• Most echinoderms develop from free-swimming, bilaterally symmetrical larvae into bottom-dwelling adults with pentaradial symmetry. The larval stage is evidence that echinoderms may have evolved from bilaterally symmetrical ancestors.
• Echinoderms have an endoskeleton made of ossicles and a water-vascular system, which includes many movable extensions called tube feet.
• The class Crinoidea includes sea lilies and feather stars, which are filter feeders that catch small organisms with their mucus-covered tube feet.
• The class Ophiuroidea consists of basket stars and brittle stars, fast-moving echinoderms with long, flexible arms.
• The class Echinoidea includes sea urchins and sand dollars, whose internal organs are enclosed inside a rigid endoskeleton called a test. Many sea urchins have long spines.
• The class Holothuroidea is made of sea cucumbers, armless echinoderms with soft bodies.
• The class Asteroidea consists of sea stars, which have from 5 to 24 arms. Two rows of tube feet run along the underside of each arm.
• The water-vascular system of a sea star consists of a network of canals that connect to bulblike ampullae. Contraction of muscles surrounding the ampullae extends the tube feet, and contraction of muscles lining the tube feet makes the tube feet retract.
• Sea stars can turn one of their stomachs inside out through their mouth to feed on prey they have captured. After the food is partially digested outside the body, it is brought inside, where digestion is completed.
• Sea stars lack circulatory, excretory, and respiratory organ systems, and they have no head or brain. They use skin gills for gas exchange and waste excretion.
• Most sea stars have separate sexes, and fertilization is external. Sea stars can also reproduce asexually by regeneration.
Vocabulary echinoderm (p. 761) ossicle (p. 762) water-vascular system (p. 762)
tube foot (p. 762) test (p. 763) pedicellaria (p. 765) madreporite (p. 766)
stone canal (p. 766) ring canal (p. 766) radial canal (p. 766) ampulla (p. 766)
cardiac stomach (p. 766) pyloric stomach (p. 766) bipinnaria (p. 767)
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