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The water vascular system of the starfish is a hydraulic system made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation and gas exchange.
Water enters the system through the madreporite, a porous, often conspicuous, sieve-like ossicle on the aboral surface.
The structures are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the interstices.
A set of radial canals leads off this; one radial canal runs along the ambulacral groove in each arm.
There are short lateral canals branching off alternately to either side of the radial canal, each ending in an ampulla.
These bulb-shaped organs are joined to tube feet (podia) on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove.
There are usually two rows of tube feet but in some species, the lateral canals are alternately long and short and there appear to be four rows.
The interior of the whole canal system is lined with cilia.
These are honeycombed structures composed of calcite microcrystals arranged in a lattice.