any of the small, abdominal appendages, or pleopods, in certain crustaceans, used primarily in swimming and for carrying eggs
See swimmeret in American Heritage Dictionary 4
swim·mer·et
noun
One of the paired abdominal appendages of certain aquatic crustaceans, such as shrimp, lobsters, and isopods, that function primarily for carrying the eggs in females and are usually adapted for swimming. Also called pleopod.