Echinoderms

Characteristics of Echinoderms

• Don't look/act like animals initially, they are
  • They move, attack prey, defend themselves, but tend to just be very slow about it
• They all share:
  • Radially symmetrical body divided into five parts
  • Move have hundreds of tiny tube feet to crawl/climb
  • Most have a water vascular system that brings oxygen to the body cells
• Echinoderms have some traits close to chordates:
  • Adult is radially symmetrical
  • Larvae is bilaterally symmetrical
  • Bilateral symmetry, along a vertical axis, is what mammals, fish, etc. have

Class Crinoidea

• Includes feather stars and sealilies
• Primary characteristics of this class:
  • Long, feather-like arms and short, hook-like legs called cirri
  • Upward facing mouths
    
  • Most nocturnal feeders, at night they unfurl their arms to capture plankton and nutrients carried into their paths by the current
  • By day, they coil up tightly and hide in the reef 
  • Most crinoids attach to the bottom by their cirri

Class Asteroidea

• Sea stars belong to this class
  
• Primary characteristics of this class:
  • Predators w/ downward facing mouths 
  • Tube feet covering their undersides
  • Usually have five arms
  • Few species have toxic spines for protection
  • Each arm carries an equal share of the animal's systems and organs
  • They can regenerate a lost limb, some grow into several new animals when cut into pieces

Class Ophiuroidea

• Sand dollars/sea urchins belong to this class
• Primary characteristics of this class:
  • Possess a five-sectioned body with no arms
  • Sand dollar/sea urchins share a disk-shaped body
  • Tube feet on the underside
  • Sea urchins graze on algae
  • Swimmers avoid sea urchins because of spines
    • Some species have toxins in the spines for self defense
    • Urchins can move their spines/tube feet for locomotion
• Shell is called a test 

Class Holothuroidea

• Sea cucumbers are part of this class
  
• Primary characteristics of this class:
  • Have elongated five segment body with tentacles around the mouth
  • Most feed by moving with their mouths open, allowing sand to flow through; a few are filter feeders
  • Some expel a sticky mass of white tubes covered in toxin
  • Protected by tough skin and the ability to expel part of their internal organs for predators while saving the rest to survive
  • They're also flshy. The things on them that look like spines are actually soft
  • Carnivorous
    • They invert their stomach through the mouth to envelope food
    • Intestine is short/missing
      • Sea urchins/sea cucumbers are the exception due to them feeding off plants
    • Body cavity is filled with a coelomic fluid
      • Coelomic - From the coelom, which is the main body cavity in most animals and is positioned inside the body to surround/contain the digestive tract and other organs.
        
    • Also serves to bring in oxygen
    • Sea cucumbers bring in water through anus to the respiratory branches
      • Extension of guts

Echinoderm Reproduction

• Echinoderms have separate sexes with 5, 10, or more gonads that shed sperm/eggs
  • Spawning
  • Gametes do not survive long in the water
    • Individual spawn all at once
  • Fertilized eggs develop into plankton and results in a ciliated larva
    • Some echinoderms carry eggs
• Asexual reproduction
  • Fission
    • Central disk splits into 2 new individuals
  • Regeneration
    • Ability to regrow missing parts
    • Requires that part of the central disk be present to grow a new individual
    • Sea stars do not require the disk