Marsupium

Marsupium is a Latin term meaning pouch or bag, used in biology to denote a pouch-like structure that encloses and protects developing offspring or their eggs. The word is applied across different animal groups to describe a chamber or sac that provides protection and, in some cases, nourishment during development.

In mammals, particularly marsupials such as kangaroos, wallabies, koalas, and opossums, the marsupium is an external abdominal pouch. After birth, the altricial young crawl into the pouch, attach to a nipple, and continue development there, sometimes for several weeks or months. The pouch is formed by skin and fur and is equipped with muscles to open and close; in many species the opening is oriented to shield the young during movement and nursing.

In crustaceans, the term marsupium describes a brood pouch carried by the female. The marsupium is formed by specialized thoracic limbs called oostegites or by modified pleopods and ventral plates, creating a chamber in which eggs or embryos are retained until hatching. The embryos may be attached to the interior surfaces by filaments and are released as free-swimming nauplius or zoea stages.

The term is also used more generally in comparative anatomy to refer to similar pouch-like structures in other organisms, reflecting its Latin root meaning pouch. In all uses, the marsupium serves to protect developing offspring and often to provide nourishment until parturition or hatching.