USE OF THE INTRINSIC MUSCULATURE FOR SIPHONAL AUTOTOMY IN THE SOLENACEA (MOLLUSCA: BIVALVIA)

SUMMARY

The anatomy of the siphons of Solen marginatus and S. capensis has been described by both light and electron microscopy. Externally the siphon is annulated, each annulation corresponding to a region within the siphon wall where connective tissue is reduced. When disturbed, animals first shed a few terminal segments of siphon before proceeding to escape by burrowing rapidly. Observations and histological sections before and after autotomy have shown that loss is achieved with rapid contraction of the circular and radial muscles of the siphon wall about one of the weakness planes. This action compresses, elongates and breaks the longitudinal muscle fibres, which run for the full length of the siphon across the autotomy planes. Although valve adductions accompany autotomy, loss of segments of siphon can occur without any mechianical aid.