This dull-coloured Scale-worm presents peculiarities of structure which
are eminently worthy of careful examination. The body, like that of the
other species we have just been considering, is composed of many rings
or segments, each of which bears on its upper surface a little wart or
tubercle on each side, to the summit of which is slightly attached the
circular leaf-like shield just described. Beneath the shadow of this
broad shield the segment projects laterally into another tubercle,
whence issue the organs of motion, which in these Annelids are of
highly curious structure. In this species they consist of a bundle of
straight bristles of exquisite tenuity, and having a silky lustre;
they are of unequal length, the upper ones being the longest, and the
pencil diminishing pretty regularly downwards. The microscope shows
them to be about one hundred in number in each bundle; and each bristle
to be a delicate oar consisting of a slender stem, apparently tubular,
dilated at the tip into a narrow blade terminating in a minute hook.
A fibrous structure is seen to run diagonally across the blade, the
edge of which is set with minute barbs pointing outwards, resembling
very fine and very short bristles. The whole bundle is connected with
a long slender rod of cartilage, which, doubtless acted upon by proper
muscles, moves to and fro through the muscular sheath, protruding the
bundle of bristles, or entirely retracting them within the body. The
bristles, when protruded, slightly diverge, so that the dilated blades
of these hundred oars strike with full force upon the water in rowing,
if such indeed is the nature of their action, as some have supposed. In
the act of withdrawal, however, into the narrow compass of the muscular
sheath, the blades slide one upon another, so as to present a surface
considerably diminished. But why is each bristle hooked and barbed?
The obvious supposition is that these organs act like the pole of a
ferry-boat in the shallows; the barbs, pointing outwards, serving to
catch any roughness of the surface, and thus to _push_ the animal by
their resistance, while the terminal hook may serve to _pull_ in an
analogous manner. But then the barbed and hooked edge is the _upper_
one of each oar, a circumstance which renders such an explanation at
first sight unsatisfactory. Perhaps, however, the habit of the animal
of living under stones, may in some measure account for it; the edge
and point of each bristle being intended to act upon the surface of the
object above it, beneath which it is forcing its way.
A secondary pencil of bristles also issues from the upper side of each
of these tubercular sheaths, so as to project diagonally upwards and
outwards beneath the edge of the shield-leaf. This pencil consists
of between thirty and forty bristles, which are connected with a
cartilaginous rod exactly similar to that of the primary bundle. These
bristles run off to a very fine point, and each one is marked on both
sides with serratures so delicate as to be only just discernible with
a magnifying power of 300 diameters. There are thus about 270 movable
bristles of exquisite structure in each segment, which multiplied by
36, the number of segments in the body, gives a total of 9720 distinct
organs of motion possessed by this animal, besides the broad leaf-like
shields, which perhaps may act as fins, and the short tentacular feet,
one of which is placed beneath the primary bristle-sheath, on each side
of the segment.
By dissection after death, I have found the proboscis in the form of
a long oval sac, very muscular and firm, of a blackish hue, lying in
the anterior part of the body, which, on being cut open, showed four
horny teeth placed in its neck so as to meet each other in the centre.
I have never seen the protrusion of this organ during life, but if it
is capable of being wholly everted, as in many species, these teeth
must be turned on the outside, where they would seem to be of little
use. Possibly, however, the version may be only just sufficient to
bring these teeth to the margin, when their action might be made very
efficient by the slightest alteration in the contraction of the sac.
Some species of this genus construct for themselves enveloping tubular
cases, out of mucus given off from their bodies, and fragments of
shells. In these they ordinarily live concealed, but are sufficiently
active when they leave their houses. Some shine with a phosphorescent
radiance in the dark.
In the great class of Worms, special organs for locomotion are often
altogether lacking; and, where found, are only rudimentary. Alternate
swellings and contractions of certain portions of the body are a
great source of progression; and these are effected by the fluids
of the interior, which are driven to a given part of the cavity,
and momentarily imprisoned there by the contraction of the circular
tegumentary muscles before and behind it; the body swelling at that
place. “The muscles of the integument are then excited to action, and
the fluid is forcibly compressed forwards or backwards, according to
the direction of the muscular agency. Nearly all Annelids are struck
with paralysis when this fluid is made to escape from its cavity by a
puncture through its external walls. The power of voluntary motion is
suspended; the body of the worm becomes passive and flaccid.”[85]