Ch. 22 - Echinoderms

Phylum: Echinodermata

  • Overview of Echinoderms

    • Includes organisms such as starfish and sea urchins.

    • Approximately 7000 species identified.

    • Characteristic features:

    • Radial symmetry found in adults.

    • Bilateral symmetry in larvae due to metamorphosis.

    • Lacks a true head and brain structure.

    • Possesses a calcareous endoskeleton composed of ossicles.

    • Skin covered with spines.

  • Water Vascular System

    • Functions include locomotion, food gathering, respiration, and excretion.

  • Hemal System

    • Characterized by minimal blood flow.

Class: Asteroidea (Sea Stars)

  • General Characteristics

    • Composed of 5 arms radiating from a central disc.

    • Contain ambulacral grooves leading to the mouth.

    • Equipped with multiple rows of tube feet used for movement and capturing food.

    • Madreporite:

    • Functions as a sieve for the water vascular system.

Key Anatomy of Starfish

  • Structure:

    • Mouth location at the center of the disc.

    • Each arm has specialized systems:

    • Tube feet for locomotion and feeding.

    • Ambulacral grooves for movement of tube feet.

    • Sensory tentacles for environmental interaction.

    • Spines present for protection.

  • Other components:

    • Central disc as the main body segment.

    • Anus located at the aboral side of the sea star.

    • Gonad and multiple digestive structures including the pyloric duct, cardiac stomach, pyloric stomach, intestinal cecum, and pyloric cecum.

Water Vascular System Structure

  • Components:

    • Madreporite: entry for water.

    • Stone Canal: path from madreporite to the ring canal.

    • Ring Canal: encircles the central disc.

    • Radial Canals: extend along each arm to distribute water.

    • Lateral Canals: connect to tube feet.

    • Ampulla: bulb-like structure that aids in moving water into tube feet.

Pedicellariae

  • Structure & Function:

    • Tiny jaws located along spines.

    • Purpose: cleaning, protecting the sea star, and capturing prey.

Feeding and Digestion

  • Feeding Mechanism:

    • Sea stars are typically carnivorous.

    • Pyloric stomach aids in digestion.

    • Sea stars exhibit a nerve net/ring for signaling and movement coordination.

Class: Ophiuroidea (Brittle Stars)

  • Regenerative Ability:

    • Brittle stars possess significant regeneration capabilities.

Class: Echinoidea (Sea Urchins)

  • Physical Characteristics:

    • Have a tough exoskeleton with a hemispherical shape and spines.

    • Possess pedicellariae equipped with glands that may produce neurotoxins.

    • Anatomy:

    • Mouth oriented ventrally and anus oriented dorsally.

Class: Holothuroidea (Sea Cucumbers)

  • Behavior and Anatomy:

    • Move slowly, typically functioning as bottom feeders.

    • Soft-bodied with reduced ossicles in the body wall.

    • Oral tentacles: modified tube feet used for foraging.

  • Respiratory Tree:

    • Functions in respiration and excretion.

Defensive Mechanisms of Sea Cucumbers

  • Eversion Mechanism:

    • Ability to rupture the body wall or expel guts through the anus when threatened.

    • Cuvierian tubules: sticky strands that can be everted when attacked to deter predators with toxins.

    • Commensal relationship with small fish (Carapus) that inhabit the cloaca/respiratory tree of sea cucumbers.

Class: Crinoidea (Sea Lilies and Feather Stars)

  • Feeding Mechanism:

    • Filter feeders using pinnules, while maintaining most echinoderm anatomical characteristics.