194 REPORT ON MARINE SPECIMENS
quickly the character of the fauna on the floor of tidal rivers
changes. Dr. Sorby has called attention to this in a paper on
"Variations in numbers and habitat of Marine Animals on the
Coast of Essex during the last ten or twelve years."2 As this
paper deals largely with the particular part of the Orwell in
which the Pycnogons were found, I venture to quote at length
as follows :—
" 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, however, I 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 exterminated most of the animals previously living at the
bottom. There had been little, if any, change in the mud banks 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 Pin Mill, though lower down the bottom
was more muddy than it used to be. In 1901, the bottom was clean, and (here
was a most extraordinary 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."
It is interesting to note that not a single specimen of Jassa
pulchella has been found in the material obtained in July last,
although the mud was quite in evidence, while specimens of
Caprella linearis, Alcyonidium, and Sponges were again plentiful.
An Amphipod (Unciola crenatipalma), found this year in the
estuaries, is of interest, as it has apparently been recorded only
once from the East Coast, at Cullercoats.
Dr. Caiman concludes his letter with the remark:—"It
would be well worth while for someone to take up the matter
seriously and to work at some of the smaller Crustacea of the
Essex Coast, for there seems to be very little known about
them."
From the records available, it would appear that, of the
Crustacea and Pycnogonida captured on this expedition, eight
species are new to the county.
The Mollusca proved particularly uninteresting, as the
majority of the species found are represented by dead shells.
2 Essex Naturalist, vol. xii. (1901), pp. 17-21.