Paired abdominal appendages under the apron of the female crab on which the eggs are carried until they hatch.

the lobster's small legs where she carries grapelike clusters of eggs

Under female apron where eggs are carried

small structures located on the ventral surface of the abdomen useful in slow swimming and, in the anterior pairs, modified for copulation

Leglike parts to help in swimming, found under a crayfish's abdomen. There are different types of swimmerets (long, short, modified)