This image shows a ventral view of a male and female preserved crayfish.  The male is easily distinguished from the female by the presence of a pair of modified first swimmerets called gonopods, which are enlarged and directed forward.  These structures are used by the male to transfer sperm to the female.  Note also the small openings to each vas deferens at the bases of the fifth pairs of walking legs (the right leg of this specimen has been removed). The vasa deferentia conduct sperm from the testes to the outside of the crayfish.  On the female crayfish observe the opening to the seminal receptacle (which receives sperm from the male) as well as the openings to the oviducts at the bases of the third pairs of walking legs.