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THE ESSEX NATURALIST
Figure 3. Semi-diagrammatic vertical section through sponge mass
showing mud tubes and individuals of Polydora and (a)
Sagartiogeton undata, (b) Diadumene cincta extending, (c) D.
cincta partly contracted. Note absence of a collar and presence
of a narrower capitular region in (b), but presence of the collar
in the more contracted anemone at (c). When fully extended
the anemone is worm-like in appearance.
The Polydora are relatively much smaller and more
numerous than they are shown.
a thickness of 10 to 12 cm., and a similar depth. Large numbers
of the two anemones Sagartiogeton and Diadumene are also pre-
sent in their own tubes and appear to have developed with the
sponge-worm association, which seems to be an annual growth.
In March, examination shows a thin layer of sponge on the piles.
In April this layer is thicker but still firm. By late July the
large commensal mass has developed and is then so heavy and
fragile that it is easily fragmented. It is not yet known how long
the mass may persist before collapse and further examinations
are needed in August, September and October. It is likely that
the anemones are unwelcome guests, predators upon Polydora.
A few members of the Crustacea have been collected. The
mysid Praunus flexuosus (Muller) is plentiful in the waters of the
creek; the amphipods Gammarus locusta (L.) and Talitrus
saltator (Montagu) have been collected in numbers from the weed.
The prawns Palaemonetes varians (Leach) and Palaemon elegans
Rathke are plentiful, as is the common shrimp Crangon vulgaris
Fabricius in the muddy sand above the dam. The only crab so
far obtained is Carcinus maenas (L.) and the most prominent of
the Isopods are the sea slater Ligia oceanica (Linnaeus) and the