CIRCULATION IN THE GIPPSLAND GIANT EARTHWORM MEGASCOLIDES AUSTRALIS

Pressures have been measured in the dorsal vessel (DV), ventral vessel (VV), and lateral hearts (LHs) of the giant earthworm, Megascolides au stralis. Mean pres- sure In the DV (22.1 +/- 9.8 cm H-2,O +/- 1 SD) wa s 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). Neith er imposing an orthostatic load by raising or lowering the tail, nor s imulated autotomy, had any effect on pressure measured in the anterior segments of the worm. Injecting the tracer (99m)technetium into the a nterior of the worm revealed that the circulation has two compartments ; blood flow was rapid in the anterior 15 or 20 segments and exception ally slow in the rest of the worm despite high blood pressure througho ut the major distributing vessel, the VV. The anterior rapid circulati on subserves the vital functions of the worm while the slow circulator y compartment is largely vegetative.