Phylum Arthropoda (2)

Regular echinoids have pentameral symmetry and move in all directions across the seafloor. The mouth is found at the center of the bottom surface and the anus is at the center of the top surface. Spines and tube feet are used for locomotion. The spines are attached to body plates with ball-and-socket joints, and muscles move the spines in all directions. Regular echinoids graze on algae or scavenge flesh on the sea floor. Their spines are common sediment components in some strata.

Irregular echinoids have bilateral symmetry, and tend to be heart shaped or oval, they burrow in sediment and are suspension or deposit feeders. The mouth and anus are at opposite ends of the bottom surface. Sand dollars are flattened irregular echinoids that live under a thin layer of sediment on the sea floor.

Class Asteroidea

Sea stars (starfish) fall within this class, which ranges from Ordovician to Recent. They possess fivefold symmetry and extended arms. The mouth is on the bottom of the body and the anus on top. The madreporite is both the entry and exit point for water to the water vascular system. Tube feet line the ambulacral grooves on the underside of the body and allow the sea stars to move freely over the seafloor.