Source : Webster's Revised Unabridged Dictionary (1913)

Swimmeret \Swim"mer*et\, n. (Zo["o]l.)
   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.

Source : WordNet®

swimmeret
     n : 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: {pleopod}]