The action of the heart in these transparent creatures is equally
visible. Below the muslin curtain with its living chambers, down at
the very bottom of the body-cavity, there is a transparent sac of
membrane, which takes the appearance of a long bag, pointed at each
end, but not closed, and strangely twisted on its long axis, so as
to make three turns. This is the heart; and within it are seen many
colourless globules, floating freely in a clear fluid, which answers
to the blood. This circulates throughout the system in the following
manner:--We see a spasmodic contraction at one end of the bag, which
drives forward the globules contained there; the contraction in an
instant passes onward along the three twists of the vessel, the part
behind expanding immediately as the movement passes on, and the
globules are forcibly expelled through the narrow but open extremity.
Meanwhile the free globules surrounding the commencing end have rushed
in as soon as that part resumed its usual width, and are in their turn
driven forward by the periodic repetition of the pulsation. The fluid,
with its globules thus put in motion, is then driven along through the
interstices of the various organs of the body, not through a system of
closed blood-vessels, some finding their way along the transverse lines
that separate the rows of gill-ovals, until they sooner or later arrive
at the point where they entered the heart, to take the same course over
again.
As in the kindred forms of animal life, the same orifice, the same
cilia, the same currents are subservient to breathing and to the
reception of food; the stomach digesting the microscopic animalcules
which are poured with the entering stream through the receiving siphon.
At some distance within the interior of this orifice there are a series
of thread-shaped tentacles, affixed in a ring, which we may suppose to
exercise some kind of superintendence, by touch or other perception,
over the atoms which indiscriminately enter upon the stream, accepting
or rejecting. Probably it is in the exercise of the latter discretion
that those irregular regurgitations of the current take place,
accompanied by a momentary closing of the mouth, that we frequently
notice.