sea mouse, marine worm of the genus Aphrodite with a short, broad, segmented body, found in moderately deep water. The upper, or dorsal, surface of a sea mouse bears 15 pairs of raised scales; the space between the scales and the body surface forms a channel for the flow of water carrying oxygen for respiration. The entire dorsal surface, including the scales, is covered by long, feltlike threads called setae, which produce a brilliant iridescence; heavier, bristlelike setae project through them. Sea mice commonly reach 6 to 8 in. (15–20 cm) in length and 2 in. (5 cm) in width. They are classified in the phylum Annelida, class Polychaeta, family Aphroditidae.