Phylum Echinodermata

• Name means spiny skin
• Examples include sea stars, urchins, sea cucumbers, sand dollars
• General characteristics: radial symmetry, non-segmented, bodies exhibit spiny skin, endoskeleton made of small bony plates, grooved bodies - usually containing tube feet, mouth centrally located on ventral surface (usually), sieve plate on the dorsal wall to provide water to a developed water vascular system, deuterostomes
• the body of the sea star is arranged around a water vascular system.
• on the dorsal surface is an inlet for water called the madreporite or sieve plate. this pressurizes the ring canal with water and distributes water to thousands of tube feet arranged along the radial canals.
• each foot acts like a tiny suction cup
• internal structural support is provided by overlapping small bony plates called ossicles. these structures are connected together by tiny muscles allowing the sea star to move and bend each arm in any direction
• the central stomach in sea stars can be inverted into the shell of a captive mollusc to digest their prey alive
• the tube feet can exert tremendous pressure on a bivalve shell, allowing for the sea star to feed
• the stomach pours out digestive enzymes, and once finished feeding, retracts back into the sea star's body.
• The anus is located on the dorsal surface.
• many echinoderms have frills of soft tissue that extend between the ossicles and provide large amounts of surface area for gas exchange. (in water vascular system)
• echinoderms have ring-shaped series of ganglia around the mouth that connect to radial nerves that extend down each arm. They have no brain or cephalization
• fluid fills the body cavity and is moved by cilia that line the coelomic space. this fluid circulates around the organs and transports nutrients, gases, and wastes. they have no blood or heart.
• asexual reproduction: fragmentation
• external fertilization: sex organs are distributed along each arm (or 'ray'), and both male and female gametes are released into water through pores on the dorsal surface.
• sea stars evolved from animals with bilateral symmetry, therefore became more simple by evolution
• this phylum is the most closely related phyla to Chordates.