Phylum Echinodermata are defined as spiny skined animals (Greek word echinos = spiny; derma = skin). Beginning their life race from lower cambrian, they are successfully continuing their biological activities. Echinodenns are at the top of invertebrate and very close to the chordates. They are unique combination of advanced and primitive characters. The advanced characters
- They ale deuterostomes,
- Presence of mesodermal calcium carbonate endoskeleton (note other invertebrates have no endoskeleton),
- Coelom enterocoelus,
- Exclusively free-living marine.
DIAGNOSTIC CHARACTERS OF PHYLUM ECHINODERMATA
- Spiny-skinned animals.
- Triploblastic deuterostomes.
- Distinct head and segmentation absent.
- Body divided into oral and aboral surfaces.
- Peculiar water vascular system for locomotion.
- Exclusively marine.
CLASSIFICATION OF PHYLUM ECHINODERMATA
SUB-PHYLUM I PELMATOZOA
- Sedentary echinoderms.
- Oral surface oriented upwards and aboral surface with jointed cirri.
CLASS I CRINOIDEA
- Commonly called as sea lilies.
- Body consists of a cup like calyx and cylindrical arms.
- Tube feet, madreporite and pedicellaria absent.
Example :- Antedon.
SUB-PHYLUM II ELEUTHEROZOA
- Free-living echinodenns.
- Oral surface directed downwards.
CLASS II ASTEROIDEA
- Commonly called as star fishes.
- Body star shaped central disc and arms not sharply demarcated
Order 1 Phanaerozonia
Order 2 Spinulosa
- Marginal plates indistinct.
- Pedicellariae pedunculate.
Example :- Solaster, Asterina.
Order 3 Forcipulata
- Pedicellariaes traught or crossed.
- Tube feet in 4 rows.
Example :- Pisaster, Asterias.
CLASS III OPHIUROIDEA
- Commonly called as brittle stars.
- Central disk and arms sharply demarcated.
Order 1 Streptophiurae
- Arms slender.
Example :- Ophiomyxa
- Arms slender.
Order 2 Zygophiurae
Order 3 Cladophiurae
- Arms much elongated and rolled as basket.
- Called as basket star.
Example :- Gorgonocephalus.
CLASS IV ECHINOIDEA
- Commonly called as sea urchins and sand dollars.
- Skeletal plates saturated forming a complete test.
- Body disk shaped, egg-shaped or heart shaped.
Sub-class A. Bothriocidarida (Extinct)
Sub-class B. Regularia
- Body globular, usually circular.
- 5 petaloids radially symmetrical.
- Locomotion by podia.
Order 1 Lepidocentroida
- Test flexible, imbricate or separate.
- Ambulacral plates continue on peristome.
Example :- Phoromosa.
Order 2 Melonechinada (Extinct)
Order 3 Cidaroidea
- Anus aboral, central.
- Peristomial gills absent.
- Rigid globular test with two rows of long narrow ambulacral plates and 2 rows of interambulacral plates.
Example :- Histocidaris, Apocidaris.
Order 4 Diadematoida
Sub-class C. Irregularia
- Body flattened on oral aboral axis in oval manner but not exactly symmetrical.
- Podia non-locomotor.
Order 1 Clypeastroida
Order 2 Spatangodia
- Commonly called as heart urchins.
- Ambulacral plates petaloid.
Example :- Echinocardium, Lovenia.
CLASS V HOTOTHUROIDEA
- Commonly called as sea cucumbers.
- Body elongated on oral-aboral axis with anterior mouth.
- Skeleton of microscopic plates embedded in body wall.
- Digestive system s-shaped Respiration by respiratory tree.
Order 1 Aspidochirota
- Tentacles branched anteriorly.
- Podia several.
- Respiratory tree present.
Example :- Holothuria, Stichopus.
Order 2 Elasipoda
- Tentacles branched distally.
- Respiratory tree absent. Tube feet.
- Modified as fins.
Example :- Pelagothuria.
Order 3 Dendrochirota
Order 4 Molpadonia
- Tentacles small hand shaped.
- Tube feet absent.
Example :- Molpadia.
Order 5 Apoda or Synaptida
- Tentacles present. Body transparent.
- Tube feet, Respiratory tree absent.
Example :- Synapta.