THE MECHANISM OF BURROWING IN THE POLYCHAETE WORM, ARENICOLA MARINA (L.)

Abstract

The burrowing activity of Arenicola marina has been studied by means of direct observations and recordings of pressure changes both internally and in the adjacent sand. Maximum coelomic pressures correspond to the swelling of the anterior segments to form an anchor (dilation anchor) which allows the posterior trunk segments to be pulled into the sand and the sides of the burrow to be compacted. This condition occurs alternately with the occurrence of flanges on the anterior segments as the worm elongates by contraction of the circular muscles. The flanges tend to form an anchor (flange anchor) from part of the anterior region so that lengthening forces the head into the substrate, as the proboscis everts. At the same time the posterior trunk region undergoes some retraction from the burrow. Essentially the same method is used by all soft-bodies animals to dig into sand, notably in bivalve molluscs. In this group a pedal anchor is formed by the foot becoming swollen by the hydrostatic pressure derived from adduction of the valves immediately before the shell is pulled down by the pedal retractor muscles. The shell is subsequently held still by the opening of the valves against the substrate (shell anchor) while the foot is protracted by the intrinsic pedal musculature.