The burrowing mechanisms of two soft-bodied marine invertebrates, Polyphysia crassa (Annelida: Polychaeta) and Priapulus caudatus (Priapulida) were re-examined and extended utilizing computer image analysis. Predetermined points on sequential photographs of animals burrowing in methylcellulose (an artificial medium of high transparency) were digitized and stored as x, y coordinates from which were calculated segment length, width, and volume in addition to producing real and straightened images of the animal on a plotter. Both species are adapted to soft substrata and both utilize a direct peristaltic wave to advance the body into a cavity or loosened area formed by anterior structures. Polyphysia displaces and loosens the substratum with lateral scraping movements of its anterior six segments (the “head region”) while Priapulus makes a large cavity anteriorly by the forceful eversion and dilation of the praesoma. Unlike Priapulus, Polyphysia utilizes a different method of locomotion when moving on the mud surface as opposed to burrowing, the former involving two direct peristaltic waves at a time and negligible internal pressures resulting in nearly continuous advance. However when burrowing, Polyphysia, like Priapulus, utilizes a single direct peristaltic wave alternating with phasic pressure pulses which advance the animal in step-wise fashion.