Sea stars and sea urchins operate their tube feet by select one: a. a hydraulic system that regulates water pressure. b. cilia transporting hemolymph from chamber to chamber. c. sticky threads leading from the mouth. d. actin and myosin fibers that slide and give muscle actions to the five arms or "rays." e. an extensive net of nerve and muscle cells.

a. a hydraulic system that regulates water pressure.

Explanation:

The thorns of starfish and sea urchins can serve both to protect the animal (some may have toxic substances) as well as to help them move around. They have internally an aquifer system, called ambulacral system. This system causes seawater to enter the animal's body and circulate through water vessels where pressure differences produce movements that help locomotion and prey capture in some species. In other words, we can say that starfish and sea urchins operate their tube feet with a hydraulic system that regulates water pressure.

The hydraulic channels are connected to small tubes, called ambulatory or ambulatory feet, which help the animal to move. Water enters the animal through one or more pores located in the so-called madrporic plaque or mother-of-stone located near the anus.

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