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The Water Vascular System
The starfish's water vascular system is composed of three intersecting canals. The radial canals extend into the starfish's feet; the circular central canal runs around the center of the star, intersecting with the radial canals at the base of the arms. The stone canal directs the flow of water toward the dorsal surface of the starfish. Ultimately, the entire series of canals converges at the madreporite, the portal between the water vascular system and the surrounding environment.
The Madreporite
As described in the article "Maintenance of fluid volume in the starfish water vascular system," the madreporite is a porous, orange disk with an opening to the water vascular system. Several sets of cilia, or small hair-like structures, line the madreporite, acting as a filter, which directs fluid into the vascular system of canals.
Ions and Osmosis
While your circulatory system contains a fluid that is significantly different from your external environment, the starfish's water vascular system contains fluid that is nearly identical to external seawater. The concentration of potassium ions is the only exception to this statement, which may be present within the vasculature at a 60% higher concentration than seawater; the previously mentioned "Nature" study describes how specialized cells actively transport potassium into the circulating fluid. Many scientists believe that this difference in ionic concentration is the primary driver of fluid absorption by the water vascular system. The surrounding seawater is pulled into the system through the osmotic pressure created by the high potassium concentration; the laws of thermodynamics favor the movement of water into the starfish, which dilutes the potassium.
Starfish Anatomy
The high potassium concentration is generated by epithelial cells, specialized structures capable of transporting ions into the vascular space. This epithelial tissue lines the inner surface of the starfish's "tube feet," or small tubular projections that each function as part of the hydrostatic skeleton generated by the water vascular system. Tube feet interact with the environment, giving the starfish traction on surfaces and the ability to grasp objects.
The Hydrostatic Skeleton in Action
The water vascular system composes the hydrostatic skeleton of the starfish; the power of locomotion originates from muscles surrounding the fluid compartment. Both circular and longitudinal muscles apply force against each other; these contractions apply pressure to the fluid within the water vascular system. When the arm is bent, the increased internal pressure will force it to straighten, thereby extending the appendage. As the pressure is released, the arm will become flexed again.