MARINE ANIMALS ON THE ESSEX COAST. 19
later years I have found one or two other animals I had never
seen before in the district.
In considering this question it is, however, impossible to be
sure that certain of the apparent changes are not due to the
remarkably local distribution of some animals, and it is possible
that the exact locality where they abound may shift somewhat
from year to year, owing to a partial migration. As bearing on
this subject, I may say that certain animals are very numerous
in one particular place, though I have met with very few or none
anywhere else over a wide district. In some cases this may
fairly be attributed to the very peculiar conditions under which
they are found. For example, I never saw a single individual of
a Phascolosoma (not yet identified) until in 1899 I hit upon a
particular locality near Brightlingsea where several specimens
may be found in each spade-full of the sandy gravel. In 1900
and 1901 I examined this place more fully, and it seems to me
that this unusual abundance in one part and absence elsewhere,
close at hand, is due to the peculiar local conditions; since the
Phascolosomae seem to occur only where this sandy gravel is kept
constantly soft by salt water draining out through it, when
the tide is low, which occurs only in one part, at a particular
level,
There is a locality in the Orwell, a short distance below Pin
Mill, usually at about the level of half tide, but perhaps varying
with the season, where a species Synapta occurs so abundantly
that specimens may be collected out of the mud by dozens ; and,
yet, I do not remember seeing a single individual elsewhere along
the coast. I cannot understand why this should be so, unless it
be due to the percolation of a certain small amount of fresh water
from the adjoining shore, which in this particular part is fringed
with reeds. In 1901 I could not find a single specimen. In the
Deben also, near Waldringfield, I have been able to collect in
less than an hour more specimens of Priapulus caudatus than I
could obtain in weeks in any other place along the coast ; and
yet I cannot even suggest a reason for this, since there do
not appear to be any conditions not met with in many other
localities.
So far most of the animals referred to are more or less fixed,
or probably do not move far, but there seems to be an equally
great variation in those which, like Medusae, float with the tide