102 THE ESSEX NATURALIST.
it is stated that on September 26, 1905. Mr. R, Vallentin col-
lected Chirocephalus in a roadside pond, between Gwinear Road
Station and Helston, but that a week later not a single speci-
men was to be found. Dr. A. S. Packard, the well-known
American authority on the Phyllopods, has also placed on record
a similar case of the rapid disappearance of these creatures.
Now. such experiences, although very disappointing from
one point of view, are also exceedingly interesting, and they
at once start us asking questions as to the reason for the
phenomena. Mr. Vallentin, in the case referred to, imagined
that the disappearance might be due to a herd of cattle on their
way to Helston market having rushed into the pond and killed
all the specimens! Dr. Packard thought that it was a question
of temperature and that with the approach of warm weather
all the animals died off.
My own opinion is that the sudden disappearance is clue
to enemies in the pond itself—Ostracods, Insect larvae, etc.
—and this leads to the conclusion that Chirocephalus probably
cannot maintain itself in an ordinary pond, where it has to com-
pete with the commoner forms of pond-life. This would explain
why it is usually recorded from small, non-permanent pools,
cart-ruts, etc., just the sort of places, in fact, where competition
is presumably at a minimum. It would also explain why
Chirocephalus appears to be so comparatively rare. It may
be supposed that it does actually commence to develop (from
eggs carried about by birds and in other ways) in many more
ponds than we imagine, but that it is soon disposed of by some
of the other inhabitants and in consequence collectors do not
chance to find it.
The sudden appearance of Chirocephalus in places where it
has never before been seen is just as remarkable a fact as the
sudden disappearance above alluded to. It is no doubt due to
the eggs retaining their vitality for long periods, even after be-
ing frozen or embedded in dried mud, and to their distribution
over wide areas by birds and other agencies.
There are a good many records now, mostly in the last fifteen
years, of the occurrence of Chirocephalus in England, but
they are mainly from the more southerly counties. The
most northerly record that I know of is from near York (speci-
mens in the British Museum). There are no records so far
from Wales or Scotland or Ireland.