Phylum Echinodermata

The "spiny skinned creatures". Our closest living relatives…ugly aren’t they?



  1. Unifying Characteristics
    1. Exclusively bilateral (as larvae) and free-living
    2. Larvae bilaterally symmetrical with a complete metamorphosis
    3. No head; secondary radial symmetry as adults
    4. Triploblastic, Dueterostomate, Coelomate
    5. Water vascular system = a division of the coelom modified into canals and tube feet; used for locomotion, food capture, and respiration; Structures: transverse tube feet, ring canal, radial canals, ampullae, podium, sucker
    6. Skeleton internal and external, composed of ossicles of calcium carbonate
    7. No excretory organs
    8. Sexes separate, fertilization internal
    9. Radial development, follows deuterostomate type development in which the anus is derived from the blastopore.



  1. Classification
    1. Class Crinoidea (Sea Lillies)
      1. most primitive class
      2. usually deep in ocean habitats
      3. have functional tube feet on oral surface and aboral surface; used for locomotion
      4. filter feed by mucus capture
    1. Class Ophiuroidea (Brittle Stars, Basket Stars)
        1. heavily ossified, not much dermal tissue exposed to environment
        2. common in all marine waters, but more common in shallow waters.
        3. central disk far from arms
        4. locomotion not be tube feet, wave arms
        5. release mucus nets to capture food
        6. most benthic
        7. smaller than Starfish
    2. Class Holothuroidea (Sea Cucumbers)
        1. most advanced class
        2. soft body, ossicles greatly reduced
        3. long, ovoid shape
        4. secondarily bilaterally symmetrical
        5. distinct, dorsal & ventral surfaces
        6. tube feet more distance on ventral surface
        7. deposit and filter feeders
        8. mouth surrounded by tentacles
        9. respiratory tree internal, used in respiration, attached to intestine
        10. defense mechanism expels respiratory tree and intestine
        11. good regenerative capabilities
    3. Class Echinoidea (Sea Urchins and Sand Dollars)
        1. sea urchins are grazers
        2. sand dollars are filter feeders
        3. fused 5 radial arms into one
        4. hard structure called test
        5. spines are modifications of ossicles
        6. have poison glands associated with spines
        7. Aristotle’s lantern is an internal structure shaped like a beak used as feeding apparatus

    4. Class Asteroidea ( Sea Stars)
        1. distinct pentamerous shape
        2. central not well set off from arms
        3. shallow intertidal marine waters on rocky substrates
        4. predacious – favorite food is bivalves
        5. evertable stomach
        6. aboral surface spiny
        7. pedicillaria – function to protect dermal glands, cleaning, and capture of small animals
        8. scissor action of arms is defensive mechanism used to prevent fouling and settling of organisms
        9. madreporite is a pressure release valve connected
        10. regeneration capability
        11. beginning of deuterostome qualities12.significant evolutionarily