The most interesting circumstances in the history of this large
jelly-fish are the wonderfully varied phases through which it passes
in the earlier stages of its existence. Along the margins of the
lengthened flaps of the polypite there are found remarkable pouches,
within which the ova are placed, and whence they are hatched in the
form of soft flat animalcules, capable of swimming by means of cilia.
This has been called a _planula_. After swimming a while, it alters
its form to that of a pear, and presently adheres by its slender end
to a sea-weed or rock under water, hanging downward. A depression now
appears in the larger end, which deepens and forms a mouth and stomach,
and the little _planula_ has assumed a polype-form. Four tiny warts now
spring from the margin of the mouth, which lengthen into tentacles;
four more then shoot in the interspaces; these eight increase to
sixteen, then to thirty-two, all at the same time acquiring great
length. In this stage, in which it is very common in our aquaria, it
has been supposed a new animal, and has been named _Hydra tuba_. The
space between the margin and the mouth has widened into an “umbrella,”
and the mouth has protruded into a polypite. The whole is of a
translucent white hue, and the body without the tentacles is ordinarily
about one-sixth of an inch high.
This stage sometimes lasts for years without further change, except
that creeping root-threads shoot from the attached base, which send
up at intervals buds that grow into _Hydræ_; and buds break out from
different parts of the body itself, which likewise develop themselves,
the form in both cases being exactly similar to that of the present
_Hydra tuba_. Thus we frequently find numerous colonies of these
tiny creatures crowded together.
At length a change takes place. The body enlarges both in length and
thickness, and begins to show traces of rings or segments, as if it
had been tied tightly round with threads at regular intervals. In this
stage it has been described under the name of _Scyphistoma_. These cuts
deepen, and the segments thus marked off become hollow; and so they
resemble a pile of tiny saucers set one within another, each of which
is now divided at its rim into eight teeth. In this stage it has been
once more named, as if an independent animal, _Strobila_.
All this time the tentacles have been set around the terminal margin,
but now these are absorbed, and a new set rapidly spring from the
basal segment. The saucers become very loosely attached; at length
the end one breaks away and swims through the sea, as a true Medusa,
though no more than a sixth of an inch wide, pumping as it goes in
proper parental wise. Others quickly follow, and thus a colony of tiny
swimming jelly-fishes are shooting hither and thither in the liveliest
manner. Strange to say, these little Medusæ, which as to details differ
much from their adult form, have been again described, under the
name of _Ephydra_; all these appellations indicating the assumptions
of various naturalists, who found the little creatures in their
respective stages, without knowing their previous history, that each
was an independent form of animal life.