Echinodermata
What Are Echinoderms?
• Definition: An invertebrate marine animal usually
characterized by a five-fold symmetry, and
possessing an internal skeleton of calcite plates,
and a complex water vascular system.
Includes:
• chinoids (sea urchins)
• crinoids (sea lilies)
• asteroids (starfish)
• Holothuroids (sea cucumbers)
• Ophiuroids (basket stars)
Sea Urchin
5 Classes of Echinoderms
1.
2.
3.
4.
5.
Class Asteroidia: Sea stars or Starfishes have arms that are supported by
elements in the body wall, not separable from a central disc. They usually have five
arms though some have more.
Class Ophiuroidea: Brittle and Basket stars have thin, flexible branching arms
that have internal structures for support and not attached to the animals' central
discs; use their entire arms, not just tube feet for locomotion.
Class Crinoidea: The Sea Lilies and Feather Stars, characterized by having 1) A
stalk and being attached to the substrate 2) Erect feeding structures on their upper
surfaces. They have upward and outward extending arms supported by calcareous
plates. 3) Crinoids are the only ones with their mouths, water-vascular system and
anus on the body surface away from the substratum. 4) Semi-rigid bodies
restricting them to suspension feeding.
Class Echinoidea: Urchins and Sand Dollar, body walls composed of plates
with moveable spines and no arms. Their overall shape is globular to oval.
Class Holothuroidea: Sea Cucumbers, feeding tentacles extending from their
circum-oral feeding ring, wide range of reduction in endoskeletons.
Symmetry, Germ Layers, and Coelum
• Adults have pentiradial symmetry
• 3 germ layers
• Considered “coelomates” because they
have a fluid-filled cavity between the
body wall and the gut
• Have well-developed coelum, forming the
• perivisceral cavity and the cavity of the
water-vascular system
Water-Vascular System
• Enclosed system within the body of an echinoderm
that contains many branches and conducts the
flow of water, which is mainly drawn into the
animal through a perforated plate. Branches
extend into the tube feet, which helps with
movement in the mobile animals
Movement
•
•
•
•
Mobile echinoderms are slow.
Some echinoderms, like sea lilies, don’t move at all
Move by using tube feet and pushing with their spines
Tube Feet are extended by using hydraulic pressure from
water drawn through the water vascular system, and
consists of two parts: ampulla and podia.
• Allow for sticking the floor of the ocean and slow movement
• Tube feet are used for both movement and feeding
• Tube Feet of a Starfish:
http://www.youtube.com/watch?v=4IRF-pKVtuU
Spines
• Many echinoderms contain internal spines
that are part of the internal skeletal
structure and are covered by the
epidermis
• These spines are used in combination with
the tube feet to propel the animals
forward
• Some, like the crown of thorns starfish,
have many external spines used for
protection
Development
• Begins with a bilaterally symmetrical embryo, with a
coeloblastula developing first
• “Second mouth” forms with gastrulation, the mesoderm
moves inwards, and the coelum forms
• Each taxon produces a distinct larvum, the left hand side of
which develops into the adult organism, the right hand side
eventually being absorbed; the left hand side typically
becomes the oral plate.
Blastulas and
gastrulas of a
starfish
Reproduction and Life Cycle
• Use both sexual and asexual reproduction
• Asexual Reproduction:
-Used mainly by sea stars and sea cucumbers
-Organisms cleave themselves into two or more parts, and
then re-grow the missing body parts
• Sexual Reproduction:
• Eggs and sperm are released into open water and fertilized
externally.
• The fertilized eggs develop into planktonic larvae and go
through two stages: bipinnaria and brachiolaria.
• Bilaterally symmetrical and have bands of cilia used in
swimming and feeding. As the larvae gradually metamorphose
into adults, a complex reorganization and degeneration of
internal organs occurs. The left side of the larva becomes
the oral surface of the adult, which faces down, and the
right side becomes the aboral surface, which faces up. The
larvae settle to the sea floor and adopt the adult pentaradial
symmetry.
Reproduction and Life Cycle
(Continued)
Starfish Digestion
• Digestion is carried out in two stomachs: the pyloric
stomach and the cardiac stomach
• Cardiac Stomach
-Can be pushed outside of the body to engulf food
-Sometimes the water vascular system is used to pry open
the shells of molusks; once inside the shell, the cardiac
stomach begins to swallow and digest the food. It is then
brought back inside the body, where the partially digested
food is sent to the pyloric stomach and then on to the
intestines and excreted through the anus or the mouth.
Starfish Stomachs
Tissues and Organs
• Contain a complete digestive tube (tubular
gut) that leads from mouth to anus
• Have an open and reduced circulatory
system but no heart
• Have a simple radial nervous system with a
modified nerve net, but no brain
• Some contain a ganglia
• Have gonads in various places in their
bodies
Interesting Facts
• Sea cucumbers:
http://www.youtube.com/watch?v=vsLBOkYLLe
I
• The word echinoderm means “spiny-skinned”
• A starfish is one of the only animals that can turn
its stomach inside out
• Echinoderms have no finite life expectancy and
no “old age”
Sources
• http://www.wetwebmedia.com/echinode.htm
• http://animaldiversity.ummz.umich.edu/site/accounts/information/Ec
hinodermata.html
• http://en.wikibooks.org/wiki/General_Biology/Animal_Evolution#Th
ree_Germ_Layers
• http://www-biol.paisley.ac.uk/biomedia/text/txt_echino.htm
• http://www.oceaninn.com/guides/echino.htm
• http://ebiomedia.com/prod/BOechinoderms.html
• http://en.wikipedia.org/wiki/Tube_feet
• http://www.dailykos.com/story/2006/10/20/201118/36
Picture
http://people.hws.edu/mitchell/oz/images96/Linckia.jpg
http://cache.eb.com/eb/image?id=72235&rendTypeId=35
http://www.oceaninn.com/guides/echino59.gif
http://oceanexplorer.noaa.gov/explorations/03edge/logs/aug27/media/sea
urchin_600.jpg