This ochre sea star is a beautiful example of pentamerism. Pentamerism is a type of radial symmetry.

Symmetry means a balanced or proportional arrangement of parts. Radial indicates a circular configuration—something with a radius, or rays. In fact, the "arms" of sea stars and other radiate organisms are called rays.

Organisms with radial symmetry have no difference between their "right" and "left" sides. They only have a top and bottom. The top part of this sea star is red (ochre), studded with white spines called ossicles. The bottom of the sea star contains its thousands of tiny sucker "feet," which the animal stretches in order to move.

Pentamerism is the most familiar type of radial symmetry. Penta- is a prefix meaning "five." Organisms displaying pentamerism have five, roughly equal sized parts (rays).

The five rays of pentamerous organisms are arranged around a central point, or axis. The rays are roughly 72 degrees apart.

Sea stars are not the only pentamers in nature! Many flowers, such as hibiscus or daffodils, are pentamers. They have five petals around a central axis. An apple, cut in half, displays pentamerism in the arrangement of its seed pods.

Noun

an invisible line around which an object spins.

flower
Noun

blossom or reproductive organs of a plant.

ochre
adjective, noun

orange-yellow color.

ossicle
Noun

small bone or bony structure.

pentamer
Noun

something made up of five parts.

radial symmetry
Noun

condition of having similar parts regularly arranged around a central axis.

radius
Noun

ray extending from the center of a circle or sphere to its surface or circumference.

ray
Noun

straight line extending from a point.

sea star
Noun

marine animal (echinoderm) with many arms radiating from its body. Also called a starfish.

seed
Noun

part of a plant from which a new plant grows.