Echinodermata

V. Echinodermata (CH16)

  • Key Morphological Features of Echinoderms:

    • Pentaradial Symmetry: Echinoderms typically exhibit a five-part symmetry, although their larval forms display bilateral symmetry.

    • Endoskeleton: calcareous endoskeleton in form of ossicles (plates)

      • providing structural support

      • (contrasting with the exoskeleton of arthropods made of chitin.)

    • Water Vascular System: A unique hydraulic system critical for locomotion and feeding.

      • Structure includes:

        • Madreporite (sieve plate): Entrance to the system for seawater.

        • Stone canal: transfers water from madreporite to ring canal

        • Ring Canal: Distributes water to radial canals leading to tube feet.

        • Radial Canals: transferes water from ring canal to tube feet

        • Tube Feet: structures used for locomotion and capturing food (suction cup-like bottom)

        • Ampulla: bulbous like structure at top of each tube foot

          • circular muscles contract around each ampulla forcing water down tube foot

          • longitudinal muscles along tube feet contract forming water back to ampulla, causing cuplike suction at distal end of tube foot

    • Complete Digestive System:

      • with two stomachs:

        • Cardiac Stomach: Muscular portion for digestion.

        • Pyloric Stomach: Receiving food, less muscular than the cardiac stomach.

    • Reproductive System: Most echinoderms exhibit dioecious reproduction, with external fertilization producing planktonic larvae, which serve as a food source for other organisms.

  • Major Classes of Echinoderms:

    • Class Asteroidea: sea stars, sea daisies

      • hard or sandy substrates

      • moveable and fixed spines

      • evert their stomachs to engulf prey

        • ingest food whole

      • diecious

      • largely regenerative

      • no head

      • sensory receptors widely distributed over body surfact

      • photoreceptors (eyes) at tips of arms

    • Class Ophiuroidea: Brittle stars

      • most diverse

      • Differentiated primarily by their long, slender arms.

        • long arms, sharply set apart from the central disk

        • arms sweep to capture food

        • wave arms to trap plankton on mucus-covered tube feed

      • no dermal branchiae or pedicellariae

      • distinct arms from central disk.

      • coelom confined to central disk

      • eggs are retained and fertilized within a bursa or released for external fertilization

    • Class Echinoidea:Sea urchins, sand dollars

      • Have globular or flattened bodies with spines.

        • allows for locomotion and burrowing

      • Utilize Aristotle's lantern for feeding.

        • Aristotle's lantern: teeth like structures in pentaradial symmetry, used to eat

      • diffusion across gill membranes surrounding mouth for gas exchange

      • dioecious

      • one gonophor in each of 5 genital ossicles

        • sand dollars have 4

      • external fertilization

    • Class Holothuroidea: Sea cucumbers

      • hard or soft-bodied

      • no arms with elongated bodies

      • one side flattened and ventral

      • bilateral symmetry

      • for defense:

        • have toxins on body wall

        • evert cuvierian tubules (respiratory tree) through anus

      • external fertilization as plankonic larvae

      • asexual reproduction

    • Class Crinoidea: Sea lilies

      • Characterized by feathery arms, resembling flowers.

        • arms used as filter feeder apparatus, feathers catch food and bring them in

      • most primitive living echinoderms

      • lessened nervous system compared to other echinoderms

        • lack nerve ring

      • mostly dioecious but can exhibit monoecious