The Collaborative International Dictionary of English v.0.48:

Swimmeret \Swim"mer*et\, n. (Zool.)
   One of a series of flat, fringed, and usually bilobed,
   appendages, of which several pairs occur on the abdominal
   somites of many crustaceans. They are used as fins in
   swimming.
   [1913 Webster]

WordNet (r) 3.0 (2006):

swimmeret
    n 1: one of the paired abdominal appendages of certain aquatic
         crustaceans that function primarily for carrying the eggs
         in females and are usually adapted for swimming [syn:
         swimmeret, pleopod]