P. H. GOSSE, _del._ LEIGHTON, BROS.
SCARLET SERPULA.
]
If we carefully break, by a moderated blow with a hammer, the shelly
tube, so as not to crush the tenant, we are able to expose the latter
to view. We then see that its length is by no means commensurate with
the length of its house, of which indeed it inhabits only the last-made
portion, having behind a roomy space into which to retire in case of
need. It is not more than an inch or an inch and a quarter long, rather
wide in proportion, and flattened, with a well-marked distinction
between the corslet and the abdomen. The former carries on each side
prominent foot-warts, which are vigorously protrusile, and within which
bundles of strong bristles are thrust to and fro. On the upper part
of each foot, extending half across the back, is a row of microscopic
hooks, wielded by long thread-like tendons, which are fixed, on
mechanical principles, to the attached end of each hook. By the aid of
these, the Serpula so cleverly withdraws with lightning-like rapidity
on alarm. By the action of muscles of indescribable delicacy, the hooks
are projected to some distance beyond the surface of the body. These
organs are formed on the model of a hedger’s bill-hook, only that the
edge is cut into long teeth. Carefully counting them I have found that
each Serpula carries about 1900 such hooks on its corslet, and that
each of these being cut into seven teeth, there are between 13,000 and
14,000 teeth employed in catching the lining membrane of the tube, and
in drawing the animal back.
The protrusion of the fore parts, which takes place in a much more
leisurely manner, is performed by quite another set of instruments,
formed on a totally different principle. Their action is a pushing or
shoving. The instruments for effecting this are the fine but strong
bristles which run through the feet. Each bristle is composed of a
strong rigid unyielding shaft, having an expanded shoulder drawn out
into a point. On one side of this pointed shoulder may be remarked a
double row of fine teeth, admirably calculated to catch against any
roughnesses of the surface with which they come into contact, against
which they then push with the force of the proper muscles. Acting
diagonally backwards, from the two sides of the animal, the result
of the combination of the forces is that the animal itself is pushed
forwards, and so protruded from the mouth of the tube. The feet on
the hinder portion of the body are, according to Dr. Williams,[142]
modified in structure with express reference to the duties of mopping,
sweeping, scraping, and wiping the inferior closed end of the
habitation. I think, however, he has forgotten that this end, formed by
the animal in its infant state, must now be very much too strait to be
reached by any portion of the body, or by any of its organs.
Dwelling in a tubular house, the Serpula would find its breathing
organs scarcely available, if these were placed, as in most _Annelida_,
in pairs on the body-segments. They are therefore much modified, and
that not only in position but in form. They consist of most elegant
comb-like filaments, richly coloured, arranged in two rows around
the front extremity, one row on each side of the mouth. They are
graduated in length, and are so affixed, that, where the rows meet
behind, they can be thrown-in, so that a vertical view of the circular
coronet shows a great sinus in it. These brilliant gill-tufts form the
most attractive feature in these elegant Worms, and are individually
most exquisite examples of mechanical contrivance. Examined under a
low microscopic power, they present a most charming spectacle. Each
filament consists of a pellucid cartilaginous stem, from one side of
which springs a double row of secondary filaments like the teeth of a
comb. Within both stem and filaments the red blood may be seen, with
beautiful distinctness, driven along the artery, and back by the vein
(which are placed close side by side) in ceaseless course, constituting
a very striking spectacle.
The exterior of these organs is set with strong cilia, so arranged that
the water-current is vigorously driven upward along one side of the
filament, and downward along the other. Yet the combined result of all
the branchial currents is to bring a powerful vortex into the enclosed
funnel, the bottom of which terminates in the mouth. The food which
sustains nutrition is thus brought to be swallowed, a large quantity
of water being at the same time constantly poured into the body; this
is discharged (by the agency of a ciliated lining of the hinder parts)
in the form of a strong current, which, impinging against the closed
end of the tube, is turned upward, carrying with it all extraneous or
fœcal matters, and is ever pouring out of the frontal extremity around
the neck of the creature. What a beautiful and effective contrivance is
this for constantly keeping in a state of the most unsullied cleanness
the interior of the house! It reminds one of the fabled Hercules
cleansing the Augean stable by driving the river Peneus through it.