Circulation in the Gippsland Giant EarthwormMegascolides australis

Abstract

Pressures have been measured in the dorsal vessel (DV), ventral vessel (VV), and lateral hearts (LHs) of the giant earthworm, Megascolides australis. Mean pressure in the DV (22.1 ± 9.8 cm H₂O ± 1 SD) was similar to that in the VV (21.8 ± 10.1 cm H₂O) although pressures were much more pulsatile in the DV than VV. Peak and minimum pressures in the DV were significantly above and below, respectively, those in the VV Contraction frequency of the DV was 6. 8 ± 1.9 ‧ min⁻¹. Neither imposing an orthostatic load by raising or lowering the tail, nor simulated autotomy, had any effect on pressure measured in the anterior segments of the worm. Injecting the $tracer^{99m}technetium$ into the anterior of the worm revealed that the circulation has two compartments; blood flow was rapid in the anterior 15 or 20 segments and exceptionally slow in the rest of the worm despite high blood pressure throughout the major distributing vessel, the VV. The anterior rapid circulation subserves the vital functions of the worm whil...