Last Updated on December 7, 2020 by Sagar Aryal
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
- They are exclusively marine and are among the most common and widely distributed marine animals.
- They occur in all seas from the intertidal zones to great depths.
- They have an organ grade system of body organization.
- Symmetry usually radial, nearly always pentamerous.
- The body is triploblastic, coelomate with distinct oral and aboral surfaces, and without definite head and segmentation.
- They are moderate to considerable size but none are microscopic.
- Body shape globular, star-like, spherical, discoidal, or elongated.
- 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.
- The body wall consists of an outer epidermis, a middle dermis, and an inner lining of the peritoneum.
- Endoskeleton consists of closely fitted, plates forming a shell usually called theca or test or may be composed of separate small ossicles.
- The coelom is spacious lined by peritoneum, occupied mainly by the digestive and reproductive system, and develops from embryonic archenteron i.e. enterocoel.
- Coelom of enterocoelous type constitutes the perivascular cavity of water vascular system; coelom fluid with coelomocytes.
- Water -vascular system of coelomic origin, including podia or tube feet for locomotion and usually with a madreporite.
- 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.
- Vascular and haemal or blood lacunar system, enclosed in coelomic peripheral channels.
- Respiratory organs include branchiae, tube-feet, respiratory tree, and bursae.
- A nervous system without a brain and with a circumoral ring and radial nerve.
- The excretory system is wanting.
- Poorly developed sense organs include tactile organs, chemoreceptors, terminal tentacles, photoreceptors, and statocysts.
- Sexes are usually dioecious with few exceptions.
- Gonads large and single or multiple; fertilization external, while few echinoderms are viviparous.
- Development is intermediate including characteristic larvae which undergo metamorphosis into the radially symmetrical adults.
- Regeneration of lost parts, a peculiarity.
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.
- 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).
- Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
- Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.