Does a Starfish Have Muscles?

Water Vascular System

• The water vascular system mostly consists of a system of tubes that run through the invertebrate's body, including the tube feet. The system is considered closed, but a starfish can open a small orifice called the madre porite to replace the fluid inside the water vascular system. The starfish can vary the water pressure within this system to control the tube feet. The fluid inside consists of sea water.

Tube Feet

• The tube feet consists of hundreds of tiny tubes. A suction cup sits on the end of each tube. Under the control of the water vascular system, a starfish can use these tube feet to slowly creep along surfaces. A starfish can even climb up vertical surfaces using the suction generated by the suction cups at the ends of the tube feet. This allows the creature to climb up the side of an aquarium. Some species of starfish use their tube feet to hold prey when they hunt.

Stomach

• The hundreds of species of starfish vary considerably in their diets and feeding behaviors. Many scavenge, eating dead debris off the substrate. Species like this typically do not require extensive muscular control over their stomachs. However, starfish from family Forcipulata, one of three starfish families, use muscular control to evert their stomachs.

Eversion

• Starfish of family Forcipulata eat bivalve mollusks like clams and mussels. In order to do so, the starfish grasp shells using their tube feet, then force the shell open slightly. Through this slit opening, starfish can evert their stomachs, pushing the organs into the shell. The stomach then proceeds to digest the bivalve within the shell.