Abstract
The structures of the siphons of the bivalve Nuttallia olivacea and changes of their states under extended conditions were examined. The inhalant and exhalant siphons had similar tissue arrangements: four circular muscle and six longitudinal muscle layers were arranged concentrically. The muscle fibers were held loosely in networks of connective tissue, which made the layers quite loosely packed, even under contracted conditions. Radial muscles divided the siphon walls into radial compartments. Both siphons had two wide dorsal and ventral hemolymph channels and had a distinct hemocoelic layer around the entire siphon wall. The inhalant and exhalant siphons had six and eight longitudinal nerve cords, respectively, that were fairly evenly spaced around the siphon wall and reached almost to the tips of the tentacles.
When the siphons were nearly fully extended, the densities of the longitudinal and circular muscle fibers decreased markedly and the spaces between these thin muscle fibers were filled with hemolymph. The longitudinal muscle fibers not only stretched and became very thin, but also slid out of place along each other longitudinally. The great extensibility of the siphons depends on such longitudinal muscle transformation, which probably makes the siphon highly vulnerable to amputation, even by small forces, such as biting by juvenile flounders.
