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This arrangement enables both easy flexion of the arms by the starfish and the rapid onset of stiffness and rigidity required for actions performed under stress.
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.
They vary in form, with some bearing external granules, tubercles and spines, but most are tabular plates that fit neatly together in a tessellated manner and form the main covering of the aboral surface.
Pedicellariae are compound ossicles with forceps-like jaws.
Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown.
Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface.
They have complex life cycles and can reproduce both sexually and asexually.
These are honeycombed structures composed of calcite microcrystals arranged in a lattice.
They are found from the intertidal zone down to abyssal depths, 6,000 m (20,000 ft) below the surface. They typically have a central disc and five arms, though some species have a larger number of arms.
The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates.
Some are thought to assist in defence, while others aid in feeding or in the removal of organisms attempting to settle on the starfish's 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.