MARINE BIOLOGY AT SKIPPER'S ISLAND
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rock worm (Marphysa sanguinea (Montagu)). The two tubes
appeared to communicate with each other beneath an empty
mussel shell at the bottom of the cleft. Did Marphysa take over
from a previous owner, or did it displace the Amphitrite which
we found?
Figure 2. Polydora ciliata, semi-diagrammatic, b, branchiae: pc,
prehensile chaetae of the fifth segment: pr, prostomium with
four small eyes : t, tentacular cirrus.
The Spionid worm Polydora ciliata (Johnston) (figure 2) is
found in large numbers on the dam. It appears to be present in
two types of habitat and shows marked differences of size in
the two situations. In the chalky blocks it forms mud-lined
tubes: here it is a fairly short, broad worm. But it also occurs
in the commensal association with Polymastia where it grows to
twice the length of the chalk inhabiting form and with a more
uniform breadth of the segments, longer tentacular cirri and
larger branchiae. Although otherwise identical with the chalk
borer it may be a different species.
The commensal association already mentioned is formed
primarily from an intertwining mass of sponge and Polydora
tubes in about equal proportions (figure 3). These masses grow
to large size, curving round the piles on the upper side of the
dam at mid-tide level. They may reach a length of 30 to 45 cm.,