Also called Pallium, plural Pallia, or Palliums, in biology, soft covering, formed from the body wall, of brachiopods and mollusks; also, the fleshy outer covering, sometimes strengthened by calcified plates, of barnacles, or the flexible body wall of tunicates.
The mantle of mollusks and brachiopods secretes the shell if one is present. It also forms a mantle cavity between itself and the body. The brachiopod mantle has a dorsal and a ventral lobe covered with small papillae (nipple-like projections) that penetrate into the shell. The molluscan mantle has a left and a right lobe and, as in bivalves, may be joined at the edge to form siphons for circulating water in the mantle cavity
Source: Taken from Encyclopaedia Britannica 2002