Phylum: Echinodermata, its classification and characteristics

• exclusively marine.
• multicellular, system grade of body organization.
• triploblastic, coelomate and radially symmetrical.
• free living, free swimming or sedentary.
• star shaped, oval shaped, globular, or discoidal with distinct oral or aboral surface.
• literally means spiny skinned animals.
• body surface is marked by five distinct ambulacral grooves and alternating five interambulacral area.
• water vascular system, consists of tubes filled with watery fluid.
• locomotion by tube feet which also helps in excretion, exchange of gases and capturing prey.
• definite excretory organs absent.
• respiration by papulae (skin gills) in star fish, periostomial gills in sea urchin, genital brussae in brittle star, cloacal respiratory tree in holothorians.
• mostly unisexual.
• development is indirect, great power of regeneration.

Image source: biologydictionary

Classification

Echinodermata is divided into two sub phylum on the basis of habit. They are:

Sub-phylum 1: Eleutherozoa

It includes all free swimming echinoderms. It is further divided into four classes. They are:

Class 1: Asteroidea

• body is star shaped with five arms.
• arms are not marked off from central disc.
• open ambulacral grooves.
• tube feet with suckers.
• examples: Asteriae (star fish), Astropecten (sea fur)

Image source: quarkscience

Class 2: Ophiuroidea

• body is flat and star shaped with five arms.
• arms are separated from central disc.
• ambulacral grooves absent or covered by ossicles.
• tube feet without suckers.
• examples: Ophioderma (Brittle star), Ophiothrix (spiny brittle star)

Class 3: Echinodea

• body is oval, globular, discoidal or heart shaped without arms.
• ambulacral grooves covered by ossicles.
• tube feet with sucker.
• examples:  Echinus (sea urchin), Echinocardium (Heart urchin)

Class 4:  Holothuroidea

• body elongated without arms and spines.
• mouth at anterior end of body surrounded by tentacles.
• longitudinal ambulacral grooves which are concealed.
• examples:  Cucumaria (sea cucumber), Holothuria, Mesothuria

Sub-phylum 2: Pelmatozoa

It consists of  stalked and sedentary echinoderms with one class.

Class 1: Crinoidea

• body is attached to substratum with aboral stalk.
• five arms bifurcated at base.
• open ambulacral grooves covered by leaf like pinules.
• tube feet without suckers.
• examples:  Antedon (sea lily), Neometra (feather star)

References: 

i) https://biologydictionary.net/echinoderm/

ii) https://www.microscopemaster.com/phylum-echinodermata.html

Phylum: Echinodermata, its classification and characteristics