Starfish Circulation: Water Vascular System Replaces Heart for Movement and Feeding

Starfish (Asteroidea) lack a heart, blood, and closed circulatory system. Instead, they possess a water vascular system (WVS) — a hydraulic network unique to echinoderms.

Components

• Madreporite: Sieve plate on aboral surface; filters seawater entry.
• Stone canal: Connects madreporite to ring canal.
• Ring canal: Circular canal around mouth (central disk).
• Radial canals: Extend from ring canal along each arm.
• Lateral canals + ampullae: Bulbous reservoirs along radial canals.
• Tube feet (podia): Extend from ampullae through ambulacral groove; terminal suction discs.

Functions

• Locomotion: Ampulla contracts → hydraulic pressure extends podium → suction disc attaches → longitudinal muscles retract → body pulls forward. Coordinated waves.
• Feeding: Tube feet pry open bivalves; stomach everts (cardiac stomach) into prey for external digestion.
• Attachment: Suction holds against waves/predators.
• Gas exchange: Diffusion across tube feet, papulae (dermal branchiae).
• Excretion: Ammonia diffusion via tube feet/papulae.

Hemal System

Separate, reduced: oral and aboral hemal rings, axial sinus — nutrient transport (no respiratory pigment).

The WVS is a brilliant evolutionary solution for a radially symmetric, benthic predator.