Zoology 250 Lecture 28 ECHINODERMATA II
- 1) Though phylogenetic relations are controversial, 2 body plans prevail
- 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)
- 2) Particle feeding is believed to be the primitive mode in echinoderms
- 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) ophiuroids capture suspended or deposited particles with simple, mucus-covered podia or spines bordering the arms or mucus nets; a bolus of particles is moved toward the mouth by podia
- 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 mouth
- 3) Some groups feed on larger items
- 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
- 4) Primitive crinoids were sessile, with jointed cirri on a long stalk
- 5) The remaining echinoderms move in a great variety of ways
- 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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