section_path: Anatomy/Torso
section_name: Torso
paragraph_text: Torso
  
  Few general characteristics describe their skeleton. In addition to unique details in the construction of the ankle, epipubic bones (ossa epubica) are observed projecting forward from the pubic bone of the pelvis. Since these are present in males and pouchless species, it is believed that they originally had nothing to do with reproduction, but served in the muscular approach to the movement of the hind limbs. This could be explained by an original feature of mammals, as these epipubic bones are also found in monotremes. Marsupial reproductive organs differ from placentals. For them, the reproductive tract is doubled. Females have two uteri and two vaginas, and before birth, a birth canal forms between them, the median vagina. In most species, males have a split or double penis lying in front of the scrotum, which is not homologous to the placental scrota.
  
  A pouch is present in most species. Many marsupials have a permanent bag, while in others such as the shrew opossum the pouch develops during gestation, where the young are hidden only by skin folds or in the maternal fur. The arrangement of the pouch is variable to allow the offspring to receive maximum protection. Locomotive kangaroos have a pouch opening at the front, while many others that walk or climb on all fours open in the back. Usually, only females have a pouch, but the male water opossum has a pouch that protects his genitalia while swimming or running.