swimmeret(n = noun.animal) pleopod - 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;
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]