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Phylum Echinodermata

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  • Last Updated : 15 May, 2022

The word Echinoderm is taken from the Greek word “Ekhinos” which means “spiny” and “derm” means “skin”. The endoskeleton of these animals consists of Calciferous ossicles, which is why these are known as Echinoderms. This Phylum has an organ system level of organization. 

Characteristics of Echinodermata

  • Habitat: Echinoderms live in the marine habitat. They generally live at the bottom of the sea. They may be Sessile (fixed in one place) and pelagic (free-swimming). 
  • Body Structure: The body shape of the echinoderms varies greatly. They may be star-shaped (starfish), cylindrical (sea cucumber), or spherical (sea urchin). Their body lacks a head. 
  • Symmetry: The symmetry of adult echinoderms is radial, while of larvae it is bilateral. Radial symmetry means that the body parts are arranged in fives or multiples of five. 
  • Body Cavity: They have a true coelom and are triploblastic (the body is derived from 3 layers of cells: Ectoderm, Mesoderm, Endoderm). 
  • Epidermis: It is ciliated and single-layered. It contains hair-like projections (cilia) that produce a waving motion and helps to carry the food particles towards the mouth and remove unwanted particles from the body. Many of these animals bear spines that are protective and help to keep the body clean from debris and tiny organisms. 
  • Digestive System: Their digestive system is complete with a mouth on the lower side (ventrally located) and anus on the upper side (dorsally located). However, in brittle stars the digestive system is incomplete. 
  • Water Vascular System: Also known as Ambulacral System. This is the most distinctive characteristic of Echinoderms. This system consists of Tube-feet that help in locomotion, respiration, and food & waste transportation. 
  • Excretory System: This system is absent in Echinoderms. However, their nitrogenous waste which is Ammonia diffuses out through the respiratory surfaces, i.e., tube feet. 
  • Sexes and Fertilization: The sexes are separate in most of the Echinoderms. The reproduction takes place sexually. The fertilization is usually external. The larva is free-swimming and its development is indirect (in this type of development, the larva is hatched from the egg and it develops into a matured individual without undergoing metamorphosis). However, some echinoderms reproduce asexually by fragmentation, eg., sea stars. 

Classification of Echinodermata

The echinoderms are classified into 5 sub-classes: 

 

Asteroidea (Sea stars)

Characteristics: 

  • These are generally freely motile and use tube feet for locomotion. (Tube feet are small suction cups that are controlled by the water vascular system).
  • They have a flattened star-shaped body with 5 arms.
  • Young ones have bilateral symmetry, while adults have radial symmetry.
  • They have regeneration ability. 
  • Pedicellaria (defensive claw-shaped appendage with movable valves) is present. They help to clean the body of small parasites and debris free.
  • E.g., Asterias, Zoroaster, etc. 

Crinoidea (sea lilies or feather stars)

Characteristics:

  • They have radial symmetry.
  • Their body shape is cup-like with 5 or more bifurcated arms. The arms have feathery projections, known as pinnules, and contain reproductive organs and numerous tube feet.
  • Their tube feet have no suckers. 
  • Pedicellariae and spines are absent.
  • E.g., Articulata, Comatulida, etc. 

Echinoidea (sea urchins and sand dollars)

Characteristics:

  • They are radially symmetrical. 
  • They have an egg-shaped body that is hemispherical. 
  • They have skeletal plates with many movable spines and 3-jawed pedicellariae. 
  • They have tube feet but lack arms. 
  • E.g., Sea Urchin, Echinus, Cidaris, etc. 

Holothuroidea (sea cucumbers)

Characteristics:

  • These are soft-bodied and lack arms. 
  • They have bilateral symmetry. 
  • They are elongated and cylindrical in shape and possess tube feet with suckers. 
  • Spines and pedicellariae are absent. 
  • E.g., Holothuria, Royal Cucumber, etc.

Ophiuroidea (brittle stars and basket stars)

Characteristics:

  • Pentaradial symmetry is present. 
  • Their tube feet do not have suckers.
  • Respiration takes place from Bursae. 
  • They have an internal calcium carbonate skeleton. 
  • E.g., Euryalida, Ophiurida, etc. 

Conceptual Questions

Question 1: What makes a sea star turn rigid? 

Answer:

Tube feet in the water vascular system of sea stars become stiff when water is pushed into them, making the sea star rigid. 

Question 2: Which type of symmetry is found in Echinoderms? 

Answer:

Radial symmetry is found in adults, while Bilateral symmetry is found in the larval stage of the Echinodermata phylum. 

Question 3: What do you mean by Pedicellariae?

Answer: 

These are the defensive claw-shaped appendages with movable jaw-like valves. These help the organism to keep the body parasite and debris free. 

Question 4: Echinoderms occur in which type of habitat?

Answer:

They occur in marine habitats and are generally found at the bottom of the sea (benthic). 

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