the brown material are vitelline glands and ducts in this adult Paragonimus westermani,a lung fluke infecting a wide array of crustacean-eating vertebrates including man.  Humans in the far east often become infected by eating "drunken crab" or raw crab which is marinated in wine.  100x
 

midbody view of P. westermani including the vitelline reservoir with ducts leading into it, the central ootype (pink) with the uterus, filled with newly-formed egg capsules emerging from it, and the spherical acetabulum to the right. 100x