Phylum Echinodermata- Characteristics, classification, examples

Echinodermata (Echinoderms) Definition

Echinoderms are enterocoelous coelomates with pentamerous radial symmetry, without distinct head or brain having a calcareous endoskeleton of separate plates or pieces and a peculiar water vascular system of coelomic origin with podia or tube-feet projecting out of the body.

Phylum Echinodermata Characteristics

1. They are exclusively marine and are among the most common and widely distributed marine animals.
2. They occur in all seas from the intertidal zones to great depths.
3. They have an organ grade system of body organization.
4. Symmetry usually radial, nearly always pentamerous.
5. The body is triploblastic, coelomate with distinct oral and aboral surfaces, and without definite head and segmentation.
6. They are moderate to considerable size but none are microscopic.
7. Body shape globular, star-like, spherical, discoidal, or elongated.
8. The surface of the body is rarely smooth, typically covered by 5 symmetrically radiating grooves called ambulacra with 5 alternating inter-radii or inter-ambulacra.
9. The body wall consists of an outer epidermis, a middle dermis, and an inner lining of the peritoneum.
10. Endoskeleton consists of closely fitted, plates forming a shell usually called theca or test or may be composed of separate small ossicles.
11. The coelom is spacious lined by peritoneum, occupied mainly by the digestive and reproductive system, and develops from embryonic archenteron i.e. enterocoel.
12. Coelom of enterocoelous type constitutes the perivascular cavity of water vascular system; coelom fluid with coelomocytes.
13. Water -vascular system of coelomic origin, including podia or tube feet for locomotion and usually with a madreporite.
14. The alimentary canal is usually a coiled tube extending from the mouth located on the oral surface to the anus on the aboral or oral surface.
15. Vascular and haemal or blood lacunar system, enclosed in coelomic peripheral channels.
16. Respiratory organs include branchiae, tube-feet, respiratory tree, and bursae.
17. A nervous system without a brain and with a circumoral ring and radial nerve.
18. The excretory system is wanting.
19. Poorly developed sense organs include tactile organs, chemoreceptors, terminal tentacles, photoreceptors, and statocysts.
20. Sexes are usually dioecious with few exceptions.
21. Gonads large and single or multiple; fertilization external, while few echinoderms are viviparous.
22. Development is intermediate including characteristic larvae which undergo metamorphosis into the radially symmetrical adults.
23. Regeneration of lost parts, a peculiarity.

Read Also: Phylum Mollusca- characteristics, classification, examples

Phylum Echinodermata Classification

The classification is adopted from Hyman, L.H. (1995). Only living classes and orders have been described.

Subphylum 1. Pelmatozoa (Gr., pelmatos=stalk+ zoon=animals)

• Mostly extinct echinoderms.
• Sedentary echinoderms.
• Body attached by the aboral surface by an aboral stalk.
• Mouth and anal aperture present on the oral surface facing upwards.
• Viscera is enclosed in a calcareous test.
• No suckers.
• Tube feet or podia are primarily food catching.
• The main nervous system is aboral.
• Has only one living class.

Class 1. Crinoidea (Gr., crinon=lily+ eidos= form)

• Commonly called Sea Lillies or feather stars.
• Both extinct and living form.
• Living members are without stalk and free moving but extinct form attached by a stalk.
• The body consists of an aboral cup, the calyx and oral cover or roof, the tegmen, and strongly pentamerous in structures.
• Mouth and anus on the oral surface.
• Arms movable, simple, mostly branched, usually 5 or 10 in numbers with or without pinnules.
• Tube feet without suckers; no madreporite, spine, and pedicellariae.
• Ambulacral grooves are open and extend along arms and pinnules to their tips.
• Sexes are separate.
• Larva doliolaria.

Order 1. Articulata

• Extinct and living crinoids.
• Non-sessile and free swimming.
• Calyx pentamerous, flexible incorporating the lower arm ossicles.
• Tegmen leathery containing calcareous particles or small plates.
• Mouth and ambulacral grooves exposed.
• Examples: Antedon (sea lily), Rhizocrinus, Metacrinus.

Subphylum 2. Eleutherozoa (Gr., eleutheros=free+ zoon= animals)

• Free-living echinoderms.
• Stem or stalk absent, usually free-living forms.
• Pentamerous body strictures.
• The oral surface bearing mouth is downward or lying on one side.
• Anus usually on the aboral surface.
• Ambulacral grooves usually not for food gathering.
• Tube feet with suckers are chiefly locomotory organs.
• The main nervous system is oral.

Class 1. Holothuroidea (Gr., holothurion=water polyp+ eidos=form)

• Commonly called sea cucumbers.
• Body bilaterally symmetrical, usually elongated in the oral-aboral axis having a mouth at or near one end and anus at or near the other end.
• Coarse body surface.
• Endoskeleton reduced to microscopic spicules or plates embedded in the body wall.
• Mouth anterior, surrounded by tentacles attached to the water vascular system.
• Ambulacral grooves concealed.
• Podia or tube feet usually present and locomotory.
• The alimentary canal is long, coiled.
• cloaca usually with respiratory trees for respiration.
• Sexes separate and gonad single or paired tufts or tubules.

Order 1. Aspidochirota

• Podia or tube feet are numerous, sometimes forming a well-developed sole.
• Tentacles are peltate or leaf-like.
• The mouth is surrounded by 10-30 mostly 20 peltate or branched oral tentacles.
• Retractor muscles of the pharynx are absent.
• A well-developed respiratory tree is present.
• Examples: Holothuria, Stichopus, Mesothuria.

Order 2. Elasipoda

• Numerous podia or tube feet.
• Tentacles leaf-like.
• Tube feet webbed together to form fins.
• The mouth is usually ventral and surrounded by 10-20 peltate or branched tentacles.
• Oral retractors absent.
• No respiratory tree.
• Deep-sea dwellers.
• Examples: Deima, Benthodytes.

Order 3. Dendrochirota

• Podia or tube feet are numerous, on the sole or all ambulacral or entire surface.
• Tentacles irregularly branched.
• Oral retractors present.
• Respiratory trees are present.
• Examples: Thyone, Cucumaria, Phyllophorus.

Order 4. Molpadonia

• Podia or tube feet are absent except as anal papillae.
• 15 digitate or finger-shaped tentacles.
• Posterior end tail-like.
• Oral retractors are absent.
• Respiratory trees are present.
• Examples: Molpadia, paracaudina.

Order 5. Apoda

• Worm-like sea cucumbers.
• No tube feet and respiratory tree.
• Body vermiform having a smooth or warty surface.
• Oral tentacles are 10-20 simple, digitate or pinnate.
• Pharyngeal retractors are present in some forms.
• The water vascular system is greatly reduced.
• Examples: Synapta, Chiridoata.

Class 2. Echinoidea (Gr., echinos=hedgehog+eidos=form)

• Commonly called Sea urchins and sand dollars.
• The body is spherical, disc-like, oval, or heart-shaped.
• The body is enclosed in an endoskeletal shell or test of closely fitted calcareous plates covered by movable spines.
• Outer calcareous plates are distinguished into 5 alternating ambulacral and 5 inter-ambulacral areas.
• Podia or tube feet come out from pores of ambulacral plates and are locomotory in function.
• The mouth is centrally placed on the oral surface and surrounded by a membranous peristome.
• Chewing apparatus of Aristotle’s lantern with teeth.
• Ambulacral grooves covered by ossicles; tube feet with suckers.
• The anus is located at the aboral pole and surrounded by a membranous periproct.
• Pedicellariae are stalked and 3 jaws.
• Sexes are separate. Gonads usually five or less.
• The development includes a free-swimming echinopluteus larva.

Subclass 1. Bothriocidaroida

• Each inter-ambulacral is with a single row of plates.
• No typical lantern.
• Radial madreporite.
• Include a single extinct Ordovician genus.
• Example: Bothriocidaris.

Subclass 2. Regularia

• Body globular, mostly circular and sometimes oval.
• Symmetry pentamerous with 2 rows of inter-ambulacral plates in existing members.
• Mouth central at the oral surface and surrounded by peristome.
• Aristotle’s lantern is well developed.
• The anus is centrally placed at the aboral pole surrounded by periproct.
• Madreporite is ambulacral.

Order 1.  Lepidocentroida

• Test flexible with overlapping plates.
• Ambulacral plates extend up to the mouth lip.
• Inter-ambulacral plates in more than 2 rows in extinct forms.
• Example: Palaeodiscus.

Order 2. Cidaroidea

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

Order 3. Aulodonta

• Test symmetrical and globular.
• Test composed of 2 rows each in an ambulacral and inter- ambulacral plates.
• Ambulacral and inter- ambulacral plates reach up to the margin of the peristome.
• Gills and sphaeridia are absent.
• Teeth of Aristotle’s lantern are devoid of the keel.
• Examples: Diadema, Astropyga.

Order 4. Camarodonta

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

Subclass 3. Irregularia

• Body oval or circular, flattened oral-aborally.
• Symmetry is bilateral.
• Mouth central or displaced anteriorly on the oral surface.
• The anus is displaced posteriorly generally marginal at the oral or aboral surface and lies outside the apical system of plates.
• Podia or tube-feet generally not locomotor.

Order 1. Clypeastroida

• Test flattened with an oval or rounded shape and covered with small spines.
• Mouth and apical system are usually central and oral in position.
• Aboral ambulacral.
• Aristotle’s lantern present.
• Gills are absent.
• Bottom dwellers.
• Examples: sand dollars: Clypeaster, Echinarachinus, Echinocyamus.

Order 2. Spatangoida

• Test oval or heart-shaped with excentric mouth and anus.
• 4 aboral ambulacral areas pataloid.
• No lantern.
• Gills absent.
• Burrowing.
• Examples: Heart urchins; Spatangus, Echinocardium, Lovenia.

Class 3. Asteroidea (Gr., aster=star+ eidos= form)

• Commonly called starfishes or sea stars.
• The body is flattened, pentagonal, or star-shaped.
• Oral and aboral; surfaces are distinct, the oral surface directed downwards and the aboral surface upwards.
• Arms 5 or more and not sharply marked off from the center disc.
• The mouth is centrally placed at the oral surface surrounded by a membranous peristome.
• The anus is small and inconspicuous located more or less eccentrically on the aboral surface.
• Tube feet in orally placed ambulacral grooves; with suckers.
• Ambulacra are restricted to the oral surface extending from the peristome top the tips of the arms.
• Endoskeleton is flexible, made of separate ossicles.
• Pedicellariae are small, movable spine-like always present.
• Respiration by papulae.
• Sexes separate, gonads radially arranged.
• The development includes bipinnaria or branchiolaria larva.

Order 1. Phanerozonia

• Body with marginal plates and usually with papulae, on the aboral surface.
• Arms are provided with 2 rows of conspicuous marginal plates.
• Oral plates are infra-marginal and aboral plates are supra-marginal.
• Pedicellariae sessile or alveolar type, not crossed.
• Podia or tube-feet are arranged in two rows without suckers.
• The mouth frame is well developed and adambulacral type.
• Mostly burrowers in soft bottoms.
• Examples: Luidia, Astropecten, Archaster, Pentaceros.

Order 2. Spinulosa

• Arms are generally without conspicuous marginal plates.
• The aboral skeleton is imbricated or reticulated with a single or group of spines.
• Pedicellariae are rare.
• Tube feet are in two rows with suckers.
• The mouth frame is of adambulacral type.
• Aboral surface with low spines.
• Ampullae single or bifurcated.
• Examples: Aesterina, Echinaster, Hymenaster, Solaster.

Order 3. Forcipulata

• No conspicuous marginal plates.
• The aboral skeleton is mostly reticulate with conspicuous spines.
• Pedicellariae are the pedunculate type with a basal piece.
• Podia or tube feet are arranged in four rows and provided with suckers.
• Papulae are on both surfaces.
• The mouth frame is of ambulacral type.
• Examples: Brisingaster, Heliaster, Zoraster, Asterias.

Class 4. Ophiuroidea (Gr., ophis=serpent+ oura=tail+ eidos= form)

• Commonly called brittle-stars and allies.
• The body is flattened with a pentamerous or rounded central disc.
• Oral and aboral surfaces are distinct.
• Body star-like with arms sharply marked off from the central disc.
• Pedicellariae absent.
• Ambulacral grooves are absent or covered by ossicles.
• No anus and intestine.
• Stomach sac-like.
• Tube feet without suckers.
• The madreporite is on the oral surface.
• Sexes are separate, gonad pentamerous.
• Bursa usually 10.
• Development included a free-swimming pluteus larva.

Order 1. Ophiurae

• Brittle and serpent stars.
• Arms are simple, mostly 5 in number, moving chiefly in the transverse plane.
• Arm ossicles articulated by pits and projections.
• Discs and arms are usually covered with distinct shields or scales.
• Spines on arms are borne laterally and are directed outwards and toward the arm tips, not downwards.
• Single madreporite.
• Examples: Ophioderma Ophioscolex, Ophiothrix, Ophiolepie.

Order 2. Euryalae

• Arms simple or branched, long and flexible, capable of coiling around objects and of rolling up in the vertical plane.
• Ossicles of arms are articulated in a streptospondylus manner.
• Discs and arms covered by soft skin.
• Spines are directed downward often forming hooks or spiny clubs.
• One madreporite in each inter-radius.
• Examples: Asteronyx, Gorgonocephalus (basket stars).

References

1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.