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( Starfish)
Brisingida (100 species[1])
Forcipulatida (300 species[1])
Paxillosida (255 species[1])
Notomyotida (75 species[1])
Spinulosida (120 species[1])
Valvatida (695 species[1])
Velatida (200 species[1]) Starfish exhibit a superficially radial symmetry. They typically have five "arms" which radiate from an indistinct disk (pentaradial symmetry). However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Starfish do exhibit some superficial remnant of this body structure, evident in their larval pluteus forms. Sea stars do not rely on a jointed, movable skeleton for support and locomotion (although they are protected by their skeleton), but instead possess a hydraulic water vascular system that aids in locomotion. The water vascular system has many projections called tube feet on the ventral face of the starfish's arms which function in locomotion and aid with feeding. Sea stars usually hunt for shelled animals such as oysters and clams. They have two stomachs. One stomach is used for digestion, and the other stomach can be extended outward to engulf and digest prey. This feature allows the starfish to hunt prey that is much larger than its mouth would otherwise allow. Sea stars 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 sea stars 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 (example Indo-pacific cushion star, Culcita novaeguineae) to gracile stars like those of the Zoroaster genus.
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