The Collaborative International Dictionary of English v.0.48:

marsupium \mar*su"pi*um\, n.; pl. marsupia. [L., a pouch],
   (Anat. & Zool.)
   (a) The pouch, formed by a fold of the skin of the abdomen,
       in which marsupials carry their young; also, a pouch for
       similar use in other animals, as certain Crustacea.
   (b) The pecten in the eye of birds and reptiles. See
       Pecten.
       [1913 Webster]