Phylum Echinodermata: Definition, Characteristics, Classification and Examples

Phylum Echinodermata (Gr., echinos = hedgehog ; derma = skin) is a group of marine invertebrates characterized by radial symmetry, a calcareous endoskeleton, and a water vascular system used for locomotion, feeding, and respiration. Members of this phylum include starfish, sea urchins, sand dollars, and sea cucumbers. They exhibit pentamerous symmetry in adults, have a unique regenerative ability, and possess tube feet for movement and adhesion. Echinoderms are exclusively marine and play essential roles in ocean ecosystems as predators, grazers, and sediment stabilizers.

* Phylum Echinodermata contains some 5300 known species and constitutes the only major group of deuterostome invertebrates.

Bather (1900) stated the phylum as “one of the best characterized and most distinct phyla of the animal kingdom”.

General Characteristics of Phylum Echinodermata

1. The echinoderms are exclusively marine and are among the most common and widely distributed of marine animals.
2. Organ-system grade of body organization.
3. Triploblastic, coelomate and radially symmetrical; often pentamerous.
4. Body unsegmented with globular, star-like, spherical, discoidal or elongated shape.
5. Head absent; body surface is marked by five symmetrically radiating area (ambulacra) and five alternating interradii (inter-ambulacra).
6. Endoskeleton of dermal calcareous ossicles with spines, covered by epidermis.
7. Coelom is spacious lined by digestive and reproductive systems.
8. Presence of water vascular or ambulacral system, which is of coelomic origin, is the most characteristic feature including the podia or tube feet for locomotion and usually with a madreporite.
9. Alimentary canal straight or coiled.
10. Respiration occurs through a variety of structures e.g., papulae in starfishes, peristomial gills in sea urchins, genital bursae in brittle stars and choacal respiratory trees in holothurians.
11. Nervous system is primitive, without brain, consisting of networks concentrated into the radial ganglionated nerve cord.
12. Sense organs poorly developed.
13. No excretory organs (excretory system is wanting).
14. Usually dioecious; fertilization external; while few echinoderms are viviparous.
15. Development indirect through free-swimming larval forms.
16. Regeneration of lost parts; a peculiarity.

Classification of Phylum Echinodermata

Note: The classification is adopted from Hyman L.H. (1955).

Subphylum I: Pelmatozoa

1. Mostly extinct echinoderms.
2. Body is attached by the aboral surface or by an aboral stalk.
3. Mouth and anal aperture present on the oral surface.
4. Viscera is enclosed in a calcareous test.
5. Tube feet or podia are primarily food-catching and devoid of suckers.
6. Main nervous system is aboral.
7. Pelmatozoa has only one living class.

* Subphylum Pelmatozoa consists of five classes: Class 1- Heterostelea; Class 2- Cystidea; Class 3- Blastoidea; Class 4- Crinoidea; Class 5- Edrioasteroidea. Out of these five classes, four classes have become extinct. Thus we will study about Class 4, i.e., Crinoidea.

Class 4: Crinoidea

1. Both extinct and living forms.
2. Living members are without stalk and free-moving, but extinct forms are attached by a stalk.
3. Body consists of an aboral cup, the calyx and oral cover or roof, the tegmen, and strongly pentamerous structure.
4. Oral surface directed upwards.
5. Mouth usually central; anus usually eccentric, present on the oral surface.
6. Arms movable, simple, mostly branched, usually five or ten in number, with or without pinnules.
7. Ambulacral grooves are open and extend along arms and pinnules to their tips.
8. Madreporite, spines, and pedicellariae are present.
9. Sexes are separate.
10. Larva is doliolaria.

Order Articulata

• Extinct and living crinoids.
• Calyx pentamerous, flexible, incorporating the lower arms ossicle.
• Tegmen leathery containing calcareous particles or small plates.
• Mouth and ambulacral grooves exposed.
• Examples: Antedon, Rhizocrinus, Metacrinus

Subphylum II: Eleutherozoa

1. Mostly living echinoderms.
2. Stem or stalk absent, usually free-living forms.
3. Body structure usually pentamerous.
4. Oral surface bearing the mouth is downward or lying on one side.
5. Anus usually on the aboral surface.
6. Ambulacral grooves are usually not for food gathering, and the tube feet with suckers are chiefly locomotory organs.
7. Main nervous system is oral.

Class 1: Holothuroidea

1. Body bilaterally symmetrical, usually elongated in the oral-aboral axis, with the mouth at or near one end and anus at or near the other end.
2. Body surface is coarse.
3. Endoskeleton reduced to microscopic spicules or plates embedded in the body wall.
4. Mouth surrounded by a set of tentacles attached to the water vascular system.
5. Podia or tube feet are usually present and used for locomotion.
6. Alimentary canal is long and coiled, and cloaca usually contains respiratory trees.
7. Sexes are usually separate, and gonads are single or paired tufts of tubules.

Order 1: Aspidochirota

• Podia or tube feet numerous.
• Mouth is surrounded by 10–30 mostly 20 peltate or branched oral tentacles.
• Retractor muscles or pharynx absent.
• A pair of well-developed respiratory trees is present.
• Examples: Holothuria, Stichopus, Mesothuria

Order 2: Elasipoda

• Numerous podia or tube feet.
• Mouth is usually ventral and surrounded by 10–20 peltate or branched tentacles.
• Oral retractors absent.
• Examples: Deima, Benthodytes

Order 3: Dendrochirota

• Podia or tube feet numerous.
• Oral tentacles are dendric or branched, like tree branches.
• Oral retractors present.
• Respiratory trees present.
• Examples: Thyone, Cucumaria, Phyllophorus

Order 4: Molpadonia

• Podia or tube feet absent except as anal papillae.
• Oral tentacles are digitate or finger-shaped.
• Oral retractors absent.
• Respiratory trees present.
• Posterior region generally tapering into a caudal portion.
• Examples: Molpadia, Paracaudina

Order 5: Apoda

• Body vermiform, having a smooth or warty surface.
• Podia or tube feet absent.
• Oral tentacles are 10–20, simple, digitate, or pinnate.
• Pharyngeal retractors are present in some forms.
• Respiratory trees are absent.
• Water vascular system greatly reduced.
• Examples: Synapta, Chiridota

Class 2: Echinoidea

1. Body spherical, disc-like, oval, or heart-shaped.
2. Body enclosed in an endoskeletal shell or test of closely fitted calcareous plates covered with movable spines.
3. Outer calcareous plates are distinguished into five alternating ambulacral and five inter-ambulacral areas.
4. Podia or tube feet come out from the pores of ambulacral plates and are locomotory in function.
5. Mouth is centrally placed on the oral surface and surrounded by a membranous peristome.
6. Anus is located at the aboral pole and surrounded by a membranous periproct.
7. Ambulacral grooves are absent.
8. Pedicellariae are stalked and three-jawed.
9. Sexes are separate, with gonads pentamerous.
10. Development includes a free-swimming echinopluteus larva.

Subclass II: Regularia

1. Body globular, mostly circular, and sometimes oval in shape.
2. Symmetry pentamerous, with two rows of inter-ambulacral plates.
3. Mouth is centrally located on the oral surface and surrounded by the peristome.
4. Anus is centrally placed at the aboral pole, surrounded by the periproct.
5. Aristotle’s lantern is well developed.
6. Madreporite is aboral.

Order 1: Lepidocentroida

• Test flexible, with overlapping or separated plates.
• Ambulacral plates continue up to the mouth lip.
• Examples: Phormosoma, Sperosoma

Order 3: Cidaroidea

• Test rigid and globular.
• Two rows of long, narrow ambulacral plates and two rows of inter-ambulacral plates present.
• Ambulacral and inter-ambulacral plates continue up to the mouth lip.
• Gills and sphaeridia are absent.
• Five bushy Stewart’s organs are present and appended to the lantern.
• Examples: Cidaris, Notocidaris

Order 4: Aulodonta

• Test asymmetrical and globular.
• Test composed of two rows each in ambulacral and inter-ambulacral plates.
• Ambulacral and inter-ambulacral plates reach up to the margin of peristome.
• Gills and sphaeridia are present.
• Teeth of Aristotle’s lantern are devoid of keel.
• Examples: Diadema, Astropyga

Order 6: Camarodonta

• Test rigid and rarely oval.
• Epiphyses of lantern are enlarged and meeting above the pyramids.
• Teeth keeled.
• All four types of pedicellariae are present.
• Examples: Echinus, Strongylocentrotus

Subclass III: Irregularia

1. Test mostly flattened, oval to circular.
2. Symmetry bilateral.
3. Mouth centrally placed on the oral surface.
4. Anus is displaced posteriorly, generally marginal at the oral or aboral surface, and lies outside the apical system of plates.
5. Podia or tube feet are not locomotory.

Order 3: Clypeastroidea

• Test flattened, oval, or rounded in shape, covered with small spines.
• Mouth and apical system are usually central and oral in position.
• Aboral ambulacral areas petaloid.
• Aristotle’s lantern present.
• Gills present.

Order 4. Spatangoida

• Test oval or heart-shaped.
• Four aboral ambulacral areas petaloid, fifth not petaloid.
• Aristotle’s lantern absent.
• Gills absent.
• Examples: Spatangus, Lovenia, Echinocardium

Class 3. Asteroidea

1. Body flattened, pentagonal or star-shaped.
2. Oral and aboral surface are distinct, the oral surface directed downwards and aboral surface upwards.
3. Five to fifty long or short rays or arms radiating symmetrically from a central disc.
4. Mouth is centrally placed at the oral surface surrounded by a membranous peristome.
5. Anus is small and inconspicuous, located more or less eccentrically on the aboral surface.
6. Ambulacra form prominent grooves provided with podia or tube feet.
7. Ambulacra are restricted to oral surface extending from the peristome to the tips of the arms.
8. Endoskeleton flexible, made of separate ossicles.
9. Pedicellariae are small, movable spine-like, always present.
10. Respiration by papulae.
11. Sexes separate, gonads radially arranged.
12. Development includes bipinnaria or brachiolaria larva.

Order 3. Phanerozonia

• Arms provided with two rows of conspicuous marginal plates.
• Oral plates are infra-marginal and aboral plates are supra-marginal.
• Pedicellariae alveolar or sessile type.
• Podia or tube feet arranged in two rows.
• Mouth frame is well developed of adambulacral type.
• Examples: Luidia, Astropecten, Archaster, Anniennea, Pentaceros

Order 4. Spinulosa

• Arms generally without conspicuous marginal plates.
• Aboral skeleton imbricated or reticulated with single or group of spines.
• Pedicellariae rarely present.
• Podia or tube feet are in two rows provided with suckers.
• Mouth frame of adambulacral type.
• Anupullae single or bifurcated.
• Examples: Asterina, Echinaster, Hymenaster, Salaster

Order 5. Forcipulata

• Marginal plates inconspicuous or absent.
• Aboral skeleton mostly reticulate with conspicuous spines.
• Pedicellariae pedunculate type with a basal piece.
• Podia or tube feet arranged in four rows and provided with suckers.
• Papulae on both surfaces.
• Mouth frame of ambulacral type.
• Examples: Brisingaster, Heliaster, Zoraster, Asterias

Class 4. Ophiuroidea

1. Body flattened with a pentamerous or rounded central disc.
2. Oral and aboral surfaces are distinct.
3. Arms usually five, rarely six or seven, are long, slender, smooth or spiny.
4. Ambulacral groove absent.
5. Anus and intestine absent.
6. Madreporite on the oral surface.
7. Sexes separate, gonads pentamerous.
8. Bursae usually 10.
9. Development includes a free-swimming pluteus larva.

Order 1. Ophiurae

• Arms simple, mostly five in number, moving chiefly in a transverse plane.
• Arm ossicles articulated by pits and projections.
• Disc and arms are usually covered with distinct shields or scales.
• Arm’s spines are borne laterally and are directed outward or toward the arm tips, not downwards.
• Single madreporite.
• Examples: Ophioderma, Ophioscolex, Ophiothrix, Ophiocoma

Order 2. Euryalae

• Arms simple or branched, long and flexible, capable of coiling around objects and of rolling up in vertical plane.
• Arm ossicles are articulated in streptospondylus manner.
• Disc and arms without or poorly developed scales or shields.
• Spines directed downwards, often forming hooks or spiny clubs.
• One madreporite in each inter-radius.
• Examples: Astrocnx, Astrophyton, Astroporpa