Both these oceanic forms of the class _Hydrozoa_ agree in being quite
unprovided with any apparatus for locomotion. Though their parts can be
moved among themselves, and sometimes with much vigour, as we have seen
in the case of the tentacles of the _Physalia_, yet the whole organism
would be absolutely confined for life to one spot, were the elements
motionless around it: it is merely passively driven hither and thither
by the winds and waves. But there are other members of the class, and
even some belonging to the same order, which have express organs of
locomotion. A most exquisite example is found in _Stephanomia_, of
which a specimen was taken in Kingstown harbour in July 1856, by Mr.
Joseph Greene, and described and figured in the “Proceedings of the
Dublin University Zoological Association” for that year. The float
(_pneumatocyst_) is a small bladder, which is filled with air, and
which has the appearance, when alive, of a globule of quicksilver;
while the slight inequalities of its surface reflect the light in such
a manner that it often looks as if a fine network of crimson veins
ramified over its surface. From the end of this oval bladder a long
fleshy tube hangs down in the water to the distance of six or seven
inches, the upper part of which is surrounded by a number--seven to
ten--of little clear transparent bells, looking very much like the
blossoms of our common Arbutus, and crowded, like flowers with short
footstalks, on the stem. These swimming bells (_nectocalyces_) are true
organs of locomotion; for by their contraction they drive out the water
contained in their cavities, and by their combined impulse, all acting
together, shoot the whole creature forward. This action I will explain
more in detail presently.
The long tube or common stem is very irritable, and under stimulus
is coiled up in a spiral, then gradually relaxed, and allowed to
hang loosely down. Attached to it throughout are the _polypites_, or
stomach-mouths, protected by leaf-like organs, which overlap them.
Numerous tentacles, too, of exceeding length and tenuity, and throwing
off branches, spring from the stem throughout its length, all the
extremities of each forming tiny oval knobs of spiral coils, the
whole capable of being thrown into the most beautiful and graceful
convolutions, now bent at right angles, and again thrown out in a
series of light airy-looking arches.
The exquisitely beautiful specimen described was taken in sultry
weather, when the sea was perfectly calm. “It lived in a large globe
of sea-water for several days, and was a most striking and beautiful
object: it generally lay quite upright in the water, the slightest
ripple upon the surface of which set it in motion; and immediately
coiling up its fishing lines, and shortening the length of its stolon
by about one-third, it threw its set of little balloons into strong
pulsations, until, weary of descending, it submitted to be guided by
its brightly glittering head, and soon ascended to the surface. When
subjected to examination it soon died, and was not long until it had
completely broken itself up.”[127]