sac

n. 囊,液囊

An organic sac or pouch, such as the udder of a cow.

（牛的）乳房有机体器官的液囊或袋，如牛的乳房

An air-filled sac or structure that aids in the flotation of an aquatic organism.

气囊水生动植物器官中用来帮助飘浮用的充满气体的囊或结构