Question

Star Fish is able to crawl over substratum by means of

• A. Arms
• B. Spines
• C. Pedicellariae
• D. Tube feet

Answer 1

Answer: (D)

Tube feet

• When an Asterias desires to move on a horizontal surface in a given direction, the arm or arms pointing in that direction is lifted.
• The ampullae of raised arm contract, the valve in the lateral canals close and the water of the ampullae is forced into the podia.
• The podia of the tube feet become extended, elongated and enlarged in the general direction of movement due to the hydrostatic pressure produced by influx of water into them.
• Subsequently, the terminal suckers of the tube feet become attached to the substratum and their central parts are withdrawn to form suction cups.
• Due to the vacuum so produced, the suckers acquire a firm grip over the substratum.
• Mucus secreted by the tips of the tube feet further aids in attachment.
• The tube feet now pivot forward on their attached suckers, assuming vertical position and thereby pushing the body forwards.
• The longitudinal muscles of the podia now contract and this forces their fluid back into the ampullae and releases their suckers.
• The ampullae then contract again and whole sequence of events is repeated.

So, the correct answer is 'D'.