Phylum Echinodermata

General Characters:

Triploblastic, enterocoelous coelomates with pentamerous radial symmetry having a calcareous endoskeleton of separate plates embedded in the skin.

Exclusively marine.

Body unsegmented with globular, star-like, spherical, discoidal or elongated shape.

Head absent; body surface is marked by five symmetrically radiating areas (ambulacra) and five alternating inter-radii (inter-ambulacra).

Many echinoderms bear spines and pincer-like pedicellariae. The spines are protective in function. The pedicellariae keep the body surface clear of debris and minute organisms.

Body wall consists of an outer epidermis (single-layered and ciliated), a middle dermis & an inner lining of peritoneum.

Alimentary canal straight or coiled.

Respiratory organs include dermal branchiae (eg. starfish), tubefeet, respiratory tree (e.g., Holothuria), bursae (e.g., brittle star) & peristomial gills system.

Presence of ambulacral system or water vascular system is the most characteristic feature. A perforated plate called madreporite, allows water into the system. Water vascular system is of coelomic origin.

Tube feet help in locomotion.

The circulatory system is greatly reduced & is of open type. It is called haemal system. Blood often lacks a respiratory pigment. There is no heart.

Excretory system is wanting. Nitrogenous waste diffuses out via gills.

The nervous system includes a nerve ring & radial nerve cords. There is no brain.

Poorly developed sense organs include tactile organs, chemoreceptors, terminal tentacles, photoreceptors & statocysts.

Reproduction both asexual & sexual. Sexes are usually separate with few exceptions. Copulation does not occur. Fertilization is external.

Development indirect through free-swimming larval forms. The bilaterally symmetrical larva undergoes metamorphosis to change into the radially symmetrical adult.

Echinoderms resemble chordates in early embryonic development.

Classification of Echinoderms

Phylum Echinodermata is divided into five classes:

Class 1 : Asteroidea

Dorsoventrally flattened having a central disc from which generally five arms arise.

The oral surface is below, which has a mouth on the central disc and an ambulacral groove in each arm. The aboral surface is above having the anus at the centre and madreporite.

Tube feet are locomotory. Pedicellariae are present. All are carnivorous e.g., Asterias (starfish), Pentaceros (sea pentagon).

Class 2 : Ophiuroidea

Brittle stars with a small central disc and five slender flexible arms. Mouth and madreporite are present on the oral surface. Anus and pedicellariae are absent.

They often capture small prey with their arms. Egestion takes place through mouth. The tube feet are reduced and are sensory. They use their arms for locomotion e.g., Ophiothrix (Spiny brittle star).

Class 3 : Echinoidea

Globular or discoidal in shape.

They have long spines used in fast locomotion.

The mouth is present at the lower oral surface while the anus and madreporite found on the aboral surface. Pedicellariae are present e.g., Echinus (Sea urchin).

Class 4 : Holothuroidea

Elongated and cylindrical body with mouth and anus at the opposite ends.

They are without spines and pedicellariae.

The body wall bears minute ossicles or spicules. The mouth is surrounded by tentacles. There are no arms. The tube feet are with suckers e.g., Holothuria, Cucunmaria (sea cucumber).

Class 5 : Crinoidea

There is present a central disc of calcareous plates from which five arms arise.

Each arm bifurcates and bears lateral pinules.

Tentacles are present. Pedicellariae, madreporite and spines are absent e.g., Antedon.

Further Reading : 

Bryophyte

Pteridophyta

Important point of Phylum Echinodermata