Metridia lucens and M. pacifica previously have been differentiated primarily by the structure of the male fifth legs. lucens originally was described to have one large spine on the inner margin of the fourth segment and M. pacifica to have two spines, one each on the third and fourth joints. Some authors have reported M. lucens with two so-called spines on the same segment and M. pacifica also with two such spines but located together on the fourth segment. This led to M. pacifica being questioned as a species by certain authors who claimed it synonymous with M. lucens. In the present study, M. lucens from the Northwestern Atlantic is compared with M. pacifica from the Northeastern Pacific and differences in size and structure are evident to separate them. The fifth pair of legs on adult males and females were examined and findings indicated errors in the original descriptions of both species. Another Metridia population, found in the coastal waters of British Columbia, was compared with the described species. It was found to be similar to M. lucens and could be, in part, the reason for the past confusion between the Atlantic and Pacific species. / Science, Faculty of / Zoology, Department of / Graduate