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Page 9
The jelly-fishes are all, in their adult state, locomotive beings.
They float freely and incessantly through the ocean, either impelled
by their own efforts, or driven by storm and billow. They for the most
part frequent the open seas, and shun the shore, their delicate frames
being endangered by the perennial strife between land and water. Being
designed for constant motion, for the navigation of the great waters,
their entire organisation is adapted to such a mode of life. We find
amongst those ocean-floaters the greatest perfection and variety of
locomotive apparatus; and they have been divided into sections,
according to the modifications of this portion of structure which they
exhibit. We shall endeavour to give a popular account of the leading
peculiarities of each, and note the most interesting points in the
history of the tribe.
In the first section, the animals are furnished with a disk or
umbrella of varying shape, which serves as a float, beneath which hang
certain processes connected with the functions of prehension and
digestion. In this division are included some of the best-known forms.
The creature, in this case, propels itself by the alternate
contraction and expansion of its disk, thus striking the water, and
driving itself forward. These movements take place at regular
intervals, and serve a double purpose. They not only propel, but at
the same time drive the water over the lower surface of the disk. Here
is situated a complicated net-work of vessels, and the fluids of the
body are thus exposed to the influence of oxygen, and receive the
needed aeration. The stroke of the disk, therefore, is not only a
locomotive, but also a respiratory act. The jelly-fishes of this
section move as they breathe, and breathe as they move. Hence the name
which has been given them--_Pulmonigrades_. We find the same admirable
economy of resources amongst the lower animalcules. The cilia which
propel them secure the aeration of the system.
It is evident that the motive apparatus in this section of the
_Acaleph�_ is but a feeble one. It only avails in calm weather. When
the sea is agitated, the jelly-fish is driven helplessly along. It
cannot choose its path. As its food, however, is everywhere abundant
around it, and it has no business that should lead it in one direction
more than another, there is no great hardship in this.
In this section are included some of the most beautiful, as well as
common of the tribe. The forms of the umbrella are often most lovely,
and present an astonishing variety. As an example of the beauty which
they sometimes display, we may refer to a species which resembles an
exquisitely formed glass-shade, ornamented with a waved and tinted
fringe. The most perfect grace of form, the transparency of the
crystal, and colour as delicate as that of the flower, combine to
render this frail being one of the loveliest of living things.
In another section, locomotion is effected by a modification of
ciliary apparatus. We have a familiar example in the _Beroe_ of our
own seas, a most attractive little being, and a prime favourite with
naturalists, who have described its habits and celebrated its beauty
with enthusiasm. We shall not soon forget the delight with which we
first made acquaintance with this graceful little rover. While
rambling along the shore in quest of marine animals, our attention was
arrested by a drop of the clearest jelly, as it seemed to be, lying on
a mass of rock, from which the tide had but just receded. On
transferring it to a phial of sea-water, its true nature was at once
revealed to us. A globular body floated gracefully in the vessel,
scarcely less transparent than the fluid which filled it. Presently it
began to move up and down within its prison-house, and the paddles by
means of which the beroe dances along its ocean-path were distinctly
visible. These paddles are nothing more or less than cilia of a
peculiar kind, ranged in eight bands upon the surface of the body.
They are set in motion at the will of the animal, and their incessant
strokes propel it swiftly through the water. By stopping some of its
paddles, and keeping others in play, the beroe can change its course
at pleasure, and so wander 'at its own sweet will,' through the
trackless waste. Beauty waits upon the course of this little crystal
globe. The grace and sprightliness of its movements must strike the
commonest observer. As the sunlight falls upon its cilia, they are
'tinted with the most lovely iridescent colours;' and at night they
flash forth phosphoric light, as though the little creature were
giving a saucy challenge to the stars.
The beroe is a most active being, its habits conforming to the
organisation with which it is endowed. Such an array of paddles
prophesies of a mercurial temperament and an energetic character. It
can, however, anchor itself and lie by when occasion offers. It is
provided with two long cables, prettily set with spiral filaments or
tendrils, by means of which it can make fast to any point. When not in
use, it can retract them, and stow them away in two _sacs_ or pouches
within the body, where they may be seen coiled up, through the
transparent walls. The mouth is a simple opening at one pole of the
globular body. No arms are needed. The beroe is spared the labour and
uncertainty of the chase. As it dances gaily along, streams of water,
bearing nutritive particles, pass through the orifice into its
stomach.
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