. Alimentary canal is confined to the disc and terminates blindly; anus and caeca absent.
6. Pedicellariae, skin gills and special sense organs absent.
7. Madreporite opens on the oral surface.
8. Tube feet arise ventrolateral from arms in two rows, usually very small, have no ampullae and terminal suckers, and not locomotory.
9. Gonads lie inter-radially in pairs in the central disc; do not open directly to the exterior but open into genital bursae, one of which opens on each side at the base of each arm on the oral surface.
10. Sexis separate; the free-swimming larva is a pluteus.
11. There are about 1,800 living and 180 fossil species; large species may have a disc 10 cm across and an arm-spread of 50 cm. In the smallest species the disc and arm together may be no more than a few millimeters.
Examples: Ophiura, Astrothorax, Astrophyton, Asteronyx, Gorgonocephalus, etc..