Chapter 23 - Echinodermata

Notes for Chapter 23:
Echinoderms

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Chapter 23 Assignment: 458-471; 476-478; RQ-23: 1-4, 6-7, 11

Introduction: A Design to Puzzle the Zoologist

Source of Image

Featured Animal: Pisaster ochraceus (ochre seastar)
More Links: 1 - 2 - 3 - 4 - 5 - 6 - 7

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The following outline of lecture notes does not follow Ch. 23 headings

I. Echinoderms:

Key terms: pentamerous symmetry, ambulacra, water-vascular system, tube feet, pedicellaria, papulae, ampullae, madreporite, ossicles, stereom, pyloric ceca, pluteus larva, bipinnaria larva, brachiolaria larva, hydrocoel, trimeric paired coeloms (a deuterostome feature)

  symmetry: think 5 except in larvae/fossils
  skeleton: endo/collagen
  water vascular system: uniquely echinoderm
  nerves: no brain, a diffuse nervous net
  respiration: extensions of coelom
  fat storage: pyloric ceca
  larvae: pluteus vs. bipinnaria

II. Introduction to Deuterostomes

Current classification of bilaterian animals:

Bilateria
  Protostomia
     Lophotrochozoa (flatworms, molluscs, annelids)
     Ecdysozoa (arthropods, nematodes)
  Deuterostomia (echinoderms, hemichordates, chordates)

Deuterostome features (see p. 459):
   • blastopore (usually) becomes the anus, never becomes the mouth
   • coelom forms through enterocoely (budded off archenteron - p. 163)
   • three pairs of coelomic pouches in larva
       (as in hemichordate larvae - p. 484)
   • endomesoderm (mesoderm from gut through enterocoely)
   • skeleton from mesoderm (not from ectoderm as in protostomes)
   • radial cleavage
   • regulative (indeterminate) cleavage
      (4-cell stage cells still form "normal" larva - see Fig. 8-14, p. 168)

III. Classification of Echinoderms (living groups only - see p. 476)
    Echinodermata
       Crinoidea (sea lilies and feather stars)
       Eleutherozoa (mouth faces down)
          Asteroidea (sea stars)
          Concentricycloidea (sea daisies)
          Ophiuroidea (brittle stars)
          Echinoidea (sea urchins)
          Holothuroidea (sea cucumbers)

IV. Diversity:
   About 6,000 living species
   About 20,000 fossil species
   "Stereom" ossicles – characteristic crystalline structure/composition
       (high-magnesium)
   Paleontologists can easily identify fossils as echinoderm ossicles
   Early Echinoderms did not have radial symmetry
   It is uncertain whether they had a water vascular system
   "carpoids" sometimes considered "calcichordates" not echinoderms
      (i.e., more closely related to chordates)

V. Echinoderm Feeding:
   Seastars
      many are predators
      stomach can come out of body

   Urchins
      scrapers, drift kelp, etc.
      5 teeth form "Aristotle’s Lantern"

    Brittlestars
      many catch food with tube feet
      others are omnivores (eat anything)
      5 jaws
      all lack an anus (secondary loss)

   Sea cucumbers
      suspension, deposit, or detritus feeders
      use tentacles - modified tube feet
      can spew guts - eviscerate

   Crinoids
      arms with pinnules
      tube feet on each pinnule catch food

Other features:
   pedicellariae – defense or surface cleaning (only in seastars and urchins)
   regeneration - widespread

VII. Echinoderm Features (more detailed)
   symmetry
      larvae are bilateral
      adults are pentamerous (but see cladogram!)
          The primitive condition was not pentamerous (p. 477)
          1st deuterostomes were bilateral
          1st echinoderms were bilateral or trimerous
          Living echinoderms all descendants of a pentamerous ancestor

   skeleton
      endoskeleton of calcareous "stereom" ossicles
      bound together with flexible/stiff collagen

   water vascular system (Figs. 23-3, 23-19)
      left hydrocoel in larva (Fig. 23-9)
      madreporite (like a master cylinder in car brake system)
      stone/ring/radial/lateral canals
      tube feet - ampulla/sucker
      ambulacrum

    nervous
      nerve net (epidermal nerve plexus)
        seastars have eyespots at tips of arms

respiration
papula (skin gills of seastars), hemal system (?)

fat storage
pyloric ceca of seastars (stores energy for gonads)

     larva (Fig. 23-10) - similar to hemichordate larva
       they start as a gastrula larva, then develop to
       a bipinnaria larva (so far, as in hemichordates)
        (Look ahead, next chapter, Fig. 24-4, p. 484)
       later, the type of larva differs, depending on the group
       urchins and brittlestars: pluteus larva
       (termed echinopluteus or ophiopluteus)
       seastars: bipinnaria larva –> brachiolaria larva

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