Echinodermata: General Characters and Classification
GENERAL CHARACTERS
1. Organ-system grade of body organization.
2. Triploblastic, coelomate and radially symmetrical; often pentamerous
3. Body unsegmented with globular, star-like. spherical, discoidal or elongated shape.
4. Head absent; body suface is marked by five symmetrically radiating areas (ambulacra) and five alternating interradii (inter-ambulacra).
5. Endoskeleton of dermal calcareous ossicles with spines, covered by the epidermis.
6. Water-vascular system of coelomic origin,including podia or tube feet for locomotion and usually with a madreporite.
7. Coelom of enterocoelous type constituting the perivisceral cavity and cavity of the water-vascular system; coelomic fluid with coelomocytes.
8. Alimentary canal straight or coiled.
9. Vascular system a haemal system, enclosed in coelomic perihaemal channels. 10. Respiratory organs include dermal branchiae, tube feet, respiratory tree and bursae.
11. Nervous system without a brain and with a circumoral ring and radial nerves.
12. Poorly developed sense organs include tactile organs, chemoreceptors, terminal tentacles, photoreceptors and statocysts.
13. No excretory organs.
14. Usually dioecious, gonads large and single or multiple; fertilization external; develop ment indirect through free-swimming larval forms.
15. Regeneration of lost parts a peculiarity. 16. Exclusively marine.
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CLASSIFICATION
Subphylum -I .Eleutherozoa
(Gr., eleutheros, free+zoios, animal) Free-living echinoderms.
Class I. Asteroidea
(Gr., aster, star+eidos, form)
1. Starfishes or sea stars.
2. Arms 5 or more and not sharply marked off from the central disc.
3. Tube feet in orally placed ambulacral grooves; with suckers.
4. Anus and madreporite aboral. 5. Pedicellariae present.
6. Free-living, slow-creeping, predaceous and scavengerous.
Subclass 1. Somasteroidea
Fossil Palaeozoic sea stars.
Platasterias latiradiata is the only living species.
Subclass 2. Euasteroidea
Living sea stars.
Order 1.. Phanerozonja
1. Body with marginal plates and usually with papulae, on aboral surface
2. Pedicellariae sessile, not crossed.
3. Tube feet without suckers.
4. Mostly burrowers in soft bottom.
Examples: Astropecten, Luidia, Goniaster, Oreaster (=Pentaceros).
Order 2. Spinulosa
1. Usually without conspicuous marginal plates and with papulae on both surfaces. 2. Pedicellariae rare.
3. Tube feet with suckers.
4. Aboral surface with low spines. Examples: Asterina, Solaster, Pieraster,Echinaster.
Order 3. Forcipulata
1. No conspicuous marginal plates.
2. Pedicellariae pedunculate and straight or crossed type.
3. Four rows of tube feet.
Examples: Asterias, Heliaster.
Class 2. Ophiuroidea
(Gr., ophis, snake+oura, tail+eidos, form)
1. Brittle-stars and allies.
2. Body star-like with arms sharply marked off from the central disc.
3. Pedicellariae absent.
4. Stomach sac-like; no anus.
5. Ambulacral grooves absent or covered by ossicles; tube feet without suckers.
6. Madreporite oral.
Order 1. Ophiurae
1. Brittle and serpent stars.
2. Small and five-armed.
3. Arms move transversely.
4. Disc and arms usually covered with plates.
Examples: Ophiura, Ophiothrix, Ophioderma, Ophlopholis.
Order 2. Euryalae.
1. Arms simple or branched.
2. Arms move vertically.
3. Disc and arms covered by soft skin.
Examples: Gorgonocephalus (basket star). Asteronyx.
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Class 3. Echinoidea
(Gr., echinos, hedgehog +eidos, form)
1. Sea urchins and dollars.
2. Body discoid, oval or semi-spherical and without arms.
3. Skeleton or test compact bearing movable spines and three-jawed pedicellariae.
4. Chewing apparatus or Aristotle's lantern with teeth.
5. Ambulacral grooves covered by ossicles; tube feet with suckers.
6. Gonads usually five or less.
Subclass 1. Bothriociduroida
1. A single row of plates in each inter ambulacral area.
2. Without typical lantern.
3. Madreporite radial.
Example: Single extinct Ordovician genus Bothriocidaris.
Subclass 2. Regularia
1. Body globular, pentamerous, with two rows of inter-ambulacral plates in existing members.
2. Mouth central.
3. Aristotle's lantern well developed.
4. Anus central on aboral surface with well developed apical plates.
5. Madreporite oral.
Order 1. Lepidocentroida
1. Test flexible with overlapping plates.
2. Ambulacral plates extend up to mouth lip. 3. Inter-ambulacral plates in more than two rows in extinct forms.
Example: Palaeodiscus.
Order 2. Melonechinoida
1. Test spherical and rigid.
2. Ambulacral plates continue to mouth lip. 3. Inter-ambulacral plates in four or more rows.
4. Wholly extinct, Carboniferous. Example: Melonechinus.
Order 3. Cidaroida
1. Test globular and rigid.
2. Two rows of long narrow ambulacral plates and two rows of inter-aubulacral plates.
3. No peristomial gills.
4. Anus aboral and central. Examples: Histocidaris, Goniocidaris.
Order 4. Diadematoida
1. Test globular usually with compound ambulacral plates.
2. Peristomial gills present. 3. Anus aboral and central.
Examples: Diadema, Echinus, Arbacia.
Subclass 3. Irregularia
1. Body oval or circular, flattened oral aborally.
2. Mouth central or displaced anteriorly on oral surface.
3. Anus marginal, outside the apical system of plates. 4. Tube feet generally not locomotor.
Order 1. Holectypoida
1. Test regular with simple ambulacra and centrally located peristome and apical system.
2. Lantern present.
3. Mostly extinct.
Examples: Holectypus, Echinoneus.
Order 2. Cassiduloida
1. Aboral ambulacral areas petaloid, forming a five-armed figure like petals of a flower.
2. Lantern absent.
3. Mostly extinct.
Example: Cassidulus
Order 3. Clypeastroida
1. Test flattened with oval or rounded shape. 2. Mouth central, anus excentric.
3. Aboral ambulacral areas petaloid.
4. Aristotle's lantern present.
5. Gills absent.
6. Bottom dwellers.
Examples: Sand dollars: Clypeaster,
Echinarachinus, Echinocyamus.
Order 4. Spatangoida
1. Test oval or heart-shaped with excentric mouth and anus.
2. Four aboral ambulacral areas pataloid.
3. Lantern absent.
4. Gills absent.
5. Burrowing.
Examples: Heart urchins: Spatangus, Echinocardium lovenia, Hemipneustes.
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Class 4. Holothuroidea
(Gr., holothurion, sea cucumber+eidos,form)
1. Sea cucumbers.
2. No arms, no spines.
3. Body elongate on oral-aboral axis; body wall leathery.
4. Mouth anterior, surrounded by tentacles. 5. Ambulacral grooves concealed; tube feet with suckers.
6. Usually with respiratory tree for respira tion.
Order 1. Dendrochirota
1. Tentacles irregularly branched.
2. Tube feet numerous, on the sole or all ambulacra or entire surface.
3. Respiratory tree present.
Examples: Cucumaria, Thyone.
Order 2. Aspidochirota
1. Tentacles peltate or leaf-like.
2. Tube feet numerous, sometimes forming a well-developed sole, 3. Respiratory tree present.
Examples: Holothuria, Actinopyga.
Order 3. Elasipoda
1. Tentacles leaf-like.
2. No respiratory tree.
3. Tube feet webbed together to form fins.
4. Deep-sea dwellers.
Example: pelagothuria.
Order 4. Molpadonia
1. 15 digitate tentacles.
2. No tube feet.
3. Posterior end tail-like.
4. Respiratory tree present. Examples: Molpadia, Caudina
Order 5. Apoda
1. Worm-like sea cucumbers.
2. Burrowing.
3. No tube feet and repiratory tree. Examples: Leptosynapta, Synapta.
Subphylum II Pelmatozoa
(Gr., pelmatos,stalk+zooios, animal) Stalked, sedentary echinoderms.
Class 5. Crinoidea
(Gr., crinon, lily-+-eidos, form)
1. Sea lillies.
2. Body attached during part or whole of life
by an aboral stalk.
3. Mouth and anus on oral surface.
4 Arms with pinnules.
5. Tube feet without suckers; no madreporite,spines and pedicellariae.
6. Ciliated ambulacral grooves on oral surface.
Order -Articulata
1. Living sea lillies and feather stars.
2 Feather stars non-sessile and free swimming
Examples: Antedon (sea lily), Neometra (feather star).
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