Examples are given to show how in the Bivalvia, (a) changes in the form of the mantle/shell may affect the position of the adductor muscles, and (b) secondary extension of the primary ligament may limit the extent of pallial attachment in the anterior and posterior embayments of the mantle.
Secondary extension of the ligament may take place by periostracum or by fusion layer but it is probable that only fusion layer is of functional significance. Where the outer surfaces of the outer folds of the mantle margins anterior and posterior to the mantle isthmus are fused, as in the formation of fusion layer, there is an accompanying loss of pallial attachment and in most, if not all bivalves, the pallial line extends to the anterior and posterior ends of the functional ligament whether this consists of primary ligament only, as in Mytilus edulis, or of primary ligament secondarily extended by fusion layer, as in Anodonta cygnea. The extent of the pallial muscles in the anterior and posterior embayments is therefore of use in distinguishing between (a) fusion of the inner surfaces of the outer folds (i.e. when the two valves are joined by periostracum) and (b) fusion of the outer surfaces of the outer folds (i.e. when the two valves are joined by fusion layer).
Elongation of the mantle/shell on either side of the demarcation line results in a corresponding extension of the region of cross fusion of the pallial muscles in the anterior and posterior embayments. For functional reasons the adductor muscles develop new the anterior and posterior margins of the shell and as a result, in many bivalves pallial attachment extends beyond the adductors to the anterior and posterior ends of the functional ligament. This feature of pallial attachment in the Bivalvia appears to have been overlooked.