BIOMECHANICS OF WATER-PUMPING BY CHAETOPTERUS VARIOPEDATUS RENIER. SKELETOMUSCULATURE AND KINEMATICS.

1. The skeletomusculature of the water-pumping segments of Chaetopterus variopedatus was examined in detail and the behavior of the segments during active water-propulsion was analyzed using cinemaphotographic techniques. 2. The musculature of the pumping segments consists largely of thin sheets of radial, transverse, and circular fibers, located immediately beneath the thin integument. Exceptions to this are the numerous isolated fibers running axially across the coelomic cavity of the parapodial rim and the major remotor muscles which originate in the neuropodial "sucker" of the adjacent posterior segment. 3. The motion of the pump segments is basically reciprocative, with the axial displacement during power and recovery strokes approximating a sine function. Centrifugal radial displacements (during the power stroke) effectively seal the lumen of the tube; centripital radial displacements (during the recovery stroke) allow water from the inlet side of the tube to by-pass the pump segment. 4. Comparison of the muscle anatomy with the displacements and shape changes which take place during water-propulsion permitted analysis of the muscle actions during the pump cycle. It is concluded that: (a) both axial and radial displacements during the power stroke are caused by a single set of muscles (remotors); (b) maintenance of the extended-disk configuration is consequent upon coelomic fluid being forced into the parapodial rim, with the axial muscle fibers acting as guy-wires to resist overexpansion; (c) most of the sheet-like muscle groups contract during the recovery stroke, thereby causing the segment to assume a nearly spherical configuration; and (d) two sets of muscles (promotors, oblique parapodial) are responsible for the axial displacement during recovery. 5. The water-pumping mechanism of Chaetopterus is compared to those of the majority of worms, which are based on persistaltic or undulatory movements.