The _nectocalyx_ or swimming-bell is, however, seen in a much more
developed condition in the delicate forms which swarm in our harbours
and about our rocky coasts in summer and early autumn, and are known
by the name of Naked-eyed Medusæ. A common and very characteristic
example of this order is the Tongued Sarsia,[128] in which a single
swimming-bell forms the greatest portion of the whole creature. It
is a tall dome of crystalline colourless flesh, thick at the summit
and thinning off at the edges; about half an inch in height. From the
interior of this dome hangs the single polypite, exactly as the clapper
hangs from the top of a bell. It is long and cylindrical, abruptly
attenuated at the upper part into a sort of foot-stalk, and at the
extremity enlarging into four fleshy, very protrusile, lips, capable
of seizing and sucking in an object much larger than the diameter of
the whole polypite. I have seen a Sarsia, in confinement in a tank,
lay hold of, and swallow, a newly-hatched fish, notwithstanding the
activity of the latter. For hours afterwards the little green-eyed fry
was visible, the engulphment being a very slow process; the Medusa,
however, never let go its hold; and gradually the tiny fish was sucked
into the interior, and passed up the cavity of the polypite, becoming
more and more cloudy and indistinct, as digestion in the stomach
gradually dissolved its tissues.
These little creatures are endowed with very effective powers of
locomotion. In the unbounded freedom of their native sea, and in the
limited dimensions of a glass vase, they are alike sprightly. By rapid
pump-like contractions of their _nectocalyx_, they dart through the
water, and shoot round and round, almost with the force and swiftness
of a swimming fish. The summit of the bell always goes foremost,
whether the direction of the movement be vertical, horizontal, or,
as is most commonly the case, oblique; and the tentacles, and the
long white polypite, drag behind in trailing lines. Now and anon, the
shooting is suddenly suspended, the bell hangs over and remains awhile
motionless, the tentacles are allowed to depend like spiders’ webs,
or are suddenly drawn up into shrivelled puckers, become mutually
entangled and intertwisted, then slowly free themselves, and hang down
again. Sometimes the motionless bell itself sinks very gradually, and
the tentacle-threads take the most elegant curves and arches in their
descent.
The manner in which the strong pulsating movements of the Medusæ are
performed depends on the position and action of certain bands of
muscular tissue. Four of these radiate from the centre of the dome
to the margin. This course is not a straight but a curved one. When,
therefore, these bands are simultaneously and forcibly contracted in
length, they are drawn from a curved into a straight line, and the
cavity, which was bell-shaped, becomes more conical, and its capacity
is considerably diminished; a portion of the water which it before held
is therefore driven out at the mouth, and by its reaction forces the
animal forward with a jerk in the opposite direction. Besides these
radiating muscles, there are circular bands which pass round the margin
and the interior walls of the dome. These by their contraction diminish
the volume of the cavity, and aid the action described above.