MARINE ANIMALS ON THE ESSEX COAST.. 21
of probably one or two square miles, so that the number may
have been something like a million. They were apparently all
males probably of the species Nereis dumerilii. Not one was seen
the day before or the day after. Some years earlier at Queen-
borough, I saw on one evening a similar large number of a some-
what different kind of Annelid, larger and coloured much more
red by haemoglobin, which were both males and females, and
though I most carefully looked for them that year and subse-
quently I never saw another. Once before I witnessed a similar
display off Sheerness. It thus seems probable that these worms
usually live in tubes which they build amongst algae, and just
occasionally, for a few hours, swim about on the surface in the
Heteronereis condition, probably when scattering the ova or
spermatozoa ; the remarkable thing being that apparently all do
so at the same time. It is impossible to say what occurs lower
down out of sight, and the swimming at the surface probably to
some extent depends on the weather being calm.
I must now describe a series of changes which seem more
easy to understand than those already noticed. Until a few
years ago the bottom of the Orwell at Pin Mill was almost
entirely free from mud, and was to a large extent covered with
Sponges, Alcyonidium, and compound Ascidians. The number
of Caprella linearis, a small Terebella (Nicolea zostericola), and the
curious worm, Siphonostoma diplochaitos, was astonishingly great;
and a larger number of different species of animals could
easily be obtained by dredging than in any other place along the
coast. In 1898 and 1899 I however found the bottom covered
with a fine tenacious mud, built into short, stout, soft tubes by
enormous numbers of the small Amphipod, Jassa pulchella, which
had increased so much as to have smothered and almost exter-
minated most of the animals previously living at the bottom.
There had been little, if any, change in the mudbanks left dry at
low water, nor any marked change in the animals living there.
To my surprise I found the bottom clean in 1900, near Tin Mill,
though lower down the bottom was more muddy than it used to
be. In 1901 the bottom was clean and there was a most extra-
ordinary number of small specimens of simple Ascidians, to the
almost complete exclusion of other animals. Probably some of
these changes have been due to the extensive dredging operations
carried on to improve the navigation to Ipswich, and perhaps
in a few years the original conditions may be restored.