In this article we will discuss about the General Characters and Classification of Phylum Echinodermata.
General Characteristics of Phylum Echinodermata:
Some of the general characters of Phylum Echinodermata are listed below:
1. Habitat:
All existing echinoderms are marine. They generally live at sea bottom borne are pelagic (free swimming in open water) and a few are sessile (attached to the substratum).
2. Body Form:
It varies considerably. The body is star-shaped, spherical or cylindrical. It is un-segmented. The body lacks head.
3. Spines and Pedicellariae:
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.
4. Symmetry:
The symmetry is bilateral in larvae but pentamerous radial in adults i. e., body parts are arranged in fives or multiples of five.
5. Body Wall:
Epidermis is single layered and ciliated. In many echinoderms there is endoskeleton of calcareous plates in the dermis which are mesodermal in origin.
6. Body Cavity:
There is a true enterocoelic coelom.
7. Ambulacral System (= Water Vascular System):
Presence of ambulacral system is the characteristic feature of phylum echinodermata. A perforated plate called madreporite is present in this system. The pores of the madreporite allow water into the system Tube teet of this system help in locomotion, capture of food and respiration. Water vascular system is of coelomic origin.
8. Digestive Tract:
It is usually complete. Brittle stars have incomplete digestive tract.
9. Haemal and Perihaemal Systems:
Instead of blood vascular system, there are present haemal and perihaemal systems which are of coelome ongin.Thus the so called circulatory system is open type and includes haemal and perihaemal systems. The so blood is often without a respiratory pigment. There is no heart.
10. Respiratory Organs:
Gaseous exchange occurs by dermal branchae or papulae in star fishes peristominal gills in sea urchins, genital bursae in brittle stars, and cloac respiratory ‘trees in holothnrians. Exchange of gases also takes place through tnbe feet.
11. Excretory Organs:
Specialized excretory organs are absent. Nitrogenous wastes are diffused out via gills. Ammonia is chief excretory matter.
12. Nervous System:
It consists of a nerve ring and radial nerve cords. Brain as such is absent.
13. Sense Organs:
They are poorly developed.
14. Sexes and Fertilization:
Except a few individuals, the sexes are separate. There is no sexual dimorphism. Fertilization is usually external.
15. Asexual Reproduction:
Some forms reproduce asexually by self-division.
16. Autotomy and Regeneration:
Phenomena of autotomy and regeneration are often well marked in echinoderms.
17. Development:
The development is indirect and includes a ciliated, bilaterally symmetrical larva that undergoes metamorphosis to change into the radially symmetrical adult. Different larval forms are found which are mentioned in the classes of Echinodermata.
Unique Features:
(i) Presence of spines and pedicellariae.
(ii) Ambulacral system (water vascular system),
(iii) Haemal system,
(iv) Mesodermal endoskeleton of calcareous plates,
(v) Bilateral symmetry in the larva and radial symmetry in the adult.
Degenerate Characters:
(i) Lack of head,
(ii) Simple sense organs,
(iii) Incomplete digestive tract in some forms,
(iv) Reduced circulatory system,
(v) Absence of excretory system.
Resemblance with Chordates:
(i) Radial and indeterminate cleavage,
(ii) Gastrulation by invagination,
(iii) Mouth derived as an ectodermal invagination,
(iv) Adult anus derived from embryonic blastopore,
(v) Mesodermal endoskeleton.
(vi) Enterocoelous coelom.
(vii) Both are deuterostomes.
From these resemblances, it is clearly proved that the Echinoderms are nearer to the Chordates than any other group. It also indicates that the chordates have been evolved from Echinoderm-like ancestors.
Classification of Phylum Echinodermata:
Phylum Echinodermata is divided into five classes.
Class 1. Asteroidea (Gk. aster- star, eidos- form):
Body is star-like. Five arms are usually present which are not sharply marked off from the central disc. Larval forms are Bipinnaria and Brachiolaria. Examples: Asterias (Star fish), Pentacews (Star fish), Astrvpecten (Star fish).
Class 2. Ophiuroidea (Gk. Ophis- snake, Oura- tail, eidos- form):
Body is star-like. Arms are sharply marked off from the central disc. Ambulacral grooves”Sre absent. Pedicellariae are absent. Larval form is Ophiopluteus.
Examples:
Ophiothrix (brittle star), Ophioderma (brittle star), Ophiocoma (brittle star), Ophiura (brittle star).
Class 3. Echinoidea (Gk. echinos- hedgehog, eidos- form):
Body is globular or dislike. Biting and chewing apparatus with teeth called Aristotle’s Lantern is present. Ambulacral grooves are absent. Larval forms are Platens and Echinopluteus.
Examples:
Echinus (sea-urchin), Clypeaster (cake urchin), Echinarachinus (sand dollar), Echinocardium (heart urchin).
Class 4. Holothuroidea (Gk. Holothurion- sea cucumber, eidos- form):
Body is elongated and cylindrical. Oral end has mouth surrounded by tentacles. Ambulacral grooves are absent. Spines and pedicellaria are absent. Larval forms are Auricularia and Doliolaria.
Examples:
Holothuria (sea cucumber), Cucumaria (sea cucumber).
Class 5. Crinoidea (Gk. Crinon- lily, eidos- form):
Body has a central disc which is attached to the substratum. Arms are branched. Spines and pedicellariae and madreporite are absent. Larval form is Doliolaria. They are commonly called feather stars or sea lilies.
Example:
Antedon (feather star). Sea lilies.
Asterias (Star Fish):
It possesses great power of regeneration and shows autotomy. At the terminal end of each ambulacral groove lies a bright red eye. The aboral surface bears many stout spines distributed irregularly. In between the spines, there are present soft dermal branchiae.
They act as respiratory and excretory organs. In between two arms near the anus, there is present a perforated circular plate, the madreporite. There are present microscopic pincer-like structures known as pedicellariae. They also act as organs of offence.
Ophiura (Brittle star):
Brittle stars also swim like snake with their arms. Anus is absent.
Echinus (Sea urchin):
It moves with the help of spines. The sea urchin has a masticatory apparatus, called Aristotle’s Lantern because of its resemblance to ancient Greek ship lantern. It is formed by five strong and sharp teeth.
Cucumaria (Sea cucumber):
The sea cucumbers respire by respiratory trees in the cloaca. For this, water is drawn in and expelled through the anus alternately. The mouth is anterior and is surrounded by tentacles.
Antedon (Feather star):
It has great power of autotomy and regeneration. The body comprises a cup shaped central disc and five slender arms. Each arm is bifurcated, bearing a row of pinnules on each side. It is attached to the substratum.