🌊 Phylum Echinodermata

Echinoderms are marine invertebrates known for their unique radial symmetry and water vascular system.

🐚 General Characteristics

• Exclusively marine animals — found only in the ocean.

• Body shapes vary:

  • Flattened (e.g., cake urchin)

  • Star-shaped with short arms (e.g., sea star/starfish)

  • Globular (e.g., sea urchin)

  • Star-shaped with long arms (e.g., brittle star)

  • Elongated (e.g., sea cucumber)

• Triploblastic, deuterostome, and coelomate animals.

🔵 Symmetry

• Larval stage → bilateral symmetry.

• Adult stage → radial symmetry (body parts arranged in five or multiples of five around an oral–aboral axis).

🦴 Skeleton

• Possess a calcareous endoskeleton made of ossicles (tiny plates).

• Ossicles are derived from the mesoderm and form spines on the body surface.

💧 Water Vascular System

• A unique system of tubes and canals used for movement, feeding, and respiration.

• Central ring canal surrounds the mouth and opens to the outside via the madreporite (a sieve-like plate).

• From the ring canal, five (or multiples of five) radial canals extend outward.

• Lateral canals arise from each radial canal and end in tube feet.

• Tube feet function by suction — they extend, attach to a surface, and pull the body forward when water is drawn back.

🍽️ Digestive System

• Complete digestive tract with:

  • Mouth → Esophagus → Stomach → Intestine → Rectum → Anus

🌬️ Respiration & Excretion

• No specialized organs for respiration or excretion.

• Exchange occurs via tube feet and body surface.

🧠 Nervous System & Sense Organs

• Poorly developed; consists of:

  • A nerve net

  • A nerve ring around the mouth

  • Radial nerves extending outward (five or more)

• Sensory receptors are distributed over the body surface and tube feet.

🧬 Reproduction

• Mostly unisexual (separate sexes).

• Asexual reproduction occurs by body division and regeneration.

• Capable of regenerating lost parts — especially in sea stars and brittle stars, which can shed body parts when threatened.

🌟 Examples

• Sea star (Starfish)

• Brittle star

• Sea urchin

• Sea cucumber

• Cake urchin