Luidiidae, Amphiuridae, Ophiactidae, Ophiuridae and Astropectinidae - Starfishes and Brittle Stars

Starfishes and brittle stars are members of the Phylum Echinodermata, which also includes sea urchins, sea cucumbers, sand dollars and feather stars. Both possess five-sided symmetry in which they are arranged in five parts around a central access. There is no anterior, posterior, head or tail. Some starfish and feather stars have a symmetry that is based on multiples of five, for example the starfish, Helicoilaster, which may have up to 50 arms and the sea lily, Comanthina schlegelii, which has 200 arms.

Starfishes and brittle stars have a skeleton made of calcareous plates, giving it an internal modular construction. The skin holds the skeleton together and contains pigment cells, receptor cells and sometimes gland cells which can secrete sticky fluids or toxins. The pigment cells can contain many vivid colors, and sometimes the colors can change quickly due to changes in light level. They also have many small tube feet, which are controlled by their vascular system. The tube feet have many functions including locomotion, wafting or moving food particles to their mouths, and aiding in respiration. Starfishes and brittle stars have two gonads in each arm whereas in sea urchins and sea cucumbers, the gonads take up the entire body cavity.

Starfishes are usually flattened with five (sometimes more) radial arms. The mouth is located on the central underside of the starfish, and the anus is located on top. Tube feet are located in a groove on the underside of each arm that runs from the mouth to the tip.

Brittle stars consist of a flat central disk with five or more long thin radiating arms. The arms are often spiny and move by undulations like a snake. At the beginning of each arm, there are usually paired scales called radial shields. As with starfishes, the mouth is on the underside of the disk and the anus is on top.

Starfishes and brittle stars are fragile and easily give up an arm or two to escape predation or danger. Most echinoderms have tremendous powers of regeneration. Many are capable of replacing lost arms or spines, and sometimes lost arms have been known to regenerate into a completely new body.