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In this article we will discuss about Echinodermata:- 1. Introduction to Echinodermata 2. History of Echinodermata 3. General Characters 4. Classification 5. Skeleton 6. Coelom 7. Relationship.
Contents:
- Introduction to Echinodermata
- History of Echinodermata
- General Characters of Echinodermata
- Classification of Echinodermata
- Skeleton of Echinodermata
- Coelom in Echinodermata
- Relationship of Echinodermata
1. Introduction to Echinodermata:
Echinodermata form a well defined and successful group of marine animals existing since the Palaeozoic, they live at the bottom of all seas creeping about slowly, though some can swim. They exhibit great diversity of form and habit, and form a peculiar group. The body is made of 10 principal divisions which radiate from a main axis, they are five radii and five inter-radii.
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The surface having a mouth is oral or ambulacral, and the opposite surface is aboral or adambulacral. Tube feet project from the ambulacral surface forming radial bands called ambulacra.
In Asteroidea and Crinoidea the tube feet of each ambulacrum project on either side of an ambulacral groove at the bottom of which lies a radial nerve cord, but in other classes the ambulacral groove is closed, so that it forms an epineural canal enclosing the nerve cord. Main axis of the body passes between these two surfaces, and the length of the axis determines the shape of the body.
The axis is short in starfishes with a lower aboral surface; in others the axis is long, in sea cucumbers the oral surface with the mouth is anterior and the animal lies with the main axis parallel to the ground, in sea lilies oral surface is uppermost.
In Asteroidea, Ophiuroidea and Crinoidea the body is prolonged into arms in the direction of radii, and ambulacral surfaces are sub-equal, but in Holothuroidea and Echinoidea the ambulacral surface extends over most of the compact body leaving only a small aboral area opposite the mouth.
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Many Echinodermata possess a faculty of self-mutilation or autotomy by which they can break off their arms or throw out their internal organs when molested, this faculty along with the capacity for regeneration is most marked in many ophiuroids, some asteroids, some holothurians and some crinoids, but it does nor occur in echinoids.
Echinodermata differ from all other coelomate animals mainly due to their radial symmetry, this symmetry is derived secondarily froma bilateral condition and it distorts all their systems. Some structures are bilateral, but externally the symmetry is never quite perfect because a madreporite or anus or a genital opening makes one of the interradii different from others.
2. History of Echinodermata:
The name Echinodermata (Gr., echinos = hedgehog + derma = skin) appears to have originated with Jacob Klein (1734), who, however, applied it only to echinoids.
Echinodermata are all exclusively marine animals living on the shore but mostly on the bottom of the sea. They are coelomate animals with pentaramous radial symmetry, that is the body can be divided into five parts arranged around a central axis, but the larva is bilaterally symmetrical.
There is no head. They have an endoskeleton of calcareous ossicles made from mesoderm, there are also external spines which may be movable or fixed. A large ciliated enterocoelous coelom forms a perivisceral cavity and several intricate systems, one of which is a water vascular system from which project delicate tube feet. Respiratory organs are minute gills protruding out from the coelom.
There is no definite blood vascular system, it is represented only by lacunar tissue, there are no definite excretory organs. Nervous system forms a ring around the mouth with nerves radiating from it, it is the principal nervous system and is in contact with the ectoderm, in addition there is a deeper nervous system lying in the mesoderm.
Sexes are usually separate but copulation does not take place, the gonads discharge to the exterior and fertilisation takes place in sea water. Echinodermata have no parasitic forms. They possess great powers of regeneration.
Echinodermata have a world-wide distribution and the phylum contains some 5,300 known species and a large number of fossil forms. The phylum is divided into two subphyla, viz., Pelmatozoa and Eleutherozoa, Pelmatozoa, has only one living class Crinoidea, while Eleutherozoa has four living classes, Holothuroidea, Echinoidea, Asteroidea, Ophiuroidea.
3. General Characters of Echinodermata:
1. The echinoderms are exclusively marine and are among the most common and widely distributed of marine animals.
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2. They occur in all seas from the intertidal zone to the great depths.
3. Symmetry usually radial, nearly always pentamerous.
4. Body is triploblastic, coelomate with distinct oral and aboral surfaces and without definite head and segmentation.
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5. They are of moderate to considerable size but none are microscopic.
6. Body shape rounded to cylindrical or star-like with simple arms radiating from a central disc or branched feathery arms arise from a central body.
7. Surface of the body is rarely smooth, typically it is covered by five symmetrically spaced radiating grooves called ambulacra with five alternating inter-radii or inter-ambulacra.
8. Body wall consists of an outer epidermis, a middle dermis and an inner lining of peritoneum.
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9. Endoskeleton consists of closely fitted plates forming a shell usually called theca or test or may be composed of separate small ossicles.
10. Coelom is spacious lined by peritoneum, occupied mainly by digestive and reproductive systems and develops from embryonic archenteron, i.e., enterocoel.
11. Presence of water vascular or ambulacral system is the most characteristic feature. It consists of tubes filled with a watery fluid.
12. Alimentary tract is usually coiled tube extending from the mouth located on the oral surface to the anus on the aboral or oral surface.
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13. Circulatory or haemal or blood lacunar system is typically present.
14. Respiration occurs through a variety of structures, i.e., papulae in starfishes, peristomial gills in sea urchins, genital bursae in brittle stars and cloacal respiratory trees in holothurians.
15. Excretory system is wanting.
16. Nervous system is primitive, consisting of networks concentrated into the radial ganglionated nerve cords.
17. Sense organs are poorly developed.
18. Sexes are usually separate (dioecious) with few exceptions. Gonads are simple with or without simple ducts.
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19. Reproduction is usually sexual, few reproduce asexually or by regeneration.
20. Fertilisation is external, while few echinoderms are viviparous.
21. Development is indeterminate including characteristic larvae which undergo metamorphosis into the radially symmetrical adults.
4. Classification of Echinodermata:
The classification is adopted from Hyman, L. H. (1955). Only living classes and orders have been described.
Subphylum I. Pelmatozoa:
(Gr., pelmatos = stalk + zoon = animal):
1. Mostly extinct echinoderms.
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2. Body is attached by the aboral surface or by an aboral stalk.
3. Mouth and anal aperture present on the oral surface facing upwards.
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.
Class 1. Crinoidea: (Gr., crinon = lily + eidos = form):
1. Both extinct and living forms.
2. Living members are without stalk and free moving but extinct forms attached by a stalk.
3. Body consists of an aboral cup, the calyx and oral cover or roof, the tegmen and strongly pentamerous in structure.
4. Oral surface is directed upwards.
5. Mouth usually central, anus usually eccentric are 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. Larva doliolaria.
10. Commonly called sea lilies or feather stars.
Order Articulata:
1. Extinct and living crinoids.
2. Calyx pentamerous, flexible incorporating the lower arm ossicles.
3. Tegmen leathery containing calcareous particles or small plates.
4. Mouth and ambulacral grooves exposed.
Examples:
Antedon, Rhizocrinus, Metacrinus.
Subphylum II. Eleutherozoa:
(Gr., eleutheros = free + zoon = animal):
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 usually not for food gathering and the tube feet with suckers are chiefly locomotory organs.
7. Main nervous system is oral.
Class 1. Holothuroidea: (Gr., holothurion = water polyp + eidos = form):
1. Body bilaterally symmetrical, usually elongated in the oral-aboral axis having 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 water vascular system.
5. Podia or tube feet are usually present and locomotory.
6. Alimentary canal is long and coiled and cloaca usually with respiratory trees.
7. Sexes are usually separate and gonad single or paired tufts of tubules.
8. Commonly called sea cucumbers.
Order 1. Aspidochirota:
1. Podia or tube feet are numerous.
2. Mouth is surrounded by 10-30 mostly 20 peltate or branched oral tentacles.
3. Retractor muscles of pharynx are absent.
4. A pair of well developed respiratory trees is present.
Examples:
Holothuria, Stichopus, Mesothuria.
Order 2. Elasipoda:
1. Numerous podia or tube feet.
2. Mouth is usually ventral and surrounded by 10-20 peltate or branched tentacles.
3. Oral retractors absent.
4. Respiratory tree is absent.
Examples:
Deima, Benthodytes.
Order 3. Dendrochirota:
1. Podia or tube feet are numerous.
2. Oral tentacles are dendritic or branched or branched like tree branches.
3. Oral retractors are absent.
4. Respiratory trees are present.
Examples:
Thyone, Cucumaria, Phyllophorus.
Order 4. Molpadonia:
1. Podia or tube feet are absent except as anal papillae.
2. Oral tentacles are digitate or finger-shaped.
3. Oral retractors are absent.
4. Respiratory trees are present.
5. Posterior region is generally tapering into a caudal portion.
Examples:
Molpadia, Paracaudina.
Order 5. Apoda:
1. Body vermiform having smooth or warty surface.
2. Podia or tube feet are absent.
3. Oral tentacles are 10-20, simple, digitate or pinnate.
4. Pharyngeal retractors are present in some forms.
5. Respiratory trees are absent.
6. Water vascular system is greatly reduced.
Examples:
Synapta, Chiridoata.
Class 2. Echinoidea: (Gr., echinos = hedgehog + eidos = form):
1. Body is spherical, disc-like, oval or heart- shaped.
2. Body is 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. Anus is located at the aboral pole and surrounded by membranous periproct.
6. Ambulacral grooves are absent.
7. Pedicellariae are stalked and three jawed.
8. Sexes are separate. Gonads are pentamerous.
9. Development includes a free swimming echinopluteus larva.
10. Commonly called sea urchins and sand dollars.
Subclass I. Bothriocidaroida:
1. Each inter-ambulacral is with single row of plates.
2. Madreporite radially placed.
3. Typical lantern absent.
4. Includes a single extinct Ordovician genus.
Example:
Bothriocidaris.
Subclass II. Regularia:
1. Body is globular, mostly circular and sometimes oval in shape.
2. Symmetry is pentamerous with two rows of inter-ambulacral plates.
3. Mouth is centrally located at the oral surface and surrounded by peristome.
4. Anus is centrally placed at the aboral pole surrounded by periproct.
5. Aristotle’s lantern is well developed.
6. Madreporite is ambulacral.
Order 1. Lepidocentroida:
1. Test flexible with overlapping or separated plates.
2. Ambulacral plates continue up to mouth lip.
Examples:
Phormosoma, Sperosoma.
Order 3. Cidaroidea:
1. Test is rigid and globular.
2. Two rows of long narrow ambulacral plates and two rows of inter-ambulacral plates are present.
3. Ambulacral and inter-ambulacral plates continue up to mouth lip.
4. Gills and sphaeridia are absent.
5. Five bushy Stewart’s organs are present appended to the lantern.
Examples:
Cidaris, Notocidaris.
Order 4. Aulodonta:
1. Test is symmetrical and globular.
2. Test composed of two rows each in a ambulacral and inter-ambulacral plates.
3. Ambulacral and inter-ambulacral plates reach up to the margin of peristome.
4. Gills and sphaeridia are present.
5. Teeth of Aristotle’s lantern are devoid of keel.
Examples:
Diodema, Astropyga.
Order 6. Camarodonta:
1. Test is rigid and rarely oval.
2. Epiphyses of lantern are enlarged and meeting above the pyramids.
3. Teeth are keeled.
4. All the four types of pedicellariae are present.
Examples:
Echinus, Strongylocentrotus.
Subclass III. Irregularia:
1. Test is mostly flattened oval to circular.
2. Symmetry is bilateral.
3. Mouth centrally placed on the oral surface.
4. Anus is displaced posteriorly generally marginal at oral or aboral surface and lies outside the apical system of plates.
5. Podia or tube feet are not locomotory.
Order 1. Clypeastroida:
1. Test is flattened, oval or rounded in shape covered with small spines.
2. Mouth and apical system are usually central and oral in position.
3. Aboral ambulacral areas petaloid.
4. Aristotle’s lantern present.
5. Gills are absent.
Examples:
Clypeaster, Laganum.
Order 2. Spatangoida:
1. Test is oval or heart-shaped.
2. Four aboral ambulacral areas petaloid, fifth not petaloid.
3. Aristotle’s lantern absent.
4. Gills absent.
Examples:
Spatangus, Lovenia, Echinocardium.
Class 3. Asteroidea: (Gr., aster = star + eidos = form):
1. Body is flattened, pentagonal or star- shaped.
2. Oral and aboral surfaces 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 the feet.
7. Ambulacra are restricted to oral surface extending from the peristome to the tips of the arms.
8. Endoskeleton is flexible, made of separate ossicles.
10. Respiration by papulae.
11. Sexes separate, gonads radially arranged.
12. Development includes bipinnaria or brachiolaria larva.
13. Commonly called starfishes or sea stars.
Order 1. Phanerozonia:
1. Arms are provided with two rows of conspicuous marginal plates.
2. Oral plates are infra-marginal and aboral plates are supra-marginal.
3. Pedicellariae are alveolar or sessile type.
4. Podia or tube feet are arranged in two rows.
5. Mouth frame is well developed and adambulacral type.
Examples:
Luidia, Astropecten, Archaster, Pentaceros.
Order 2. Spinulosa:
1. Arms are generally without conspicuous marginal plates.
2. Aboral skeleton is imbricated or reticulated with single or group of spines.
3. Pedicellariae are rarely present.
4. Podia or tube feet are in two rows provided with suckers.
5. Mouth frame is of adambulacral type.
6. Ampullae single or bifurcated.
Examples:
Aesterina, Echinaster, Hymenaster, Solaster.
Order 3. Foreipulata:
1. Marginal plates are inconspicuous or absent.
2. Aboral skeleton is mostly reticulate with conspicuous spines.
3. Pedicellariae are pedunculate type with a basal piece.
4. Podia or tube feet are arranged in four rows and provided with suckers.
5. Papulae are on both surface.
6. Mouth frame is of ambulacral type.
Examples:
Brisingaster, Heliaster, Zoraster, Asterias.
Class 4. Ophiuroidea:
(Gr., ophis = serpent + oura = tail + eidos = form):
1. Body is 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 grooves are absent.
5. Anus and intestine are absent.
6. Madreporite is on the oral surface.
7. Sexes are separate, gonads pentamerous.
8. Burase usually 10.
9. Development includes a free swimming pluteus larva.
10. Commonly called brittle stars.
Order 1. Ophiurae:
1. Arms are simple, mostly five in number, moving chiefly in transverse plane.
2. Arm ossicles articulated by pits and projections.
3. Disc and arms are usually covered with distinct shields or scales.
4. Spines on arms are borne laterally and are directed outward or toward the arm tips, not downwards.
5. Single madreporite.
Examples:
Ophioderma, Ophioscolex, Ophiothrix, Ophiolepie.
Order 2. Euryalae:
1. Arms are simple or branched, long and flexible, capable of coiling around objects and of rolling up in vertical plane.
2. Ossicles of arms are articulated in streptospondylus manner.
3. Disc and arms are without or poorly developed scales or shields.
4. Spines are directed downward often forming hooks or spiny clubs.
5. One madreporite in each inter-radius.
Examples:
Asteronyx, Astrophyton, Astoporpa.
5. Skeleton in Echinodermata:
The mesoderm forms a skeleton of ossicles lying in the dermis, the ossicles may be few and scattered so that they impart a leathery consistency to the body wall, or they may be united by muscles as a definite skeleton, or they may be firmly jointed to form a shell.
Some ossicles usually project as spines over which the epidermis is lost. Two or three spines may be arranged so as to work as pincers, these form pedicellariae of various types, they occur only in Asteroidea and Echinoidea.
The primitive skeleton had two series of plates forming oral and apical systems, but in present day Echinodermata the apical plates are absent or reduced or replaced by accessory plates, e.g., corona of sea urchins. The oral system forms five plates around the mouth, it is fully developed only in Crinoidea forming a calyx. Lime is deposited not only in the skeleton but it may be found in any organ of the body.
6. Coelom in Echinodermata:
A coelom is formed from paired pouches which arise as lateral out-pushings of the embryonic archenteron, thus, the coelom is enterocoelic. The pouches undergo a constriction so that each forms an anterior and a posterior sac, the posterior sacs grow and form coelomic cavities and the anterior sacs become the rudiments of a water vascular system, they are called hydrocoel sacs.
The left hydrocoel sac acquires a stone canal which communicates with the body wall, the right hydrocoel sac disappears, but recent evidence shows that the right hydrocoel sac is represented by the dorsal sac of the axial sinus. Thus, the entire water vascular system is formed from the left hydrocoel and it assumes a radial arrangement of its parts.
Besides the water vascular system the coelomic cavities form a perivisceral cavity containing the main viscera, a perihaemal system which encloses a vascular system, and its aboral sinus extension enclosing the gonads, and an axial sinus of varied development in different classes, but it has an opening forming a madreporite.
7. Relationship of Echinodermata:
The free Eleutherozoa have been derived from attached pelmatozoic ancestors. Echinodermata show no close relationship to any invertebrates, except with Hemichordata and Pogonophora.
These three phyla have a number of common features, among which are the formation of coelom by enterocoel retention of blastopore as the site of the future anus, in having a dipleurula-like larva at some stage, and in having a heart vesicle which may represent the right anterior coelom. The larvae of some echinoderms (auricularia larva of holothurians) closely resemble the tornaria larva of Saccoglossus.
But they are closely related to hemichordates because of the following reasons:
1. The mesoderm is derived from cells around the lips of the blastopore.
2. They possess a mesodermal endoskeleton, whereas the invertebrates have an ectodermal exoskeleton.
3. The blastopore becomes the adult anus as in chordates, in invertebrates (annelids, molluscs) the blastopore becomes the mouth.
4. Mouth arises as a new structure from the ectodermal stomodaeum as in chordates.
5. The coelom is formed from paired lateral diverticula of the archenteron and is enterocoelic.
The many resemblances between echinoderms and hemicordates Auricularia larva, are neither accidental nor due to convergent evolution, but because the two phyla are closely related and both arose from some common ancestor. Hemichordates are closer to this common ancestor, while echinoderms have deviated greatly because they have arisen as a blind branch from the ancestral type.
