250 MR. A. T. WATSON ON THE STRUCTURE 
muscular exertion. The necessity for the unusually ample 
equipment of uncini in the “abdominal” region, in order to 
carry the tube with the body, is thus explained. 
The rate of burrowing is not, of course, rapid when a con- 
siderable thickness of sand has to be penetrated, ten hours being 
required, under favourable circumstances, to bore through 
150 mm., though 20 mm. would be run through in a few minutes. 
As the tube is very similar in colour to the sand in which the 
animal dwells, observation upon its displacements was at first im- 
possible ; but, by the admixture of a sufficiently large proportion 
of white (Calais) sand, I ultimately ascertained the burrowing 
act to consist of a combination of an undulatory with a screwing 
motion. When engaged in this operation, the animal occupies 
the anterior end of the tube, the conical tip of which is filled by 
the densely-packed tentacles; while the first row of shelly frag- 
ments, spread out, possibly forms a cutting edge. By twisting 
the anterior portion of the body and tube round in the sand, the 
animal practically converts them into a short hollow screw, 
about 10 to 15 mm. in diameter, which, working somewhat spas- 
modically at intervals of a minute or two, to afford time for rest 
and stretching the tube, gradually makes its way to the surface. 
On reaching this, the action of the worm confirms the observa- 
tion in a very curious manner, as, in order to get the desired 
length of tube exposed, it bends hard over, and pressing on 
the surface of the sand, describes another revolution with its 
anterior end, which results in a further stretching of the tube, 
accompanied by the formation of a saucer-like hollow in the 
sand. It will be noticed that I speak of stretching the tube; and 
in explanation I may state that, as a rule, the animal does not 
carry the whole tube bodily with it to the surface, but attains 
its end by causing it to stretch the required distance. In some 
of my experiments, a tube apparently only 65 mm. long stretched 
to a length of 200 mm., but when again uncovered contracted to its 
original dimensions. Occasionally, however, the animal, for the 
purpose of changing its habitat, quits the sand, and in this case 
it brings the whole tube with it. This is an act which probably 
sometimes costs the animal its life, for McIntosh states (10. p. 103) 
that Owenia filiformis, with its gravelly tube, is a favourite food 
of haddocks, cod, and flounders. In order to re-bury itself, the 
worm reverses its position, and, protruding the branchial ten- 
tacles through the opening at the posterior end of the tube, 
