Internal Anatomy
As echinoderms, starfish possess a hydraulic water vascular system that aids in locomotion. This system has many projections called tube feet on the starfish's arms which function in locomotion and aid with feeding. Tube feet emerge through openings in the endoskeleton and are externally expressed through the open grooves present along the oral surface of each arm.
The body cavity also contains the circulatory system, called the hemal system. Hemal channels form rings around the mouth (the oral hemal ring), nearer to the aboral surface and around the digestive system (the gastric hemal ring). A portion of the body cavity called the axial sinus connects the three rings. Each arm also has hemal channels running next to the gonads. These channels have blind ends and there is no continuous circulation of the blood.
On the end of each arm is a tiny simple eye which allows the starfish to perceive the difference between light and darkness. This is useful in the detection of moving objects. Only part of each cell is pigmented (thus a red or black colour) and there is no cornea or iris. This eye is known as a pigment spot ocellus.
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