a) discrete arms with ambulacra (V-shaped groove containing the radial canal) restricted to the oral surface and clear pentaradial symmetry (crinoids, asteroids, ophiuroids)
b) arms absent, and ambulacra extend from the oral to aboral pole; varying degrees of bilateral symmetry (echinoids, holothuroids)
a) crinoids capture suspended particles with simple, mucus-covered podia bordering the pinnules of long feather-like arms; ciliated grooves move food to the mouth
b) the oral surface is oriented up in attached forms and down in most mobile forms
c) holothuroids capture suspended or deposited particles with large, branched, mucus-covered tentacles (modified buccal podia); the entire tentacle is drawn through the mouth to ingest food
d) irregular echinoids (sand-dollars, heart urchins) live in sediment; many small podia underneath spines move particles to branching grooves in the test and then along grooves to the mout
a) asteroids are mostly scavengers or carnivores; primitively, they swallow prey whole, but most can evert their stomach to feed
b) regular echinoids ('true' urchins) feed on algae & encrusting invertebrates with a remarkable scraping device: Aristotle's lantern
a) slender, jointed appendages (cirri in mobile crinoids)
b) coordinated tube feet (asteroids, regular echinoids, holothuroids)
c) rapid whip-like movement of arms (ophiuroids)
d) undulation of entire body (some holothuroids)
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Copyright © 1998 by A. Richard Palmer. All rights reserved.(revised April 3, 1998)