甲殻類

Hard-shelled, segmented animals evolved from segmented worms. Characterized by multiple limbs and eyes, lack of a nerve braid (there is a nerve cord) and molting. Capable of swimming, though many larger specimens walk on the seafloor.

—Enormous biomass—tiny organisms akin to krill and shrimp fill the seawater. They feed on plankton and in turn are fed upon by fish, making them a critical link in the food web.

—Diverse forms—range in size from fractions of a millimeter to larger than an elephant.

Earth analog: crustaceans. Segmentation is a common evolutionary step from early vermiforms.