Phylum Echinodermata


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Detailed description of the phylum Echinodermata along with classification


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Phylum Echinodermata : 

Phylum Echinodermata Pre-Medical Presentation Diversity in Living World Dr. R. K Pillai, Dept of Zoology, Hindu College, Moradabad

General Characters : 

General Characters 1. Number of species about 7,000 (living) and 13,000 (extinct). 2. The term ‘Echinodermata’ (Gr., echinos, spiny or hedgehog; derma, skin) coined by Jacob Klein in 1734. 3. The ‘Echinodermata’ (spiny-skinned animals) recognized as distinct taxon by Frey and Leuckart in 1847. 4. Comprises sea lilies, feather stars, sea stars, brittle stars, sea urchins, sand dollars and sea cucumbers. 5. Triploblastic, radially symmetrical often pentamerous, larva with bilateral symmetry, sessile or creeping at sea bottom. 6. Exclusively marine deuterostomes. 7. All benthic (except few pelagic sea cucumbers).

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8. Distinctly different from other metazoans; body is unsegmented. 9. Uncephalized (head absent), body parts organized about an oral-aboral axis. 10. An endoskeleton composed of unique calcareous ossicles bearing protective spines. 11. A complete digestive system with straight or coiled alimentary canal. 12. An outstanding characteristic is their complex coelom (enterocoel) and its derivatives: the water vascular system, perivisceral coelom and haemal system. 13. The water vascular system, a unique organ system that functions in locomotion, feeding, respiration and excretion. 14. Water vascular system is connected to outside through a perforated plate known as madreporite. 15. Locomotion by external tube feet connected with water vascular system.

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16. Circulatory structures, when present, compose hemal system derived from coelomic cavities and sinuses. 17. Respiratory organs include dermal branchiae, tube feet, respiratory tree and bursae. 18. Excretory system absent. 19. The nervous system diffuse, decentralized, usually with a nerve net, nerve ring and radial nerves, but no brain. 20. Sense organs poorly developed with tactile tentacles, pigment eye-spots and statocysts. 21. Mostly dioecious, fertilization external. 22. Development direct or indirect through free swimming larval stages. 23. Possess the remarkable power of regeneration.


CLASSIFICATION Classification is based on the shape of the body and characteristics of digestive system. Two subphyla and five classes of Echinodermata are: Subphylum I ELEUTHEROZOA Class I Asteroidea Class II Ophiuroidea Class III Echinoidea Class IV Holothuroidea Subphylum II PELMATOZOA Class V Crinoidea

Subphylum I ELEUTHEROZOAFree-moving echinoderms : 

Subphylum I ELEUTHEROZOAFree-moving echinoderms Class I Asteroidea (Gr. aster, star; eidos, form) 1. Starfishes and sea stars. 2. Body flattened, star-shaped, ambulacral canal on oral side, with tube feet. 3. Anus and madreporite aboral surface. 4. Minute pincers called pedicellariae are present. 5. Starfish is benthonic (bottom dwelling) carnivorous animal. 6. Starfish is nocturnal and eats its prey.

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7. Starfish feeds on molluscs and destroys pearl oyster. 8. Starfish can evert stomach out of the body, feeds and digests food outside the body. 9. Starfish has great power of regeneration. When injured, the animal readily breaks off its own arm. This process is called ‘autotomy’; it serves as a means of protection to the animal. 10. About living 1,500 species.


Subphylum I ELEUTHEROZOA Class II Ophiuroidea (Gr., ophis, snake; oura, tail; eidos, form) 1. Brittle-stars and basket stars. 2. Body with a central disc and five distinct slender unbranched or branched jointed arms. 3. Anus and pedicellariae are absent. 4. Tube feet nonlocomotory without suckers. 5. Madreporite on oral surface. 6. About living 2,000 species.


Subphylum I ELEUTHEROZOA Class III Echinoidea (Gr., echinos, spiny; eidos, form) 1. Sea urchins and sand dollars. 2. Body globose, hemispherical or discoidal and without arms. 3. Skeleton bearing movable spines and three-jawed pedicellariae. 4. A biting and masticatory apparatus or ‘Aristotle’s lantern’ present. 5. About 950 living species


Subphylum I ELEUTHEROZOA Class IV Holothuroidea (Gr., holothurion, sea cucumber; eidos, form) 1. Sea cucumbers. 2. No arms, no spines, no pedicellariae. 3. Body fleshy, cylindrical and elongate on oral-aboral axis. 4. Tube feet some times absent, madreporite internal. 5. Mouth anterior surrounded by feeding tentacles, anus at posterior end. 6. Usually with respiratory tree for respiration. 7. About 1,150 living species.

Subphylum II PELMATOZOAStalked, sedentary echinoderms : 

Subphylum II PELMATOZOAStalked, sedentary echinoderms Class V Crinoidea (Gr., crinon, lily; eidos, form) 1. Sea lilies and feather stars. 2. Body form as a cup or calyx, attached to substratum by aboral surface (calyx) or by stalk. 3. Mouth and anus on oral surface. 4. No external madreporite. 5. Arms bear pinnules. 6. Tube feet without suckers, no madreporite, spines and pedicellariae. 7. Ciliated ambulacral grooves on oral surface. 8. About 625 living species.

General Points : 

General Points Concentricycloidea is a new class of echinoderms established by Baker et al. in 1986. Concentriclycloidea comprises " sea daisies" , body discoidal (< 1 cm diameter), with ring of marginal spines, but without arms or rays, two ring canals with hydropore, gut absent or incomplete. e.g., Xyloplax. Classification of "sea daisies" is problematic, many authorities treat them under class Asteroidea. Pedicellaria were discovered by O. F. Muller (1778) who considered them as parasitic polyps.

Larval forms of Echinodermata : 

Larval forms of Echinodermata Asteroidea — Dipleurula, Bipinnaria, Brachiolaria Ophiuroidea — Ophiopluteus Echinoidea — Echinopluteus Holothuroidea — Auricularia Crinoidea — Doliolaria, Pentacrinoid

Taxonomic Summary : 

Taxonomic Summary Phylum Echinodermata Subphylum ELEUTHEROZOA Class Asteroidea Asterias — Sea star (Starfish) Astropecten —Sea star (Starfish) Pentaceros— Sea pentagon Class Ophiuroidea Ophiothrix — Brittle star Gorgonoephalus— Basket star Class Echinoidea Echinus — Sea urchin Echinocardium —Heart urchin Clypeaster — Sand dollar Class Holothuroidea Cucumaria — Sea cucumber Subphylum PELMETOZOA Class Crinoidea Antedon — Feather star Holopus — Sea lilies

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END Thanks Dr. R. K Pillai, Dept of Zoology, Hindu College, Moradabad

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