It is of interest to consider a common animal like the jellyfish
Aurelia. It is admirably suited for a leisurely life in the open sea,
where it swims about by contracting its saucer-shaped body, thus driving
water out from its concavity. By means of millions of stinging cells on
its four frilled lips and on its marginal tentacles it is able to
paralyse and lasso minute crustaceans and the like, which it then wafts
into its mouth. It has a very eventful life-history, for it has in its
early youth to pass through a fixed stage, fastened to rock or seaweed,
but it is a successful animal, well suited for its habitat, and
practically cosmopolitan in its distribution. It is certainly an
old-established creature. Yet it is very variable in colour and in size,
and even in internal structure. Very often it is the size of a saucer or
a soup-plate, but giants over two feet in diameter are well known. Much
more important, however, than variation in colour and size are the
inborn changes in structure. Normally a jellyfish has its parts in four
or multiples of four. Thus it has four frilled lips, four tufts of
digestive filaments in its stomach, and four brightly coloured
reproductive organs. It has eight sense-organs round the margin of its
disc, eight branched and eight unbranched radial canals running from the
central stomach to a canal round the circumference. The point of giving
these details is just this, that every now and then we find a jellyfish
with its parts in sixes, fives, or threes, and with a multitude of minor
idiosyncrasies. _Even in the well-established jellyfish there is a
fountain of change._
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