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Phylum Echinodermata

Introduction:

  • There are about 6000 species belonging to phylum Echinodermata.
  • The phylum includes spiny skinned animals.

Topics covered

  • General characteristics of phylum Echinodermata
  • Classification of phylum Echinodermata
  • Examples

General characteristics of phylum Echinodermata
 

Detailed explanation:
 

Habit and habitat

  • Exclusively marine.
  • They are slow moving and generally found in the bottom of sea.

Level of organisation

  • Organ-system level of organisation.

Symmetry

  • Their adults are radially symmetrical but their larvae are bilaterally symmetrical.

Germ layers

  • Triploblastic animals i.e., the body consists of three germ layers.

Body form

  • Unsegmented body without distinct head.

Body plan

  • Tube-within-tube body plan.

Body wall

  • Body wall consists of a single layered epidermis that is usually ciliated.
  • Body surface has spines and an endoskeleton of calcareous ossicles which helps in protection.
  • Body of some Echinoderms also bears pincer-like pedicellariae which keeps their body surface clear of debris and minute organisms.

Coelom (body cavity)

  • Eucoelomates i.e true coelom is present.
  • Enterocoelomates like chordata.

Digestion

  • Digestive system is complete.
  • Mouth is present on the ventral (lower) surface while anus is present at the dorsal (upper) surface.

Respiration

  • Exchange of gas occurs through tube feet.
  • Respiration also takes place by body surface or gills called dermal branchiae or populae.eg., star fish
  • Respiration occurs by peristomial gills in sea-urchin.

Excretion

  • The specialized excretory organ is absent in Echinoderms.
  • Excretion takes place through general body surface
  • Ammonia is the chief nitrogenous waste.

Circulatory system

  • Circulatory system is a reduced and open type that lacks a heart or pumping vessel.

Nervous system

  • Nervous system is a simple and diffused type which consists of a nerve ring and radial nerves.
  • They lack a brain and head.
  • Sense organs absent.

Reproductive System

  • Reproduction is sexual.
  • Mostly unisexual i.e sexes are separate in them.
  • Fertilization is external.

Development

  • Development is indirect.
  • Free swimming larvae bear similarity with the larva of Echinodermata and Chordata.

Special characteristics of phylum Echinodermata

  • Autotomy and regeneration
    - It is a well-marked characteristic feature of phylum Echinodermata.
    - As defence response organisms remove their own body parts,if caught by Predator.
    - Removed part regenerates later.
     
  • Water Vascular System or Ambulacral System
    - It is the most distinctive 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 feet of this system help in locomotion, capture of food and respiration.
    - It is of coelomic origin.

    water-vascular-system

     
  • Haemal and Perihaemal Systems
    - As the blood vascular system is absent.
    - The circulatory system which is open type and includes haemal and perihaemal systems, which are of coelomic origin
    - Blood is without a respiratory pigment.

Classification of phylum Echinodermata
 

Detailed explanation:
 

Phylum Echinodermata is classified into 5 classes -
 

1. Asteroidea

  • Their bodies are star-like.
  • Five arms are present which are not sharply marked off from the central disc.
  • Larvae formed are Bipinnaria and Brachiolaria.
  • Examples : Asterias (Star fish)

2. Ophiuroidea

  • Their bodies are also star-like but their arms are sharply marked off from the central disc.
  • Pedicellariae are absent.
  • The Larva formed is Ophiopluteus.
  • Examples : Ophiothrix (brittle star), Ophiura (brittle star)

3. Echinoidea

  • Their body is globular or disc- like.
  • Aristotle's Lantern is present which contains teeth for biting and chewing.
  • The Larva formed is Echinopluteus.
  • Examples : Echinus (sea-urchin),

4. Holothuroidea

  • Their body is elongated and cylindrical.
  • Structures like Spines and pedicellariae are absent.
  • The Larva formed are Auricularia and Doliolaria.
  • Examples : Holothuria (sea cucumber), Cucumaria (sea cucumber)

5. Crinoidea

  • They are commonly called feather stars or sea lilies.
  • Their body consists of a central disc which is attached to the substratum.
  • Structures like spines, pedicellariae and madreporite are absent.
  • The Larva formed is Doliolaria.
  • Examples : Antedon (feather star), Sea lilies.

Examples
 

Detailed explanation:
 

1. Asterias

  • They are commonly known as Starfish.
  • It possesses a great power of regeneration and shows autotomy.
  • The aboral surface bears many stout spines which are distributed irregularly and in between the spines soft dermal branchiae are present.
  • The soft dermal branchiae helps in respiration and excretion.

    asterias
     

2. Ophiura

  • They are commonly known as brittle star.
  • They swim like snakes with the help of arms and anus is absent in them.

    ophiura
     

3. Echinus

  • They are commonly known as sea urchin
  • Movement takes place with the help of spines.
  • Arms are absent in them.
  • Mouth is Aristotle's Lantern which is formed by five strong and sharp teeth.

    echinus
     

4. Cucumaria

  • They are commonly known as Sea cucumber.
  • They respire by respiratory trees in the cloaca.

    cucumaria
     

5. Holothuria

  • Commonly known as Sea cucumber.
  • Arms are absent in them.

6. Antedon

  • Commonly known as Sea lily.

    antedon
     

7. Ophiothrix

  • They are commonly known as Brittle star.

Frequently Asked Questions: FAQ
 

Q1. Explain Autotomy and regeneration
Ans :

  • It is a well-marked characteristic feature of phylum Echinodermata.
  • As defence response organisms remove their own body parts,if caught by Predator.
  • Removed part regenerates later.

Q2. Explain Water Vascular System or Ambulacral System.
Ans :

  • It is the most distinctive 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 feet of this system help in locomotion, capture of food and respiration.
  • It is of coelomic origin.

Q3. What are Haemal and Perihaemal Systems ?
Ans :

  • As the blood vascular system is absent in organisms belonging to phylum Echinodermata.
  • The circulatory system which is open type and includes haemal and perihaemal systems, which are of coelomic origin.
  • Blood is without a respiratory pigment.

Q4. Which is the larvae formed in the life cycle of star fish ?
Ans :

  • Larvae formed in the life cycle of star fish are Bipinnaria and Brachiolaria.

Q5. What is the common name of Cucumaria ?
Ans :
Sea cucumber

Q6. Which is the respiratory organ of the star fish ?
Ans :

  • Respiration in starfish takes place by body surface or gills called dermal branchiae.
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