THE ADDUCTOR AND RETRACTOR MUSCLES OF THE VELIGER OF PECTEN MAXIMUS (L.) (BIVALVIA)

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Abstract
In Pecten maximus (L.), retractor and adductor muscles become functional in the early veliger larva. The twelve-day-old veliger has four pairs of velar retractors, three pairs of retractors attached to the posterior body wall and an anterior adductor. The pediveliger has in addition, pedal retractor muscles and a posterior adductor. The retractors consist of striated muscle: the adductors have both smooth and striated portions. The retractors attach near the hinge, branch to a greater or lesser extent, then attach to specific areas of the velum, posterior body wall and foot. Some features of the branching and of the disposition of points of attachment form a pattern which exhibits mirror symmetry about the plane between the two shell valves. This pattern is characteristic of the species. It is deduced that retraction and protraction of the velum result from co-ordinated sequences of muscle contractions.

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