The Basics:
  • Echino” – Spiny, “Derm” – Skin
  • Many have spiny skin (i.e. sea urchins, or the internal skeleton of sea stars) but many do not.
  • Appeared between 541-488 Million Years Ago (Cambrian Explosion – same as chordates)
  • Coelomates with deuterostome development.
  • Most closely related phylum to chordates!

A. Evolution of Body Plan

1. Deuterostome development

    • During embryonic development the blastopore develops into the anus.
    • Largest group of deuterostomes outside of Chordates

2. Water Vascular System

  • Unique system of fluid-filled canals and tubes that help with feeding, movement and respiration.
  • Only found in echinoderms.

Major Parts of the water vascular system:

  • Tube Feet – Have a ampullae a “balloon” that fills with water causing the foot with a suction cup to extend and stick to prey/ground.
  • Sieve Plate – The entrance for water into the water vascular system (also called a madreporite).
  • Ring Canal – Central canal where all canals branch from.

3. Endoskeleton – Always covered in a thin layer of skin therefore it is a endoskeleton (like us!)

  • Made of calcium carbonate
  • Can be loosely connected for movement (sea stars)
  • Fused to form a rigid skeleton (sea urchins)

4. Radial Symmetry Symmetry around a central axis. In echinoderms they have 5 symmetrical sections (pentaradial symmetry)

Important Note:

It is important to note that echinoderms developed from a bilateral ancestor and developed radial symmetry secondary.

  • We can see this in their larval stages which are all bilaterally symmetrical!

B. Feeding and Excretion

One-Way digestive system

  • Typically have two stomachs – a cardiac stomach which can be moved outside of the body begin digestion and a pyloric stomach.

Due to this, sea stars can digest prey much larger than their stomach since they begin digestion outside their body.

  • Powerful enzymes liquify prey – digested completely within the stomachs.


  • Digestive waste exits the anus on opposite side of mouth.
  • No distinct excretory organs – nitrogenous waste is removed from the water vascular system via diffusion through tube feet or with digestive waste.
C. Respiration and Circulation


  • Use water vascular system in respiration and circulation.
  • Gas exchange occurs through simple gills and diffusion through tube feet.


  • Reduced circulatory system.
  • All fluids and “blood” circle throughout the hydraulic-like water vascular system. Blood has no pigment (i.e. no colour)
  • No heart!

D. Reproduction

Sexual Reproduction:

  • Male and female are separate individuals.
  • External fertilization through broadcast spawning
  • Females can release up to 100 million eggs at one time.

Sexual Reproduction – brief life cycle:

  • Fertilized embryo develops into bilateral, planktonic larvae.

The planktonic larvae gradually change into radially symmetrical adults under the course of two-months and settle on the ocean floor.

Asexual reproduction:

  • Asexual reproduction through fragmentation or fission (rare)
  • Due to repetition of organs in each area and a nerve disk, echinoderms can regenerate if they are fragmented with a piece of their central disk.

E. Major Groups of Echinoderms

Class Asteroidea (Sea Stars)

  • Predatory
  • Two-stomachs (pyloric and cardiac)
  • 5 or more arms that are wide near the central disk and gradually get thinner.

Class Opiuroidea (Brittle Stars)

  • 5 or more thin, flexible arms.
  • Many are filter feeders.
  • All organs in central disk – not in arms

Class Echinoidea (Sea Urchins)

  • Round body covered in spines
  • Hard, fused endoskeleton

Class Holothuroidea (Sea Cucumbers)

    • Appear bilaterally symmetrical, although they are still radially symmetrical (dorsal-ventrally stretched).
    • Tube shaped detritivores.

Class Crinoidea (Crinoids – feather stars)

  • Cup-shaped body with many feathery arms.
  • Filter feeders