The analysis under a scanning electron microscope of young larval and postlarval shells of Mytilus edulis and Margaritifera margaritifera reveals some primitive characters. Among these features, the most important are the ligamental formations which are described in detail during ontogenesis. In M. edulis a primitive ligament appears in a pit under the cardinal plateau immediately following metamorphosis. A second ligament later develops on the posterior edge of the shell, and becomes the only ligament present in the adult. In M. margaritifera a provisional internal structure acts as a ligament in the young glochidia fish parasites. A definitive external ligament only develops following metamorphosis of the glochidia.
The similar pattern of development of these ligaments in both species may represent the development of the ligament in the first bivalves.