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Phylum Echinodermata

  • All marine
  • Known as spiny-skinned animals
  • Endoskeleton known as the test is made of calcium plates or ossicles with protruding spines
  • Includes sea stars, brittle stars, sand dollars, sea urchins, & sea cucumbers
  • Undergo metamorphosis from bilateral, free-swimming larva to sessile or sedentary adult
  • Larval stage known as dipleurula or bipinnaria
  • Adults have pentaradial ( 5 part) symmetry
  • Lack segmentation or metamerism
  • Coelomate
  • Breathe through skin gills as adults
  • Capable of extensive regeneration

Bipinnaria Larva

  • Ventral (lower) surface called the oral surface & where mouth is located
  • Dorsal (upper) surface known as aboral surface & where anus is located
  • Have a nervous system but no head or brain in adults
  • No circulatory, respiratory, or excretory systems
  • Have a network of water-filled canals called the water vascular system to help move & feed
  • Tube feet on the underside of arms help in moving & feeding
  • One-way digestive system consists of mouth with oral spines, gut, & anus
  • Deuterostomes (blastopore becomes the anus)
  • Separate sexes
  • Reproduce sexually & asexually
  • Includes 5 classes:
    * Crinoidea – sea lilies & feather stars
    * Asteriodea – starfish
    * Ophiuroidea – basket stars & brittle stars
    * Echinoidea – sea urchins & sand dollars
    * Holothuroidea – sea cucumbers

Class Crinoidea

  • Sessile
  • Sea lilies & feather stars





  • Have a long stalk with branching arms that attach them to rocks & the ocean bottom
  • Can detach & move around
  • Mouth & anus on upper surface
  • May have 5 to 200 arms with sticky tube feet to help capture food (filter feeders) & take in oxygen
  • Common in areas with strong currents & usually nocturnal feeders

Class Asteroidea

  • Usually sedentary along shorelines
  • Starfish or sea stars
  • Come in a variety of colors
  • Prey on bivalve mollusks such as clams & oysters

Starfish Feeding on Clam

  • Have 5 arms that can be regenerated
  • Arms project from the central disk
  • Mouth on oral surface (underside)


Class Ophiuroidea

  • Largest class of echinoderms
  • Includes basket stars & brittle stars





  • Live on the ocean bottom beneath stones, in crevices, or in holes
  • Have long, narrow arms resembling a tangle of snakes
  • Arms readily break off & regenerate
  • Move quicker than starfish
  • Feed by raking in food with arms or trapping it with its tube feet

Class Echinoidea

  • Includes sea urchins & sand dollars





  • Internal organs enclosed by endoskeleton or test made of fused skeletal plates
  • Body shaped like a sphere (sea urchin) or a flattened disk (sand dollar)
  • Lack arms
  • Bodies covered with movable spines
  • Have a jawlike, crushing structure called Aristotle’s lantern to grind food
  • Use tube feet to move
  • Sea Urchins:
    * Spherical shape
    * Live on ocean bottom
    * Scrape algae to feed
    * Long, barbed spines make venom for protection
  • Sand Dollars:
    * Flattened body
    * Live in sand along coastlines
    * Shallow burrowers
    * Have short spines

Class Holothuroidea

  • Includes sea cucumber


  • Lack arms
  • Shaped like a pickle or cucumber
  • Live on ocean bottoms hiding in caves during the day 
  • Have a soft body with a tough, leathery outer skin
  • Five rows of tube feet run lengthwise on the aboral (top) surface of the body
  • Have a fringe of tentacles (modified tube feet) surrounding the mouth to sweep in food & water
  • Tentacles have sticky ends to collect plankton
  • Show bilateral symmetry
  • Can eject parts of their internal organs (evisceration) to scare predators; regenerate these structures in days

Structure & Function of Starfish
Body Plan

  • Range in size from 1 centimeter to 1 meter
  • Mouth located on oral surface (underside)
  • Have an endoskeleton made of calcium plates
  • Sharp, protective spines made of calcium plates called ossicles found under the skin on the aboral (top) surface


  • Have pedicellariae or tiny, forcep-like structures surrounding their spines to help clean the body surface

Water Vascular System

  • Network of canals creating hydrostatic pressure to help the starfish move


  • Water enters through sieve plate or madreporite on aboral surface into a short, straight stone canal
  • Stone canal connects to a circular canal around the mouth called the ring canal
  • Five radial canals extend down each arm & are connected to the ring canal
  • Radial canals carry water to hundreds of paired tube feet


  • Bulb-like sacs or ampulla on the upper end of each tube foot contract & create suction to help move, attach, or open bivalves
  • Rows of tube feet on oral surface (underside) are found in ambulcaral grooves under each arm

Tube Feet in Ambulcaral Grooves

Feeding & Digestion

  • Tube feet attach to bivalve mollusk shells & create suction to pull valves apart slightly
  • Starfish everts (turns inside out) its stomach through its mouth & inserts it into prey
  • Stomach secretes enzymes to partially digest bivalve then stomach withdrawn & digestion completed inside starfish

Other Body Systems

  • No circulatory, excretory, or respiratory systems
  • Coelomic fluid bathes organs & distributes food & oxygen
  • Gas exchange occurs through skin gills & diffusion into the tube feet
  • No head or brain
  • Have a nerve ring surrounding the mouth that branch into nerve cords down each arm
  • Eyespots on the tips of each arm detect light
  • Tube feet respond to touch


  • Separate sexes
  • Two gonads (ovaries or testes) in each arm produce eggs or sperm
  • Have external fertilization
  • Females produce up to 200,000,000 eggs per season
  • Fertilized eggs hatch into bipinnaria larva which settles to the bottom after 2 years & changes into adult
  • Asexually reproduce by regenerating arms