||Zoology 250 (2017)
Feeding, locomotion, development
(Echinoderm study images;
click on tree to see full cladogram)
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- 1) Particle feeding is believed to be the primitive mode in echinoderms; the ancestral use of podia (tube feet) was for feeding not locomotion!
a) crinoids capture suspended particles with mucus-covered podia on pinnules of feather-like arms; ciliated grooves move food to mouth
b) ophiuroids capture suspended or deposited particles with simple, mucous-covered podia or spines bordering the arms or mucous nets; a bolus of particles is moved toward the mouth by podia; curiously, they lack an intestine and anus!
c) irregular echinoids (sand-dollars, heart urchins) live in sediment; small podia under spines move particles along grooves to mouth
d) holothuroids capture suspended or deposited particles with large, branched, mucous-covered tentacles (modified buccal podia); the entire tentacle is drawn through the mouth to ingest food
- 2) 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 and encrusting invertebrates with a remarkable scraping device: Aristotle's lantern
- 3) Primitive crinoids were sessile, with jointed cirri on a long stalk; but 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 arm movement (ophiuroids), swimming (crinoids)
d) undulation of entire body (some holothuroids)
- 4) Respiration and excretion: all use skin and podia; some have other structures; a lack of excretory structures precludes life in fresh water
- 5) Most possess a tissue with odd abilities: mutable collagenous tissue
- 6) All have spectacular ability to regenerate body parts; some sea cucumbers can eviscerate or shoot Cuverian tubules if attacked
- 7) Sexes are separate and fertilization is typically external
a) early larva is bilaterally symmetrical and primitively exhibits a tri-partite coelom (axocoel, hydrocoel, somatocoel): fused axo-hydrocoel yields water-vascular system, somatocoel yields perivisceral coelom
b) many groups have complex bilaterally symmetrical(!) feeding larvae (echinoplutei, ophioplutei, auricularia, bipinnaria); some larvae undergo dramatic metamorphosis; some larvae show dramatic metamorphosis; some feeding larvae can clone themselves!
c) in all classes except crinoids, feeding larvae can clone themselves!
||View a stunning movie of fertilization, cleavage, and metamorphosis in a sea biscuit (irregular urchin) by Bruno Vellutini.
- 7) Phylogenetic relations among classes fairly stable
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Copyright (c) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016)