Tube feet of Echinoderms

1. Tube foot:- A podium or tube foot is a short, hollow, elastic, thin walled, closed tube present in the ambulacral groove of arms in echinoderms.
Each tube foot extends through a gap, called ambulacral pore, which lies between 2 ambulacral ossicles. 
Structure:-
Each tube foot can be distinguished into 3 regions:-
1. Ampulla:- A rounded sac like structure.
Position:- Situated above the ambulacral ossicles & projects into the coelom.
Wall:- The walls of ampulla possess circular muscles which are provided vertically.
2. Podium:- It is the middle tubular portion.
Position:- It is extending through the ambulacral groove.
Wall:- The wall of each podium is covered on the outside by an ciliated epithelium & internally with peritoneum. Between these 2 layer lie connective tissue & longitudinal muscle fibres.
3. Sucker:- The lower end of the podium is flattened forming a cup like structure called sucker.
Function:-
1. Tube feet help in locomotion by anchoring the substratum tightly.
2. These also helps in capturing  & handling the food.
3. Thin walls of tube feet may serve for respiratory exchange of gases.





Mechanism of locomotion in echinoderms:-
            The entire w.v.s. acts as a hydraulic system during locomotion.
1. Water enters through the madreporite to different canals such as, stone canal → ring canal → radial canals → lateral canals.
2. From lateral canals water enters into the ampulla of the tube feet.
3. Body is moved by the stepping action of tube feet which are alternately adhered & raised from the substratum.
4. One or 2 arms, in the desired direction of movement, are raised from the substratum. 
5. Simultaneously the vertical circular muscles of the ampullae of the tube feet of these arms contract & the valves of the lateral canals close.
6. This increases hydrostatic pressure within the tube feet.
7. The tube feet consequently elongate, extend forward.
8.  Podial suckers adhere to the substratum by suction force as well as the adhesive secretory products of the tips of tube feet.
9. Then by muscular activity, tube feet assume a vertical posture, dragging the body forward.
10. Tube feet then shorten by contracting their longitudinal muscles, forcing the water back in to the ampulla.
11. Consequently the suckers release their hold on the substratum.
 During movement 1 or 2 arms act as leading arms& all the tube feet extend in the same direction in a coordinated manner.