Larval development of the shell and the shell gland inMytilus (Bivalvia).

In the newly hatched larva ofMytilus galloprovincialis the larval shell field almost totally invaginates to form a shell gland. Only very few cells remain outside and they meet at one point. Only these cells secrete the pellicle, which in the beginning is protected by the glycocalyx and the microvilli of the surrounding cells. The invaginated cells do not participate in pellicle secretion. The secretion partly passes through infoldings of the apical cell membrane. Mineralization of each valve begins along the hinge line, continues to the valve margins and includes the valve centre.