General Characters and classification of Echinodermata.

General characters.

Phylum Echinodermata contains some 5300 known species and constitutes the only major group of deuterostome invertebrates. Bather (1900) states the phylum as "one of the best characterized and most distinct phyla of the animal kingdom.Some of the general characteristics of Echinodermata are distinguished below.

1. Exclusively marine.
2. Organ-system grade of body organization.
3. Triploblastic, coelomate and radially symmetrical; often pentamerous.
4. Head absent; body surface is marked by five symmetrical; radiating areas and five alternating interradii (inter-ambulacra).
5. Endoskeleton of dermal calcareous ossicles with spines, covered by the epidermis.
6. Coelom of enterocoeles type constituting the perivisceral cavity and cavity of the water-vascular system; coelomic fluid with coelomocytes.
7. Body segmented with the globular, star-like, spherical, discoidal or elongated shape.
8. Water vascular system of coelomic origin, including podia or feet for locomotion and usually with a madreporite.
9. Alimentary canal straight or coiled.
10. Vascular system and haemal system, enclosed in coelomic peri haemal channels.
11. No excretory organs.
12. Respiratory organs include dermal branchiae, tube feet, respiratory tree, and bursae.
13. A nervous system without a brain and with a circumoral ring and radial nerves.
14. Poorly developed sense organ include tactile organs, chemoreceptors, terminal tentacles, photoreceptors, and statocysts.
15. Usually dioecious, gonads large and single or multiple; fertilization external; development indirect through free-swimming larva forms.
16. Regeneration of lost parts, a peculiarity.

Classification of Echinodermata.

Phylum Echinodermata has been classified into four classes.General character and classification of each class are summarized below.

Subphylum I. Eleutherozoa.

(Gr., eleuthero, free+zoios, animal)

1. Free-living echinoderms.

Class 1. Asteroidea

(Gr., aster, star+eidos, form)

1. Free-living, slow-creeping, predaceous and scavengers.
2. Pedicellariae present.
3. Starfishes or sea stars.
4. Arms 5 or more and not sharply marked off from the central disc.
5. Tube feet in orally placed ambulacral grooves; with suckers.
6. Anus and madreporite aboral.

Subclass 1. Somasteroidea

Fossil Palaeozoic sea stars. Platasterias lati radiate is only living species.

Subclass 2. Easter idea

Living sea water.

Order 1. Phanerozoic

1. Body with marginal plates and usually with papulae, on aboral surface.
2. Mostly burrowing in soft bottom.
3. Pedicellariae sessile, not crossed.
4. Tube feet without sucker.

Example: Astropecten.

Order 2. Spinulosa

1. Aboral surfaces with low spines.
2. Tube feet ith suckers.
3. Usually without conspicuous marginal plates and with papulae on both surfaces.
4. Pedicellariae rare.

Example: Asterina, Solaster.

Order 3. Forcipulata

1. Four rows of tube feet.
2. No, conspicuous marginal plates.
3. pedicellariae pedunculate and straight or crossed type.

Example: Asterias.

Class 2. Ophiuroidea

(Gr., his, snake +oura, tail+eidos, form)

1. Madreporite oral.
2. Stomach sac-like; no anus.
3. Pedicellariae absent.
4. Body star like with arms sharply marked off from the central disc.
5. Madreporite oral.

Order 1. Ophiurae.

1. Brittle and serpent stars.
2. Small and 5 armed.
3. Disc and arms usually covered with plates.
4. Arms move transversely.

Examples:

Ophiura, Ophiura, Ophiothrix.

Order 2. Euryale

1. Arms move vertically.
2. Arms simple or branched.
3. Disc and arms covered by soft skin.

Examples: Gorgonocephalus

Class 3. Echinoidea

(Gr., echinus, hedgegog+eidos, form)

1. Sea urchins and dollars.
2. Body discoid, oval or semi-spherical and without arms.
3. Skeleton or test compact bearing movable spines and three jawed pedicellariae.
4. Chewing apparatus or Aristotle's lantern with teeth.
5. Gonads usually five or less.

Subclass 1. Bothriocidaroida

1. A single row of plates in each interambulacral area.
2. Without typical lantern.
3. Madreporite radial.

Example: Bothriocidaris.

Subclass 2. Regular

1. Madreporite oral.
2. Mouth central.
3. Body globular, pentamerous with two rows of inter- ambulacral plates in existing membrane.
4. Aristotle's lantern well developed.
5. Anus central on an aboral surface with well-developed apical plates.

Order 1. Lepidocentroida

1. Test flexibly with overlapping plates.
2. Ambulacral plates extend up to mouth lip.
3. Interambulacral plates in more than two rows in extinct forms.

Example: Palaeodiscus.

Order 2. Melonechinoida

1. Text spherical and rigid.
2. wholly extinct, carboniferous.
3. Ambulacral plates continue to mouth lip.
4. Interambulacral plates in four or more rows.

Example: Melonechinus.

Order 3. Cidaroida

1. No peristomial gills.
2. Anus aboral and central.
3. Test globular and rigid.

Example: Histocidaris.

Order 4. Diadematoida

1. Peristomial gills absent.
2. Test globular usually with compound ambulacral plates.
3. Anus, aboral and central.

Example: Diadema.

Subclass 3. Irregular

1. Body oval or circular flattened oral-aborally.
2. Mouth central or displaced anteriorly on oral.
3. Anus marginal, outside the apical system of plates.
4. Tube feet generally not locomote.

Order 1. Holectypoida

1. Test regular with ambulacra and centrally located peristome and apical system.
2. Mostly extinct.
3. Lantern present.

Example: Holectypus.

Order 2. Cassidulopida

1. Mostly extinct.
2. Lantern extinct.

Example: Cassidulus.

Order 3. Clypeastroida

1. Gilla absent.
2. Mouth central, anus excentric.
3. Aboral ambulacral areas petaloid.
4. Bottom dwellers.

Example: Echinocyamus.

Order 4. Spatangoida

1. Gills absent.
2. Burrowing.
3. Lantern absent.
4. Four aboral ambulacral areas petaloid.
5. Test oval or heart-shaped with excentric mouth and anus.

Example: Spatangus.

Class 4. Holothuroidea

(Gr., holothurians, sea cucumber+eidos, form)

1. Sea cucumbers.
2. No arms, no spines.
3. Body elongated on the oral-aboral axis; body wall leathery.
4. Mouth anterior, surrounded by tentacles.
5. Ambulacral grooves concealed; tube feet with suckers.
6. Usually with a respiratory tree for respiration.

Order 1. Dendrochirota

1. Tentacles irregular branched.
2. Tube feet numerous on the sole or all ambulacral on entire surfaces.
3. Respiratory tree present.

Example: Cucumaria.

Order 2. Aspidochirota

1. Tentacles peltate or leaf-like.
2. Tube feet numerous, sometimes forming a well-developed sole.
3. Respiratory tree present.

Example: Holothuria

Order 3. Elasipoda

1. Tentacles leaf-like.
2. No respiratory tree.
3. Tube feet webbed together to form fins.
4. Deep-sea dwellers.

Example: Pelagothuria.

Order 4. Molpadonia

1. 15 digitate tentacles.
2. No tube feet.
3. Posterior end tail-like.
4. Respiratory tree present.

Example: Molpadia.

Order 5. Apodaca

1. Worm-like sea cucumbers.
2. No tube feet and respiratory tree.
3. Burrowing.

Example: Synapta.

Subphylum II. Pelmatozoa

(Gr., pelmatos, stalk+zooios, animal)

Stalked, sedentary echinoderms.

Class 5. Crinoidea.

(Gr., crimson, lily+eidos, form)

1. Sea lilies.
2. Body attached during part or whole of life by an aboral stalk.
3. Mouth and anus on oral surfaces.
4. Tube feet without suckers; no madreporite, spines and pedicellariae's.
5. Ciliated ambulacral grooves on oral surfaces.
6. Arms with pinnules.

Order Articulata

1. Living sea lilies and feather stars.
2. Feather stars non -sessile and free swimming.
3. Example: Antedon.

Reference:

Aggarwal Sarita. A Text Book of Biology,New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

https://en.wikipedia.org/wiki/Echinoderm