Asteroids 2 - hydraulic systems
Tube feet (podia) are supported by a hydraulic and connective-tissue skeleton under the control of antagonistic
muscle pairs systems. The essential components are shown in the diagram above and explained below.
Components of the tube-foot system:

A fluid-filled cavity. Contraction of the ampulla pump by ampulla muscles increases the pressure in
the ampulla cavity, which forces its fluid into the fluid-filled cavity of the podium. The resultant
increase in pressure within the podium causes tube-foot extension.

Connective tissue skeleton. Tube-foot extension under pressure is permitted and limited by radial
hoops of connective tissue fibres arranged in series down the length of the podium, and connected
by a longitudinal or axial series of hinges. The hinges allow the hoops to move together when the
podium retracts and to separate by a finite amount when the podium extends. The hoops prevent
wasteful radial extension of the podium when it is under pressure.

Retractor muscles. These longitudinal muscles shorten the podium by contracting, when the
ampulla muscles relax. The resultant pressure moves fluid back into the ampulla. The retractor
muscles and ampulla muscles are antagonistic.

Orienting or postural muscles. These work in antagonistic pairs to move the tube foot forwards and
backwards. When the podia are attached to the substrate, co-ordinated movements forwards or
backwards will propel the echinoderm.

Sucker. The terminal sucker of the podium enables the podium to attach to the substrate and
hence to apply force to the substrate.

Disc levator muscles. Not shown in the diagram, these muscles also attach to the terminal plate.
Contraction of these muscles breaks the sucker seal by lifting the terminal plate, enabling the
adhered tube foot to detach from the substrate. Lifting the plate without tilting it relative to the
substrate creates a cavity between the sucker and the substrate, resulting in suction. Suction is also
enhanced by the secretion of a sticky liquid from the podium.

Terminal plate. A skeletal plate in the centre of the disc, to which the disc levator muscles attach.
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Coordinated action of the tube feet brings about slow creeping locomotion. One arm temporarily dominates and
leads the way, according to which arm receives the strongest positive stimulus (induced arm dominance).
Alternatively, in some starfish one particular arm may dominate most of the time (intrinsic arm dominance). The
tube-feet execute coordinated (though not necessarily synchronised) walking movements: a tube foot detaches
from the substrate, shortens, swings forward, elongates and then reattaches to the substrate further forward and
then swings backwards whilst still attached to the substrate. Together, the many hundreds of tube-feet move the
starfish along. Starfish can also crawl over vertical surfaces.

The tube-feet also function in suction, whether as part of locomotion, getting a grip on an oyster shell, or
preventing the starfish being swept off the rocks in rough seas. They also have a key function in feeding. In
mucus-ciliary feeders they pass food particles to the mouth. In many starfish, small animals, even fish, captured
by the pedicellaria are passed to the podia which convey them to the mouth. In predatory starfish, the podia may
be used to help gain access to food, for example by prizing the valves of muscles and oysters apart. A single
podium may exert a force of 25-30 grams and working together they may exert a pressure of nine pounds per
square inch.

In burrowing starfish, such as
Astropecten, the tube-feet lack suckers and are used for both walking and
burrowing into sand.