Shape of bivalve shell determined by mode of growth
Schneider, Erich Robert
Zeitschr Wiss Biol Abt A Morph U Okol Tiere 9(5): 681-709
The soft body of the mussel may be regarded as a bullated form which is expanded both on the right and left sides. The part responsible for formation and constant growth of the valves is a ring-shaped zone in the median plane; growth of the mussel shell is, therefore, mainly circular. In this zone are located the centers of development for the 2 outer layers which mainly determine the size of the shell: the conchiolin (periostracum) and the prismatic layer. The outer layer or periostracum, after decalcification, remains as a hull around the soft parts. The median narrow field erected above the comb-shaped central position is thicker than the lateral parts. In the so-called shell ligament, the conchiolin layer pre- dominates, and the calcareous layers are strongly or entirely reduced. The 3rd layer of the mussel shell, the nacreous layer, is not connected with growth in size of the shell; it is deposited by the greater portion of the surface of the pallium which is expanded from the borders of the median plane both to the right and left, which accounts for the thickening of the shell later on. The general style of structure of the shell and its radial ornaments and ribs on the surface is in connection with the general outline and with the specialization of the lateral parts of the circular zone of the median plane. The region of the lock of the shell also shows the reprint of the soft comb of the lock; the teeth and grooves not only grow larger but also shift more apart as the ribs and grooves of the shell increase in breadth and depth with increasing distance from the back. During the growth of the animal itself, an increasing distance of the 2 valves from the median plane can be observed, produced by the constant addition of new rings which are broader near the open margin than they are near the lock. The muscle impression increases in size with the general growth of the animal and moves farther apart.