Sea urchin Tube feet extended past the Spines

Tube feet (technically podia) are small active tubular projections on the oral face of an

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...

, whether the arms of a

starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...

, or the undersides of sea urchins, sand dollars and

sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuri ...

s; they are more discreet though present on brittle stars, and have only a feeding function in feather stars. They are part of the water vascular system.

Structure and function

Tube feet function in locomotion, feeding, and respiration. The tube feet in a starfish are arranged in grooves along the arms. They operate through

hydraulic pressure Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...

. They are used to pass food to the oral mouth at the center, and can attach to surfaces. A starfish that is inverted turns one arm over and attaches it to a solid surface, and levers itself the right way up. Tube feet allow these different types of animals to stick to the ocean floor and move slowly. Each tube foot consists of two parts: the ampulla and the podium. The ampulla is a water-filled sac contained in the body of the animal that contains both circular muscles and longitudinal muscle. The podium is the tube-shaped structure that protrudes from the body and contains longitudinal muscle only. When the muscles around the ampulla contract, they squeeze water from the ampulla into the connected podium, causing the podium to elongate. When the muscles around the podium contract, they squeeze the water back into the ampulla, causing the podium to contract. The podia use a chemical adhesive (no suction) to attach to the substratum. Image:Podia de Colobocentrotus atratus.JPG, The shingle urchin ('' Colobocentrotus atratus'') has among the most powerful podia of all echinoderms. File:Star Fish Tube Feet.jpg, Starfish attaching to aquarium glass using tube feet. Image:T. gratilla podia.jpg, Collector urchins (''

Tripneustes gratilla ''Tripneustes gratilla'', the collector urchin, is a species of sea urchin. Collector urchins are found at depths of in the waters of the Indo-Pacific, Hawaii, the Red Sea, and The Bahamas. They can reach in size. Description Collector urchin ...

'') have elongated podia. Image:T. gratilla podia (1).jpg, Close-up on the same specimen. Image:Mithrodia clavigera bras.jpg, The nail starfish (''

Mithrodia clavigera ''Mithrodia clavigera'' is a species of tropical starfish in the family Mithrodiidae The Mithrodiidae is a family of starfish in the order Valvatida. Members of this family are big to huge tropical sea stars with 5 arms, rounded in cross-sect ...

'') has particularly strong podia. Image:Pycnopodia helianthoides on glass.jpg, Oral face of a sunflower sea star, ''

Pycnopodia helianthoides ''Pycnopodia helianthoides'', commonly known as the sunflower sea star, is a large sea star found in the northeast Pacific. The only species of its genus, it is among the largest sea stars in the world, with a maximum arm span of . Adult sunflow ...

''. Image:Pycnopodiahelianthoides-tubefeet.jpg, Close-up on a '' P. helianthoides''. Image:Holothuria forskali Rab2011 l 4213.JPG, Podia of a sea cucumber ('' Holothuria forskali'')

References

{{reflist Echinoderm anatomy