Burrowing Habit and the Early Evolution of Body Cavities

Abstract

RECENT researches on burrowing1 have substantiated Clark's suggestion that the method of penetration into a soft substrate is essentially the same in all soft-bodied animals2. The basis of this mechanism (Fig. 1) is the formation of two types of anchors, first by dilation of part of the body above the distal extremity to form what may be termed the “penetration anchor”, and second, by the distension of the extremity, the “terminal anchor”. These terms are here introduced to describe the anchorages from a functional aspect in all soft-bodied animals and respectively supersede terms used in previous accounts for restricted groups of animals3,4. When the circular muscles contract, the tip of the animal elongates and the penetration anchor enables this to be extended downwards by preventing the animal from being thrust upwards out of the sand. The terminal anchor, which is formed by part of the body-wall at the lower extremity of the animal pressing outwards against the substrate, allows the upper part to be drawn into the burrow by the contraction of longitudinal or retractor muscles. These anchorages are successively applied until burial is complete. Before the animal has entered the burrow, the penetration anchor cannot be applied and the animal is only prevented from being pushed backwards by its weight, while making rather weak penetrative or scraping movements.