. Beginners' Zoology . stonecanal; ap, ampulla. 36 BEGINNERS ZOOLOGY also called ambulacra! feet (Latin ambulacra, forest walks \^ There is a water holder (ampulla), or muscular water bulb at the base of eachtube foot (). These con-tract and forcethe water intothe tube feet andextend cuplike ends of thetubes cling tothe ground bysuction. Thefeet contain delicate musclesby which theycontract andshorten. Thusthe animal pullsitself slowly along, hundreds of feet acting together. The tube feet, for their own protection, may contract and retire into the groove, the water which extend
. Beginners' Zoology . stonecanal; ap, ampulla. 36 BEGINNERS ZOOLOGY also called ambulacra! feet (Latin ambulacra, forest walks \^ There is a water holder (ampulla), or muscular water bulb at the base of eachtube foot (). These con-tract and forcethe water intothe tube feet andextend cuplike ends of thetubes cling tothe ground bysuction. Thefeet contain delicate musclesby which theycontract andshorten. Thusthe animal pullsitself slowly along, hundreds of feet acting together. The tube feet, for their own protection, may contract and retire into the groove, the water which extended them being sent back into the ampulla. This system of water vessels (or water- _ |_ vascular system) of the echinodermata is characteristic of them; is not found elsewhere in the animal kingdom. The grooves and the plates on each side of them occupy the ambulacral ajras. The rows of spines on each side of the grooves are freely movable. (What advantage ?) The spines on the aboral surface are not freely Fig. 57. — Starfish, from below; tube feet extended.
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