The ciliated rim of the velum of larvae of Pecten maximus (bivalvia Pectinidae)

During the early development of Pecten maximus, the prototroch of the trochophore becomes the rim of the velum of the veliger. The prototroch consists of a tract of randomly-distributed cilia, but in the veliger an ordered pattern of ciliation with some compound cilia develops. The thin epithelium connecting the velum to the body of the larva bears no cilia, nor does the upper surface of the velum (except for an apical tuft); the much thicker epithelium of the velum rim, however, is profusely ciliated. The cilia are arranged in five bands or rings each extending round the rim of the velum. The ring closest to the upper (i.e. ventral) surface of the velum is the inner preoral ring of single cilia. Below this are two rings of much longer cilia grouped to form blade-shaped cirri, which each consist of 2 or 3 rows of 10-15 cilia. The cilia substructures indicate that the direction of active beat of the cirrus is along the axis of the rows. This beating generates the main swimming current. The energy demands of beating are reflected in the numerous large mitochondria in the cells bearing the cirri. Nerve processes in the velum may control beating. Below the cirri are an adoral tract of shorter cilia and then a ring of postoral cilia. The velum anatomy is that of a typical bivalve veliger, but some features distinguish Pecten maximus from other bivalves. The onfiguration of the bands of cilia and the orientation of their beating suggest that the veliger captures food particles by the 'opposed band' method. This configuration is likely to be homologous with those of other spiralian larvae.