THE ESSEX FIELD CLUB. 107
among the whole of the group of the Cladocera in which this occurred, and then
only in the generation arising from the winter eggs.
Mr. Scourfield said that he had, of course, seen many other kinds of
Entomostraca that afternoon, but he did not think that any of them were
sufficiently noteworthy to form the subject of any special remarks. He was in
great hopes, however, that, when he came to examine his collections under the
microscope, he would find some specimens of a minute and very rare species,
known as Anchistropus emarginatus, which had previously been collected from
these reservoirs. The facts concerning Anchistropus were somewhat peculiar. It
was first seen in this country by Robertson, in the Paisley Canal, in 1863, but
was not recorded again until 1900, when it was found by Mr. Kane, in Lough
Erne. In 1901, Mr. Sidwell found it in the reservoir close to where they were
then assembled, and the next year the speaker found it in the same piece of
water. In 1903, Mr. Gurney found it in the Broads District in Norfolk. On each
occasion, only one or two or a few specimens were obtained, and the question
naturally arose as to the meaning of these extremely few records in spite of the
evidently-wide distribution of the species. The most likely explanation seemed
to be that Anchistropus was a form having some very peculiar mode of life,
such as a close association with a particular plant or animal (the fresh-water
sponge was suggested as a possibility), but that the true habitat had not yet
been discovered, the few individuals so far taken being merely stray specimens.
With regard to the beautiful polyzoan Cristatella mucedo, some specimens of
which had been obtained that afternoon, Mr. Scourfield said that a few years
ago the "statoblasts" of this species were so abundant in one of the reservoirs
that they formed a dark red scum along the margins, and could literally be
collected in handfuls. A solid mass of these statoblasts, taken on the occasion
referred to (1902) and almost filling a two-ounce bottle, was exhibited.
Purification of Water by Living Organisms.—At a later stage of the
proceedings, Mr. Scourfield dealt with the role played by living organisms in the
purification of water by filtration on a large scale. He said that the sand-filters,
when newly prepared, allowed a great deal of sediment and large numbers of the
smaller organisms to pass; but that, after tweny-four hours or so, they became much
more efficient, and the water issuing from them could then be used with safety.
The explanation of this change was that not only did the fine spaces between the
sand-grains become more and more clogged and so enable the filter-bed to act
better merely as a strainer, but that a layer of living organisms gradually collected
on the surface of the sand and formed a kind of felt. The principal organisms
concerned in this were various species of filamentous diatoms and other algae
such as Melosira, Fragilaria, Spirogyra, &c., but many other species of plants
and animals were also to be found in this living filtering layer. The action of
this layer upon the water was undoubtedly something more than a mere straining,
and it seemed probable that the more minute particles in the water were stopped
by adhering to the gelatinous envelopes with which most of the organisms
forming the felt were provided. That this sort of action did take place was
shown pretty conclusively by the fact that even a bed of coarse gravel could
(after a day or two) reduce the amount of suspended matter in the water, including
even the bacteria, very considerably. The stones became coated with a thin
gelatinous layer of organic origin which entrapped a certain proportion of even
the smallest particles present in the water, notwithstanding the fact that the
spaces between the stones were relatively very large.