paramere

from The Century Dictionary.

• noun In biology: A radiated part or organ; one of a set of radiating parts arranged like the spokes of a wheel about a common center; an actinomere: correlated with antimere, metamere, etc. The arms or rays of a starfish arc parameres in this sense.
• noun Either half, right or left, of a bilaterally symmetrical animal: now oftener called antimere.
• noun Either half, right or left, of one segment or somite of a bilaterally symmetrical animal.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) One of the symmetrical halves of any one of the radii, or spheromeres, of a radiate animal, as a starfish.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology One of the symmetrical halves of any one of the radii, or spheromeres, of a radiate animal, such as a starfish.