20 VARIATIONS IN NUMBERS AND HABITAT OF
over great distances. In such cases it is not a question of
occasional inspection or of looking for them or dredging, since
they are visible at any time in suitable weather from the deck of
the yacht, when they are present. There seems to be great
variation in the numbers of the very common Medusa, Aurelia
aurita, in different years, unless it be that in some years they
swim near the top and in others low down out of sight. This
they certainly do according to the weather, coming towards the
top when fine and smooth, but such a difference could scarcely
occur throughout an entire season. In July, 1888, I saw in the
course of a few days in the Alde below Orford, more deep blue
specimens of Cyanea lamarkii than I have seen anywhere before or
since, though year by year I have looked for them most atten-
tively and not seen one. This handsome species had not been
previously observed within some hundreds of miles of the English
coast. Many years ago I saw at Queenborough a great many
specimens of the beautiful Medusa, Chrysaoya isoceles, but lately I
have year after year seen comparatively very few. Last year
(1900) I think they were somewhat more common; but, strange
to say, they were all a very pale variety, with none of the dark
radiating stripes on the upper surface. In the last few years
good fine specimens of Pleurobranchia pileus have been more and
more scarce, whereas formerly they were common. Time alone
will show whether such changes as these just described are
permanent or only temporary. The great variation in the
numbers of fish and shrimps is, of course, well known.
Of a totally different character is the remarkable occasional
appearance of Heteronereis. I have never found specimens in
mud, but in dredging I have obtained a few, very imperfectly
developed. I have occasionally found them in my aquarium,
into which they must have been introduced when young, living
in tubes amongst algae, before assuming the adult form. The
remarkable fact is the manner in which they are now and then
seen swimming at the surface of the sea water. I must have
lived at anchor in the Orwell and Stour for a total period of
between one and two years, and yet I never saw Heteronereis in
that locality except off Harwich on July 16th, 1898, and perhaps
once in the Orwell at Pin Mill. I do not know how early they
appeared, but from the time I turned out at 5 a.m. until 7 a.m.,
they were swimming about on the surface in such numbers that
they were only a few yards apart, and, as I judged over an area