This feature allows the starfish to hunt prey that is much larger than its mouth would otherwise allow. Starfish are able to regenerate lost arms. A new starfish may be regenerated from a single arm attached to a portion of the central disk. Starfish are composed of a central disc from which arms sprout in pentaradial symmetry. Most starfish have 5 arms, but some have more or fewer. Some starfish have shown differing numbers of limbs within a single species.
The mouth is located underneath the starfish, on its ventral surface. The spiny upper surface is called the aboral or dorsal surface. On the aboral surface there is a structure called the madreporite, a small white spot located slightly off-center on the central disc which acts as a water filter and supplies the starfish's water vascular system with water to move.
Porcellanasteridae employ additional cribriform organs used to generate current in the burrows made by these infaunal starfish. While having their own basic body plan, starfish radiate diversely in shapes and colors, the morphology differing between each species. A starfish may have dense rows of spines as a means of protection, or it may have no spines at all.
Ranging from nearly pentagonal to gracile stars like those of the Zoroaster genus. The body cavity also contains the water vascular system that operates the tube feet, and the circulatory system, also called the hemal system.
The anatomy visible from both of these surfaces has been identified and defined. Eye: The common sea star has five eye spots on the tip of each of its five rays. These eye spots can see shadows and light. Tube feet can be visible from the aboral surface as they stretch out to move the sea star from one location to another.
The surface of the tube feet can exchange gases and nitrogen waste. Rays: Common sea stars have five rays, unless they lose one or grow an extra. Most sea stars have rays, but sunflower sea stars can have up to rays. The hemal system is shown below. There are hemal channels forming rings around the central part of the body around the mouth the oral hemal ring , closer to the upper surface the aboral hemal ring , and a third ring around the digestive system the gastric hemal ring.
These are connected by the axial sinus. There are also radial hemal channels running down the rays next to the gonads which are also located in the rays. A dorsal sac attached to the axial sinus pulsates, sort of like a very inefficient heart inefficient because it lacks a one-way valve system.
The hemal system seems mostly organized to distribute nutrients from the digestive tract. The water vascular system uses cilia and the constant contraction of ampulla to extend and retract the tube feet also helps keep things moving. There is an ionic imbalance that causes water to flow into the water vascular system through the madreporite, and then the Tiedemann's bodies divert some of it into the perivisceral coelom. Circulation in the perivisceral coelom is mostly by ciliary beating.
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