pentamerous

Pentamerous is a term used in biology to describe five-part organization or symmetry. In botany, pentamerous flowers have floral organs arranged in whorls of five. This commonly means five petals and five sepals, and often five stamens as well, though fusion or reduction can alter the appearance of the whorls. Pentamerous flowers are typical of many dicot angiosperms, including numerous members of families such as Rosaceae and Ranunculaceae. The term helps distinguish flowers that are five-merous from tetramerous (four-part) or trimerous (three-part) flowers. Variation occurs through fusion of parts or secondary loss, but the five-part plan remains a useful diagnostic feature for many plants.

In zoology, pentamerous describes fivefold radial symmetry, especially in adults of echinoderms such as starfish (Asteroidea), sea lilies, sea urchins, and brittle stars. Their body plan centers on five (or multiples of five) radiating zones, though larval forms are typically bilaterally symmetrical and symmetry can be modified in some groups.

Etymology: from Latin pentamerus, from Greek pente 'five' and meros 'part'. The term emphasizes the recurrent five-part pattern that characterizes certain flowering plants and certain echinoderm body plans, and it is often contrasted with other merism such as tetramerous or trimerous.