If we watch our Ascidia for a few minutes, we perceive that at
irregular intervals one or both of the gaping orifices are suddenly
closed and contracted, commonly both at the same instant. They
are, however, soon opened again; and we may discern, especially if
the specimen is in a glass vessel, and we watch it by the aid of
a lens, with the light of a window at its back, that a current of
the surrounding water flows from all sides to the taller orifice,
and pours down its tube; while occasionally we see the ejection of
a stream from the orifice of the shorter tube. Thus we have here a
receiving and a discharging tube, the exact representatives of the
two siphons in such bivalves as _Pholas_, _Venus_, etc. The former
leads down into a capacious sac in the interior, the walls of which
constitute the breathing apparatus. The inner surface is marked by
regular parallel ridges which run in a horizontal direction; and these
are again connected by vertical ridges at right angles, very numerous,
enclosing a vast number of oval compartments. The sides of these are
richly ciliated; and if the whole apparatus be carefully dissected out,
and laid upon the stage of the microscope, the course of the ciliary
currents may be distinctly seen, continuing with unabated vigour
and with unfaltering precision for a long time after the severance
of the organ from the body of the animal. But all this is seen to
most advantage, if we select one of the smaller species, which are
brilliantly transparent, such as one which grows in groups of elegant
tall vases, about an inch in height, around the edges of our rocky
pools,[150] or a tiny thing which forms a little heap of transparent
globules, like pins’ heads, attached to sea-weeds.[151] In either of
these, placed in a stage-trough of sea-water, we can watch at leisure
the performance of the various vital functions in healthy action, with
the knowledge that the little subject has not been martyred to science,
but is all the while enjoying its humble life with perhaps as much zest
as if it were still environed by the rough walls of its little native
basin of rock.
In the tiny pin-head of clear jelly, the microscope displays the
branchial sac hanging free in the cavity, like a bag of clear muslin.
The oval cavities divided-off by the rectangular ridges are about forty
in number, around each of which the ciliary waves incessantly roll,
as running spots of black. It is a very charming spectacle to see so
many oblong figures set symmetrically, all furnished on their inner
surface with what look like the cogs or teeth of a mill-wheel, dark
and distinct, running round and round with an even, moderately rapid,
ceaseless course. These black, well-defined, tooth-like specks are
merely an optical effect; they do not represent any actual objects,
but only the waves which the cilia make: the cilia themselves being
hairs, so fine as to be defined only with high powers. Occasionally we
see one or other of the ovals suddenly cease its movement, while the
rest go on; and now and then the whole are arrested simultaneously, and
presently all start off again together, with a very pleasing effect, as
if we were looking at the wheels of a very perfect and complex piece of
machinery. These phenomena appear to indicate that the movements are
under the control of the animal’s will, capable of being suspended or
continued, wholly, or in any degree, at pleasure; which is not the case
in the higher animals; our own respiratory movement, for example, as
well as the pulsations of the heart, going on without the concurrence
of our will, and even without our consciousness.