THE MECHANISM OF BURROWING IN THE POLYCHAETE WORM, ARENICOLA MARINA (L.) E. R. TRUEMAN Zoology Department, The University, Hiill, England New techniques of recording pressure changes and activity have increased our knowledge of the fluid dynamics of burrowing in Arenicola marina (L.) (Trueman, 1966a). The new information, together with an understanding of the habits of the lugworm, derived from the extensive researches of Wells (1961), allow an assess-ment of the mechanism of burrowing to be made. Initial entry of a worm into sand is brought about by the eversion of the proboscis at comparatively low coelomic pressures (2-6 cm. water pressure) and when several segments have passed beneath the surface a series of pressure peaks commence, each of about two seconds duration. They occur at intervals of 5-7 seconds, and as burrowing progresses increase in amplitude up to 110 cm. During burrowing, waves of peristaltic contraction pass along the trunk from the posterior segments to the anterior buried region of the worm where they appear to develop into high pressure peaks by the synchronous contraction of the longitudinal muscles of all or part of the trunk segments. The fluid of the essentially single trunk coelom acts in a hydraulic system which allows the force produced by the longi-tudinal muscles of the posterior trunk to be transferred to the anterior end, there to be utilized in burrowing. The principal function of the high pressure is to anchor the anterior end during the contraction of the longitudinal muscles. Each time the pressure increases the anterior end is pressed firmly against the substrate, while the posterior trunk is pulled into the burrow. The purpose of this article is to consider further observations of the movements made by Arenicola during burrowing and to compare the mechanism with that of other animals, in particular bivalve molluscs. MATERIAL AND METHODS Observations were made of the burrowing of Arenicola of 15-20 cm. length both at Hull and at the Marine Biological Laboratory, Millport, using specimens which would burrow rapidly. Direct visual observation of burrowing into sand could be made from above or from the side, through the glass of an aquarium tank. The latter was largely, but not always, unrewarding as even when the worm was close to the glass a thin layer of sand could obstruct detailed observation. Accordingly a technique (Trueman, 1966a) of continuously recording the pressure imparted to the sand by a burrowing worm was further developed by use of a more sensitive pressure transducer (Statham, Model P 23 BB, maximum sensitivity 0.4 cm. pressure/cm, pen deflexion) which was coupled to a multichannel pen recorder. Both instruments were obtained from E. & M. Instrument Company Inc. 369
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