Metroid Cocoon

A Metroid Cocoon is a breeding hive for Infant Tallon Metroids in Metroid Prime 2: Echoes. They are only seen in Biostorage Station, and hang on ceilings. According to the scan, they are used to transport the Metroids. They can take a beating to destroy.

Logbook entry
Metroid Prime 2: Echoes: "Morphology: Metroid Cocoon. Storage chamber for young Tallon Metroids. Biomass to support Tallon Metroid infants. Lacks armor or shielding."

"The Space Pirates transport young Tallon Metroids in an organic "cocoon" designed to sustain the small creatures. Each cocoon can support numerous infants for several cycles. The creatures are normally content to remain in the cocoon, but will emerge when they detect the presence of Phazon."