Echinoderms “Life with Spiny Skin”
A RadicalRadial Change • Worms, mollusks, and arthropods all have bilateral symmetry. • So do echinoderm larvae. • What kind of symmetry do adult echinoderms have? Bipinnaria larvae (sea star) Radial symmetry What other animal phyla display radial symmetry?
Five Alive • Most echinoderms show pentamerous radial symmetry. • This means there are usually five arms (or legs) or they are found in multiples of five. 2 1 3 5 4
Spiny Skin – but only the skin! • Echinoderm means “spiny skin.” • Echinoderms typically have: • well developed digestive tracts • well-developed coelom • Internal skeleton called an endoskeleton • Spines or bumps are anchored in the endoskeleton but may protrude through the skin.
What No Top and Bottom? Echinoderms have no head; therefore, there is no anterior or posterior. Rather than use the term dorsal, the side of the echinoderm without a mouth is called the aboral side. The echinoderm mouth is usually on the bottom. This is termed the oral side. aboral oral
Water vascular system • A network of water-filled canals that function in movement, feeding, and excretion. Water enters the echinoderm through the madreporite (mother pore) or sieve plate. Water is then forced through individual tube feet allowing them to move.
Types of Echinoderms • Approximately 7,000 species – all marine • Located from the poles to the tropics • Five major classes • Asteroidea (sea stars) – not starfish –they are definitely not “fish” • Ophiuroidea (brittle stars) • Echinoidea (sea urchins and sand dollars) • Holothuroidea (sea cucumbers) – not a vegetable! • Crinoidea (feather stars and sea lilies)
Asteroidea • Endoskeleton rather flexible to allow for movement. • Most have five arms radiating from a central disk.
Asteroidea • Hundreds of tube feet are found in channels called ambulacral grooves radiating from the central disk.
Asteroidea • Naturally the anus is on the aboral (top) side. • Aboral surface covered with pedicellariae – tiny pincer like organs that keep the sea star clean.
Ophiuroidea • Brittle stars • Most numerous class of echinoderms. • Characterized by thin, very flexible arms. • Eat particulate matter on the ocean floor. • No anus. • Often hidden. Ophiothrix spiculata
Echinoidea • Sea urchins & sand dollars • Endoskeleton is a rigid, shell-like “test.” • Covered with movable spines – used in locomotion and defense. • Grazers – feed on algae and dead organic matter. aboral oral
Echinoidea • An intricate mouth and jaw system called the Aristotle’s lantern consists of 50 bones and is controlled by over 60 muscles.
Echinoidea • Not all “urchins” have prominent spines. • Sand dollars have flattened bodies and tiny spines. • They use a mixture of mucus and physics to capture food.
Holothuroidea • Sea cucumbers • Elongated version of the pentamerous body plan. • Lie on side with five rows of tube feet on bottom. • Tough skin supported by calcareous spicules.
Holothuroidea • Tube feet near the mouth are modified into tentacles for feeding. • Some burrow and capture food while others ingest sand and filter out detritus and small organisms.
Holothuroidea • Defense • Secrete toxins • Discharge sticky toxic filaments • Eviscerate – eject a portion of the internal organs to confuse an attacker. Sea cucumber evisceration
Crinoidea • Feather stars, basket stars, sea lilies • Suspension feeders • Oral surface on top • Can have from 5 to 200 arms! • Have claw like appendages that hold the aboral surface to the substrate.