Phylum Echinodermata- Definition, Classification, Characteristics, Examples

Definition

Echinoderms are the marine animals that are triploblastic deuterostomes.

The larva is bilaterally symmetrical and the adult has modified pentamerous radial symmetry.

The integument contains spines and dermal ossicles. They have peculiar locomotory organs, the tube feet and peculiar water vascular system.

Some 5,500 living species are known.

Brief History

It was Jacob Klein (1738) who coined the word Echinodermata. Leukart (1847) raised this group to the status of phylum and gave it the modern form.

Characteristics Features of Echinodermata

  • The members of Phylum-Echinodermata (Gr. Echinos = hedgehog, Derma = skin), are characterized by the presence of spiny skin.
  • They are triploblastic and coelomate and have pentamerous radial symmetry.
  • The body is unsegmented, globular, star-like, spherical or discoidal in shape.
  • The Head is absent, the body surface is marked by five symmetrically radiating areas (ambulacra) and five inter-radiating areas (Interambularca).
  • The endoskeleton comprises dermal calcareous ossicles with spines covered by the epidermis.
  • Water vascular system comprising podia for locomotion.
  • The alimentary canal is straight or coiled, the anus is absent in ophiuroids.
  • The vascular system is the haemal system enclosed in coelomic canals.
  • Respiratory organs include dermal branchiae, tube feet, respiratory tree and bursae.
  • A district brain is absent, circumoral ring and radial nerves are present.
  • Sense organs are poorly developed and include chemoreceptor, photoreceptor and statocysts.
  • Tube feet are used for locomotion.
  • Dioecious, gonads are large, fertilization is external.
  • Larval forms are present.
  • Regeneration power is highly developed.
  • They are exclusively marine.
  • Examples of Echinoderm: Sea Star, Sea Urchin, Sea Cucumber, Brittle Star, Antedon etc.

Subphylum 1-Elutherozoa

  • Free-living echinoderms.

Class 1-Asteroidea

  • Starfishes.
  • Arms 5 or more and not sharply marked off from the central disc.
  • Tube feet in ambulacral grooves on the oral surface.
  • Anus and madreporite on aboral side.
  • Pedicellariae present.
  • Free-living slow, creeping, predaceous and scavenging.

Order 1-Phanerozonia

  1. Body with marginal plates and papulae.
  2. Pedicellariae sessile, not crossed.
  3. Two rows of tube feet without suckers.
  4. Burrowing animals.
  5. Examples: Astropecten, Pentaceros.

Order 2-Spinulosa

  1. Marginal plates are not conspicuous.
  2. Pedicellariae are rare.
  3. Two rows of tube feet with suckers.
  4. ExamplesAsterina, Solaster.

Order 3-Forcipulata

  1. No conspicuous marginal plates.
  2. Pedicellariae stalked and crossed.
  3. Four rows of tube feet with suckers.
  4. Example-Asterias.

Class 2-Ophiuroidea

  • Brittle stars with a flattened body.
  • Arms sharply marked off from the disc.
  • Pedicellariae are absent.
  • Stomach sac-like and anus is absent.
  • The madreporite is oral.

Order 1-Ophiurae

  1. Brittle and serpent stars.
  2. Simple, small and five-armed.
  3. Arms move transversely.
  4. Disc and arms usually covered with spines.
  5. Madreporite single.
  6. Examples: Ophiura, Ophioderma.

Order 2-Euralae

  1. Arms simple or branched.
  2. Arms move vertically.
  3. Disc and arms covered with soft skin.
  4. Madreporite single.
  5. Example: Gorgonocephalus.

Class 3-Echinoidea

  • Sea urchins, Sand dollars.
  • Body discoid, oval or semispherical, absent.
  • Pedicellariae three jawed and stalked.
  • Aristotle’s lantern for chewing food is present.
  • Ambulacral grooves covered by ossicles and tube feet.

Order 1-Lepidocentroida

  1. Test flexible with overlapping plates.
  2. Ambulacral plates extend up to the mouth.
  3. Interambulacral plates in two rows.
  4. Example-Palaeodiscus.

Order 2-Melonechinoida

  1. Test spherical and rigid.
  2. Four rows of interambulacral plates.
  3. Wholly extinct, were present in the carboniferous period.
  4. Example-Melonechinus.

Order 3-Cidaroida

  1. Test globular and rigid.
  2. Two rows of long, narrow, ambulacral plates and two interambulacral plates.
  3. No peristomial gills.
  4. The anus is aboral and central.
  5. Example-Histocidaris.

Order 4-Diadematoida

  1. Test globular, compound ambulacral plates.
  2. Peristomial gills present.
  3. The anus is aboral and central.
  4. Examples Echinus.

Order 5-Holectypodia

  1. Test regular with simple ambulacral and centrally located peristome.
  2. Lantern present.
  3. Mostly extinct.
  4. Example-Holectypus.

Order 6-Cassiduloida

  • Aboral ambulacral areas petaloid forming a five-armed figure, like petals of a flower.
  • Lantern absent.
  • Mostly extinct.
  • Example-Cassidulus.

Order 7-Cypeastroida

  1. Test flattened with an oval or rounded shape.
  2. Mouth central, anus eccentric.
  3. Aboral, ambulacral, areas petaloid.
  4. Aristotle’s lantern present.
  5. Gills absent.
  6. Example-Clypeaster.

Order 8-Spatangoida

  1. Test oval, heart-shaped with excentric anus and mouth.
  2. Aristotle’s lantern absent.
  3. Gills absent.
  4. Four ambulacral areas petaloid.
  5. Example-Spantangus.

Class 4-Holothuroidea

  • Sea cucumbers.
  • Arms, spines absent.
  • Body elongated on oral and aboral axis.
  • The body wall is leathery.
  • Mouth anterior surrounded by tentacles, anus posterior.
  • Ambulacral grooves concealed, tube feet with suckers.
  • The respiratory tree is present.

Order 1-Dendrochirota

  1. Tentacles are irregularly branched like branches of tree.
  2. Tube feet numerous on the sole.
  3. Respiratory tree present.
  4. Example-Cucumaria.

Order 2-Aspidochirota

  1. Mouth surrounded by branched tentacles.
  2. Tube feet numerous, forming a sole.
  3. Example-Holothuria.

Order 3-Elasipoda

  1. Tentacles leaf-like.
  2. The respiratory tree is absent.
  3. Tube feet united to form fins.
  4. Example-Pelagothuria.

Order 4-Molpadonia

  1. Fifteen digitate tentacles.
  2. Tube feet absent.
  3. Posterior end tail-like.
  4. Respiratory tree present.
  5. Example-Molpadia.

Order 5-Apoda

  1. Worm-like sea cucumbers.
  2. Tube feet and respiratory tree are absent.
  3. Burrowing animals.
  4. Body surface smooth or warty.
  5. Respiratory tree is absent.
  6. Example-Synapta.

Subphylum 2-Pelmatozoa

  • Stalked, sedentary echinoderms.

Class-Crinoidea

  • Sea lilies.
  • Body attached during part or whole of life by an aboral stalk.
  • Mouth and anus on the oral surface.
  • Arms with pinnules.
  • Tube-feet without suckers, no madreporite spines and ambulacral grooves on the oral surface.

Order-Articulata

  • Living sea lilies and feather stars.
  • Calyx pentamerous and tegument leathery.
  • Example-Antedon (sea lily).

Some Examples of Echinodermata

These are some of the main examples of Echinodermata.

  1. Asterias
  2. Ophioderma
  3. Echinocardium
  4. Holothuria
  5. Cucumaria
  6. Antedon
  7. Echinus

Let us discuss about them briefly one by one.

ASTERIA (Star Fish)

Classification of Asteria

PhylumEchinodermata
SubphylumEleutherozoa
ClassAsteroidea
OrderForcipulata
GenusAsterias

Distribution:

Cosmopolitan in distribution. Specially found in India, Japan and USA.

Habits and Habitat

  1. Marine form, occurs from tide marks to deep waters, crawls about over rocks and shells.
  2. Voracious carnivore, feeds on crustaceans and molluscs.

Characteristics of Asterias

  • It is commonly known as star fish.
  • The body contains central disc from which five radiating arms project out, which are broad at their bases and tapering towards the extremities.
  • The arms are not clearly separated from the central disc.
  • The body surface is divisible into an oral surface with a mouth directed downwards and an aboral surface with the anus directed upwards.
  • The oral surface has a central mouth from which five ambulacral grooves extend into the arms.
  • Tube feet with suckers are arranged in rows on either side of the ambulacral groove forming the water vascular system.
  • The aboral surface is pink in colour and contains spines all over. The madreporite is a thick circular plate that is interradial on the aboral side.
  • Branchiae and pedicellariae are present in the body.
Asterias (Star Fish)
Asterias (Star Fish)– External Features (A) Aboral View (B) Oral View

OPHIODERMA (Brittle Star)

Classification of Ophioderma

PhylumEchinodermata
SubphylumEleutherozoa
ClassOphiuroidea
OrderOphiurea
GenusOphioderma

Distribution:

Found along the coast of North Atlantic Ocens and Mediterannean Sea.

Habits and Habitat

  • Marine form.
  • Carnivorous inhabit.

Characteristics of Ophioderma

  • Commonly known as Brittle star.
  • The body consists of a small pentagonal disc set by closely set granules. Oral papillae are numerous and arranged in series.
  • The body is clearly differentiated into oral and aboral surfaces with a distinct central disc and five serpentine arms. The arms are covered by shields or calcareous spines.
  • The mouth is star-shaped, present on the oral surface and covered by numerous ossicles. The anus is absent, Madreporite is oral.
  • Calcareous plates or ossicles are present on the aboral side.
  • There are two slits in each bursa, one situated orally and the other peripherally.
Ophioderma (Brittle Star)
Ophioderma (Brittle Star)

ECHINOCARDIUM (Heart Urchin)

Classification of Echinocardium

PhylumEchinodermata
SubphylumEleutherozoa
ClassEchinoidea
OrderIrregularia
GenusEchinocardium

Distribution:

Cosmopolitan in distribution.

Habits and Habitat

  1. The marine deep-sea form usually found buried in sand at the bottom with extended tube feet for capturing food.
  2. It forms a small chamber-like burrow that opens to the surface by a chimney-shaped structure.

Characteristics of Echinocardium

  • It is commonly known as Heart urchin due to its peculiar shape.
  • It shows an extreme modification of the body.
  • The test is large in size with four well-developed petaloid and is differentiated into oral and aboral surfaces and anterior and posterior sides.
  • Tubercles are absent on aboral side.
  • The apical central plate contains four gonopores.
  • The peristome becomes transversely extended, and the mouth is centric, lying near the anterior side.
  • Aristotle’s lantern is absent.
  • Ambulacral areas bear short tube feet and small spines.
  • Interambulacral areas bear long spines.
  • Echinopluteus larva is present.
  • In defence, the emission organs are thrown out and the enemy is entangled in sticky threads.
Echinocardium (Heart Urchin)
Echinocardium (Heart Urchin) Labelled Diagram

HOLOTHURIA (Sea Cucumber)

Classification of Holothuria

PhylumEchinodermata
SubphylumEleutherozoa
ClassHolothuroidea
OrderAspidochirota
GenusHolothuria

Distribution:

Found in India, Florida and West Indies near sea coasts.

Habits and Habitat

  1. Marine bottom dweller, moves slowly on the sea bottom with the help of tube feet and contraction of the body wall.
  2. It feeds on microorganisms.

Characteristics of Holothuria

  • Commonly known as Sea cucumber.
  • The body is elongated, cylindrical and soft, measuring about 30 cm in length.
  • The mouth and anus are located at opposite ends of the body.
  • The body surface is divisible into five ambulacral areas with double rows of podia and interamularcral ones, which are not recognisable externally.
  • The retractile tube feet are densely scattered all over the body surface.
  • The mouth is surrounded by a circular lip and a thin peristome. Buccal tentacles are present around the peristome, which are modified podia.
  • The water vascular system is well developed.
  • The madreporite is internal, Polian vesicles one to many.
  • The respiratory tree is present.
  • Sexes are separate. Auricularia larva is present.
Holothuria (Sea cucumber)
Holothuria (Sea cucumber)

CUCUMARIA

Classification of Cucumaria

PhylumEchinodermata
SubphylumEleutherozoa
ClassHolothuroidea
OrderDendrochirota
GenusCucumaria

Distribution:

Cosmopolitan in distribution and specially found in Europe, U.S.A. and India.

Habits and Habitat

  1. Marine, deep-sea form, bottom dweller.
  2. Sluggish, found partially or completely buried in sand, feed on micro-organism.

Characteristics of Cucumaria

  • Commonly known as Sea cucumber.
  • The mouth and anus are placed at the opposite ends.
  • The body is cylindrical, slightly pointed posteriorly.
  • The oral end contains a whorl of 10 bushy, pinnately branched tentacles surrounding the mouth.
  • The trunk is differentiated into ambulacral and 5 interambulacral cones. Numerous tube feet emerge on the body wall in ambulacral zones. Tube feet are few in number in the interambulacral area.
  • The region at the base of tentacles is smooth, thin, collar like the introvert.
  • The genital pore is mid-dorsal in position, present between interambulacral tentacles near the mouth.
  • The animal rests on a flat ventral surface enclosing three ambulacral areas and the remaining two ambulacral areas from the dorsal surface.
  • The water vascular system is well developed and the respiratory tree is present.
  • Hermaphrodite, Auricularia larva is present.
Cucumaria labelled diagram
Cucumaria labelled diagram

ANTEDON (Sea Lily)

Classification of Antedon

PhylumEchinodermata
SubphylumPelmatozoa
ClassCrnoidea
OrderArticularia
GenusAntedon

Distribution:

Cosmopolitan, specially found along Atlantic Coast in 25 to 500 fathoms from Chesapeake Bay to coast of New Foundland.

Habits and Habitat

  1. Marine, found attached to rocks by cirri from a central disc.
  2. Gregarious feeds upon microscopic organsims.

Characteristics of Antedon

  • Commonly known as Feature star, or Sea Lily.
  • Appears somewhat like a plant.
  • Small cup-shaped disc or crown with the lower cup-shaped aboral surface, the calyx and upper oral surface, tegmen.
  • Calyx has a knob-like sting on the stalk on the centrodorsal side.
  • From the Centro-dorsal ossicle arise 5 arms, which bifurcate and possess slender jointed pinnules on each side.
  • Arms supported by a jointed skeleton of vertebra-like plates.
  • Oral surface tegmen directed upwards.
  • Tegmen has an excentric anus on a papilla and a central mouth.
  • Viscera extend into arms, gonads present on pinnule.
  • Sexes are separate, crystida and crinoid larva present.
  • Five ambulacral grooves bifurcate and enter the two branches of arms.
  • Spines, pedicellariae and madreporite absent.
Antedon (Sea Lily)
Antedon (Sea Lily)– (A) Entire (B) Central Disc in Aboral view

ECHINUS (Sea Urchin)

Classification of Echinus

PhylumEchinodermata
SubphylumEleutherozoa
ClassEchinoidea
OrderRegularia
GenusEchinus

Distribution:

Widely distributed in Atlantic, Mediterranean and Pacific water.

Habits and Habitat

  • It is Marine, benthonic animals generally inhabit rocky or hard bottom.

Characteristics of Echinus

  • It is commonly known as Sea Urchin.
  • Body is spherical with flat, oral surface and domed aboral surface. Body is enclosed in a rigid, globular test or corona which is composed of closely fitting plates or ossicles.
  • The surface of shell from oral to aboral pole divides into 5 ambulacral and 5 interambulacral zones. Water vascular system is well developed.
  • Oral surface has peristome, the edge of which forms lips surrounding the mouth which forms Aristotle’s lantern, helping in grinding the food.
  • Tube feet with spines form five double rows in the ambulacral area.
  • Around the peristome 10 oral tentacles and 10 groups of dermal papillae present.
  • Periproct with anus on the aboral surface.
  • Around periproct 5 genital plates present inter-radially.
  • Out of these genital plates, one bears the madreporite.
Echinus (Sea urchin)
Echinus (Sea urchin)– (A) Oral View (B) Aboral View

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