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In bivalves, the mantle consists of two lobes of tissue that completely enclose the animal within the shell (Figure 2.6). Between the mantle and the internal organs is a capacious mantle cavity. Unlike in other marine bivalves, the mantle in mussels contains most of the gonad. Gametes proliferate within the mantle and are carried along ciliated channels to paired gonoducts that discharge through the exalant opening of the mantle (see Chapter 5). The colour of the mantle varies from a creamy white to pink, brown or orange, depending on the stage of gametogenesis. After mussels have released their gametes, the mantle is thin and transparent. The mantle consists of connective tissue with haemolymph (‘blood’) vessels, nerves and muscles that are particularly well developed near the mantle margins. The mantle edge is usually darkly pigmented, which may give protection from the harmful effects of solar radiation. Cilia on the inner surface of the mantle play an important role in directing particles on to the gills and in deflecting heavier material along rejection tracts toward the inhalant opening, the entry point on the mantle for incoming water (see Chapter 4). Periodically, the rejected material is discharged by sudden and forceful closure of the shell valves; this is sufficient to blow the rejected material out of the mantle cavity through the inhalant opening.