and/or from the collapse of old burrow walls. There is also avalanching of 

 surface floe into efferent and afferent burrow openings. The volume of 

 suspended sediment transported in this manner into the deeper benthos then is 

 proportional to total burrow volume. 



Method of Burrow Building 



New burrow building is accomplished as N. incisa first penetrates the wall of 

 its existing burrow with an undulating proboscis, displacing small amounts of 

 sediment. Worm position for this initial step is maintained through hydrostatic 

 enlargement of the posterior segments. The worm next penetrates the sediment 

 by lengthening the anterior segments and finally, as the head penetration 

 reaches its forward limit, the pharynx everts, creating a bulbous cavity in the 

 sediment. This type of sediment penetration has been described as "bolting" 

 by Schafer (1962). He states that this is a common form of sediment 

 penetration and is accomplished by contracting all body musculature. The 

 resulting pressure forces coelomic fluid into the anterior region, forcing those 

 segments without longitudinal muscles to expand and finally everting the 

 pharynx at peak pressure. When the pharynx is re-inverted, TV. incisa swallows a 

 slurry of sediment which was created as the compacted sediment was 

 penetrated. The whole sequence is repeated until the worm occupies the new 

 burrow fully and has established a new opening to the surface. The entire 

 process can usually be accomphshed in less than an hour. 



Spatial Extent of Burrowing 



A series of observations were made to determine if burrowing followed 

 some functional pattern vs random burrowing and also to determine the 

 horizontal and vertical scope of burrowing. A typical sequence of new burrow 

 formation is illustrated in Figure 20-3. This figure is a two-dimensional 

 reconstruction of a three-dimensional activity which is typically only partially 

 visible against the glass wall. It represents an example of burrowing but does 

 not indicate a predictable pattern of burrowing. Figure 20-3 shows actual 

 tracings from weekly photographs of a different burrow building sequence (a 

 single worm over a six-week period at 18°C). Observations were also made with 

 worms in large dishes of sediment so that horizontal movement could also be 

 assessed vdthout wall interference. Each new afferent burrow opening was 

 mapped, with new openings connected as a series of vectors (Figure 20-4). At 

 tlie time of the last recording, the worm is probably lying in a burrow between 

 opening 10 and 11, with the head located toward opening 1 1. The magnitude 

 of each horizontal vector was found often correlating with the size of the 

 worm (Figure 20-5), with burrow length approximately 2-3 times the length of 

 A^. incisa. Yet exceptions do occur, as shown in Figure 20-4 by the short 

 distance to afferent openings 2 and 4. By observing 30 six-week sequences of 



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