Study Guide: Echinoderms

Echinoderms (Phylum: Echinodermata)

Habitat: All are marine (from shallow shores to deep sea)

Symmetry:

Larvae: bilateral

Adults: pentaradial symmetry (five-part)

• Endoskeleton: Made of calcium carbonate ossicles 

• Water Vascular System:

  • Hydraulic system used for movement, feeding, and gas exchange

  • Includes tube feet

No true brain: Nerve ring with radial nerves

Regeneration: Many can regrow lost body parts. Sea stars require part of central disc.

Coelomate, deuterostomes: Closely related to chordates evolutionarily

1. Water Vascular System

Components:

   Madreporite (sieve-like entry point for water)

   Stone canal

   Ring canal

   Radial canals

   Ampullae

   Tube feet (podia)

• Functions:

  • Locomotion

  • Feeding

  • Respiration

2. Body Structure

• Oral surface: Side with mouth

• Aboral surface: Opposite side (often contains anus)

• Ossicles: Internal skeletal plates (may form spines)

3. Digestive System

Usually complete (mouth → anus), though modified in some classes

Extant Classes of Echinoderms

1. Asteroidea (Sea Stars / Starfish)

• Star-shaped with 5+ arms

• Distinct central disc

• Tube feet with suction

• Eversible stomach (can digest prey externally)

• Predatory (e.g., mollusks)

Key Traits:

• Regeneration ability

• Ambulacral grooves on underside

2. Ophiuroidea (Brittle Stars & Basket Stars)

• Slender, flexible arms distinct from central disc

• Move by arm rowing, tube feet no used for locomotion due to lack of suckers/suction

Key Traits:

• Fast movement

• No anus (waste exits through mouth)

• Often scavengers or suspension feeders

3. Echinoidea (Sea Urchins & Sand Dollars)

• No arms; body forms a rigid test (shell)

• Covered in movable spines

• Tube feet extend through pores

Key Traits:

• Specialized feeding structure: Aristotle’s lantern (in urchins)

• Sand dollars are flattened for burrowing

4. Holothuroidea (Sea Cucumbers)

• Soft-bodied, elongated

• Reduced ossicles (not rigid)

• Tube feet often modified into feeding tentacles

Key Traits:

• Deposit or suspension feeders

• Can eject internal organs (evisceration) for defense

• Gas exchange via respiratory tree

5. Crinoidea (Sea Lilies & Feather Stars)

Most primitive group

Feather-like arms used for filter feeding

Key Traits:

• Mouth faces upward

• Tube feet capture food particles

• Sea lilies are stalked; feather stars are free-moving