300
THE ESSEX NATURALIST
Paludestrina stagnalis
Table IV shows that this minute gastropod has a very limited
distribution, occurring only on the long transect. Where it is
present it is found in considerable numbers. It avoids almost com-
pletely the areas in which both species of Corophium are found.
These observations seem to confirm the conclusions of Hart (1930)
who observed at Whitby that 'the small gastropod P. stagnalis
occurred in mud of a slightly different nature, where C. volutator
was absent, and here they were found in countless thousands'.
Arenicola marina
On the muddy beaches of Shoeburyness this species occurs in
vast numbers, the whole beach being covered by their casts. In
Benfleet Creek, however, they are almost absent. Where they
are present they seem to be confined to the bottoms of temporary
pools which form in small hollows in the mud in a narrow belt
in the Leigh Island side of area B—station 27 is on the edge of
one such pool.
Sabellid tubes
The identity of these tubes is not certain as none were found
to be inhabited. They are composed of fine sand particles, bound
together by mucus and with no calcium in their composition—
this suggests that they are some form of Sabellid tube. They are
limited in their distribution in the Creek, being found in two
distinct zones on the steep slopes of area B. Table IV shows that
these belts are found roughly from station 3-7 and 18-24. They
were also found in a small part of area A. The tubes project
vertically about 1 cm. above the surface of the substratum and
extend down to about 10 cms. depth at which level a layer of
sand concretion exists, very like an iron pan.
OTHER SPECIES
Carcinus maenas
This species of crab could be included either here or in the
above section because although it feeds and lives on the surface it
rapidly burrows into the mud, just below the surface, when
slightly disturbed. The most noticeable concentrations of C.
maenas are in the deep channels of the saltings where the mud is
extremely soft and where there is usually a small trickle of surface
water. They are also to be found in the main channel at low
tide.
Mussel Bed
The extent of this 'mussel bed' is shown in figure 2 by area D,
in the vicinity of station 2 on the long transect. It is situated
in the broad, flat bottom of the stream bed in a region of very soft
mud. The whole area is, however, covered with a deposit of
broken shells of a number of molluscs, mainly S. plana, and there
are extensive deposits of small pieces of anthracite coal which