The mantle is the thin epithelial tissue that secretes the shell and covers the mussel’s gills and visceral mass (i.e., central large body containing major organs). Mantle tissue helps to protect organs from soil abrasion and contamination. The mantle is comprised of three layers: the outer, middle, and inner folds. The outer fold secretes the shell, the middle fold is sensory, and the inner fold (the thickest of the three) contains the circular and radial muscles. The mantle forms two openings: the inhalant (water entering) and exhalant (water exiting) siphons. The space within the mussel that is filled with water and lined by the mantle tissue is referred to as the mantle cavity.
The inhalant is the larger of the two siphons, typically having 80 to 100 tentacles that aid in the filtration of food particles. The exhalant siphon lacks tentacles and is located just above (dorsal to) the inhalant siphon. The only other opening in the mantle is the pedal gape, located on the ventral surface of the shell; this is the opening through which the byssus is extruded for attachment.
The inward flow of water into the inhalant siphon and finally out through the exhalant siphon is the result of the beating of millions of microscopic, hair-like cilia located on the gills and lining the mantle cavity. The beating of these cilia, which cover the gill filaments, causes water to be drawn through the inhalant siphon, into the mantle cavity, through the gill ostia (openings), into the water tubes of the gills, into the suprabranchial cavities, and finally out through the exhalant siphon. For more information on water circulation, see Silverman et. al. (1996).