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swimming leaf that ornaments each segment and the lower
cirrus. The bristles are of the compound jointed form,
but the joint is fixed in a peculiar manner. The basal
portion is drawn out into a very slender long straight
shaft, terminating in a knob somewhat resembling the end
of a limb- bone. This is slit in one direction to receive the
terminal piece, which is shaped somewhat like a lance-
head, and is inserted into the slit exactly as a knife-blade
is fixed into the haft. The head is in fact a knife-blade,
with a thickened back and a very thin edge, which is
notched into teeth of the most exquisite delicacy. The
blade is slightly curved, and drawn out into a long acute
point; and the whole bristle is formed out of an elastic
horny substance (probably chitine) that rivals in trans-
parency and brilliancy the purest flint-glass.

I might adduce a vast variety of examples of these
organs in the Marine Worms, all of which would charm
you by their elegance and by their extreme diversity; but
I have other things to show you in this interesting class
of animals, which fortunately are so common on all our
shores that you will have no difficulty in procuring plenty
of specimens for your private observation and study. And
if you need intelligent guidance, you cannot have a better



304 EVENINGS AT THE MICROSCOPE

mentor than Dr. Williams, whose admirable "Report on
the British Annelidas" I have just cited.

Before we dismiss our little Phyllodoce to its home in
the aquarium, we must try to get a sight of its pretty
mouth. The Worms are somewhat wayward in display-
ing this part of their charms, sometimes exposing it at
intervals of a second or two for very many times in suc-
cession, at others sullenly keeping it closed; and no efforts
that I am aware of on our part will induce the display: we
must await their pleasure. It is, in fact, a turning of the
throat inside out. In most of the Worms the head is
minute, and what seems to be the mouth is but the orifice
from which the throat or proboscis is everted. In the
Phyllodoces this organ is a great muscular sac, in some
species equalling in length one-fourth of the whole body.

fla! there it appears! What a chasm yawns in the
under side of the head, as the interior begins rapidly to
protrude, turning inside out as it comes forth, like a living
stocking, until it assumes the form of an enormous (com-
paratively enormous, of course) pear-shaped bag, the sur-
face of which is beset with a multitude of secreting warts
or glands, somewhat like the papillae which stud the
tongue in higher animals! The extremity, which is per-
forated, is surrounded by a muscle, by means of which it
contracts forcibly on whatever it is applied to, and thus
holds it firmly, while the reinversion of the sac drags it
into the body to be digested.

But this huge proboscis disappears as rapidly and as
wonderfully as it was revealed. Commencing at what
is now the outer extremity, which is quickly turned in,
the whole swiftly returns to its cavity in the inverse order
to that in which it was extended; and now that it is all



WORMS 305 _

engulfed, we marvel that so vast a sac can be packed away
in so slender a case.

In this instance the armature of the proboscis is feeble;
but we have species which are very elaborately armed.
There is a minute species of Lombrinereis, which com-
monly appears in our aquaria after they have been some
time established, and breeds in vast numbers on the floe-
cose matter that clogs the bottom and sides. In this tiny
Worm there is a formidable array of jaws, resembling
black hooks, which we may discern through their pellucid
tissues, snapping and cutting viciously like so many pairs
of hooked scissors. Though I have often had this little
species in my tanks in copious abundance, I regret to say
I cannot find any at this moment for our examination, and
shall therefore content myself with translating for you
MM. Audouin and Milne-Edwards' description of the jaws
as they appear in a closely -allied form, but of far greater
dimensions, Eunice.

"The proboscis is not very protrusile; when it is with-
drawn its external orifice is longitudinal, and the jaws are
fixed on each side, all facing the medial line. When it is
projected, however, the two margins of the longitudinal
cleft become transverse in separating, and the jaws follow
the same movement, and diverge in the ratio of their for-
wardness. A kind of lower lip which is affixed to the
under side of the proboscis is composed of two horny
blades united toward their front extremity, and prolonged
behind into points. The jaws are to the number of seven;
three on the right and four on the left; the two upper ones
are perfectly alike, and mutually opposed; they are large,
narrow, pointed, recurved hook-wise at the tip, and jointed
at their hinder ends on a double horny stem shorter than



306 EVENINGS AT THE MICROSCOPE

themselves. The second pair of jaws are large, broad and
flat, mutually alike, and jointed on the lower side of the
first pair; . . . their internal edge is straight and cut into
deep teeth. The third pair are small, thin, concave, and
notched; they are affixed by their inferior edge outside
and in front of the second pair, which they conceal during
repose. Finally, the supernumerary jaw, which is found
on the left side only, is small, semicircular, toothed, and
placed between the second and third pairs. All these
pieces are surpassed by the margin of the proboscis, which
is often hard and black." l

From this complex and formidable mouth we will pass
to one of quite another form, not less effective, perhaps
more formidable, but ordained by the goodness of God to
be a most valuable agent in the relief of human suffering.
I mean the Medicinal Leech, of which we can readily pro-
cure a specimen from our friend the apothecary.

Here it is. There is no protrusile proboscis, but the
throat is spacious, and capable of being everted to a slight
degree. The front border of the mouth is enlarged so as

to form a sort of upper lip, and
this combines with the wrinkled
muscular margin of the lower
and lateral portions to form the
sucker. With the dissecting
scissors I slit down the ventral
margin of the sucker, exposing
THBOAT OF LEECH LAID OPEN. fa e whole throat. Then, the
edges being folded back, we see implanted in the walls
on the dorsal region of the cavity three white eminences

1 "Litt. de la France, " ii. 138.




WORMS 307

of a cartilaginous texture, which rise to a sharp crescentic
edge; they form a triangular, or rather a triradiate, figure.

Now, if you recollect, this is the figure of the cut made
in the flesh wherever a Leech has sucked, as it is of the
scar which remains after the wound has healed. For these
three little eminences are the implements with which the
animal, impelled by its blood-sucking instincts, effects its
purpose. But to understand the action more perfectly,
we must use higher powers.

I dissect out of the flesh, then, one of the white points,
say the middle one, and laying it in water in the compres-
sorium, flatten the drop, but use no more pressure than
just enough for that. Now I apply a power of 150 diam-
eters, and we will look at it in succession. You have
under your eye a sub-pellucid mass, of an irregular oval
figure, and of fibrous texture, one side of which is thinned
away apparently to a keen edge of a somewhat semicircular
outline. But along this edge, and as it were imbedded
into it for about one-third of their length, are set between
seventy and eighty crystalline points, of highly refractive
substance, resembling glass. These points gradually de-
crease in size toward one end of the series, and at length
cease, leaving a portion of the cutting edge toothless. At
the end where they are largest, they are nearly close
together, but at length are separated by spaces equal to
their own thickness. The manner in which they are in-
serted closely resembles, in this aspect, the implantation
of the teeth in the jaw of a dolphin or crocodile.

But this appearance is illusory. By affixing the little
jaw to the revolving needle, we bring the edge to face our
eye. It is not an edge at all; but a narrow parallel-sided
margin of considerable breadth. And the teeth are not




308 EVENINGS AT THE MICROSCOPE

conical points, as they seemed when we viewed them side-
wise, but flat triangular plates, with a deep notch in their
lower edge. Thus they partly embrace, and are partly
inserted in, the margin of the jaw.

Observe now how beautifully this apparatus subserves
the purpose for which it is in-
tended. By means of its sucker,
the Leech creates a vacuum upon
a certain part of the skin, exactly
like that produced by a cupping-
glass. The skin covered is drawn
into the hollow so far as to render

JAW or I.EECH (in part). ft quite tense, by the pressure of
the surrounding air. Thus it is brought into contact with
the edges of the three jaws, to which, by means of power-
ful muscles attached to them, a see -saw motion is com-
municated, which causes the little teeth soon to cut through
the skin and superficial vessels, from which the blood be-
gins to flow. The issue of the vital fluid is then promoted
by the pressure around, and so goes on until the enormous
stomach of the Leech is distended to repletion.

It has been suggested that this whole contrivance, with
the instinct by which it is accompanied, is intended for
the benefit of Man, and not of the Leech. Blood seems
to be by no means the natural food of the Leech; it has
been ascertained to remain in the stomach for a whole
twelvemonth without being digested, yet remaining fluid
and sound during the entire period: while, ordinarily, such
a substance cannot in one instance out of a thousand be
swallowed by the animal in a state of nature. Whether
this be so or not whether man's relief under suffering
were the sole object designed, or not it was certainly one



WORMS 309

object; and we may well be thankful to the mercy of God,
who has ordained comfort through so strange an instru-
mentality.

The progress of marine natural history, as studied in
the aquarium, has made our drawing-rooms and halls
familiar with a multitude of curious and beautiful creat-
ures which a few years ago were known only, and that
very imperfectly, to the learned professors of technical
science. Among the forms which embellish our tanks are
several species of Serpula, and Worms allied to it. The
shelly contorted tube which this painted Sea-worm in-
habits, and which it has built up itself around its own
body, with stone and cement which that body supplied,
is well known to you; as is also the curious conical stop-
per with which it closes up its bottle as with cork, when
safe at home, and the lovely crown of gorgeously colored
fans which it expands when it takes ("the air," I was
about to say, but rather) the water. You are familiar,
too, with the lightning-like rapidity with which, while in
health and vigor, the Serpula, on the slightest alarm,
retreats into his fortress, taking care to clap the door to
after him. But perhaps you have never had an oppor-
tunity of examining the mechanism by which this rapid
flight is effected.

As there are two distinct movements performed by the
Worm the slow and cautious and gradual protrusion, and
the sudden and swift retreat so there are two distinct sets
of organs by which they are performed. Shall 1 sacrifice
one from this fine group to demonstrate the mechanism?
Well, then, I carefully break the shelly tube, and extract
the Worm uninjured.

Its form is, you perceive, much shorter and more dumpy



310 EVENINGS AT THE MICROSCOPE

than you would have supposed from looking at the tube;
and it is somewhat flattened, having a back and a belly
side. On the former there is a sort of shield, the sides of
which bear wart-like feet about seven pairs in all -which
are perforated for the working of protrusile pencils of
bristles, similar in structure and in function to those
which we lately examined.

Here is one of the pencils extracted. To the naked
eye it is a yellowish body with a satiny lustre; and this
effect depends upon the light being reflected from a num-
ber of nearly parallel lines the staves of the spear-like
bristles, which the eye cannot resolve in detail. A drop




of the caustic solution of potash cleanses the bundle from
the fleshy matter which would otherwise obscure the vision,
and now I place it on the stage.

With this power of 400 diameters you see a multitude
some twenty or thirty, or more of very long, slender,
straight rods, of a clear yellowish horny substance, set
side by side, like a sheaf of spears in an armory. Each
one merges, at its upper end, into a sort of blade, wnich
is slightly bent, and which tapers to an exceedingly fine
point. But its chief peculiarity is that the blade has a
double edge, not like a two-edged sword, the edges set
on opposite faces, but on the same face, set side by side,
with a groove between them; and each edge is cut with



WORMS 311

the most delicate and close -set teeth, the lines of which
pass back upon the blade, as in our reaping-hooks.

These pencils of spear-like bristles are the organs by
which the protrusion of the animal is performed. Their
action is manifestly that of pushing against the walls of
the interior, which on close examination are seen to be
lined with a delicate membrane, exuded from the animal's
skin. The opposite feet of one segment protrude the pen-
cils of bristles, one on each side, the acute points and teeth
of which penetrate and catch in the lining membrane; the
segments behind this are now drawn up close, and extend
their bristles ; these catch in like manner ; then an elongat-
ing movement takes place ; the pencils of the anterior seg-
ments being now retracted, they yield to the movement
and are pushed forward, while the others are held firm by
the resistance of their holding bristles; thus gradually the
fore parts of the animal are exposed.

But this gradual process would ill suit the necessity of
a creature so sensitive to alarm, when it wishes to retreat.
We have already seen how, with the fleetness of a thought,
its beautiful crown of scarlet plumes disappears within its
stony fastness: let us now look at the apparatus which
effects this movement.

If you look again at this Serpula recently extracted,
you will find, with a lens, a pale yellow line running along
the upper surface of each foot, transversely to the length
of the body. This is the border of an excessively delicate
membrane; and on placing it under a higher power (say
600 diameters), you will be astonished at the elaborate pro-
vision here made for prehension. This yellow line, which
cannot be appreciated by the unassisted eye, is a muscu-
lar ribbon over which stand up edgewise a multitude of



312



EVENINGS AT THE MICROSCOPE



what I will call combs, or rather sub-triangular plates.
These have a wide base; and the apex of the triangle is
curved over into an abrupt hook, and then this is cut
into a number (from four to six) of sharp and long teeth.
The plates stand side by side parallel to each other, along
the whole length of the ribbon, and there are muscular
fibres seen affixed to the basal side of each plate, which
doubtless give it independent motion. 1 have counted 136
plates on one ribbon; there are two ribbons on each tho-
racic segment, and there are seven such segments hence




HOOKS OF SBRPULA.



we may compute the total number of prehensile comb-like
plates on this portion of the body to be about one thou-
sand nine hundred, each of which is wielded by muscles
at the will of the animal; while, as each plate carries on
an average five teeth, there are nearly ten thousand teeth
hooked into the lining membrane of the cell, when the
animal chooses to descend. Even this, however, is very
far short of the total number, because long ribbons of
hooks of a similar structure, but of smaller dimensions,
run across the abdominal segments, which are much more
numerous than the thoracic. No wonder, with so many
muscles wielding so many grappling hooks, that the retreat
is so rapidly effected!



SEA-URCHINS AND SEA-CUCUMBERS 313



CHAPTER XYI

SEA-URCHINS AND SEA-CUCUMBERS

PEERING- about among the rocks to-day at low tide,
I found, on turning over a large stone, an object
which, though familiar enough to those who are
conversant with the sea arid its treasures, would surprise
a curious observer fresh from the fields of Warwickshire.
It is a ball, perfectly circular and nearly globular only
that its under part is a little flattened hard and shelly
in its exterior, which is, however, densely clothed with a
forest of shelly spines, each one of which has a limited
amount of mobility on its base. On attempting to remove
it, I find that it adheres to the stone with some firmness,
and that on the exercise of sufficient force it conies away
with a feeling as if something were torn, and I find that
a multitude of little fleshy points are left on the stone.
Having dropped my prize into a glass collecting- jar of
sea-water, I presently see that it is slowly marching up
the side, sprawling out on every side a multitude of trans-
parent hands, with which it seems to feel its way, and
which are evidently feet also, for on these it crawls along
at its own tortoise-pace. And I now see that it is the
knobbed ends of some of these feet which were torn away
by my forcible act of ejectment, , and left clinging to the
stone.

It was not the first time that I had seen the Sea-urchin

15 SCIENCE 14



314: EVENINGS AT THE MICROSCOPE

(Echinus miliaris); and I might have passed it by with a
feeling of satiated curiosity, had I not recollected our
evening's amusement. Oh, ho! said I, what a fund of
microscopic entertainment is enclosed in this stone box!
So I brought it home, and now produce it as the text of
our conversazione.

Every part is a wonder; but we mast examine each in
order. Take the spines first.

As we examine these organs on the animal crawling at
ease over the bottom of a saucer of sea-water, using this
triple lens, we see that each is a taper pillar, rounded at
the summit, and swelling at the base, where it seems to be
inserted into a fleshy pedestal, on which it freely moves,
bending down in all directions, and describing a circle
with its point, of which the base is the centre. Each
spine is for the greater portion of its length of a delicate
pea- green hue, but the terminal part is of a fine lilac or
pale purple. The whole surface appears to be fluted, like
an Ionic column, but this is an illusion, as you will see
presently,

I now detach one of the spines, cutting it off with fine-
pointed scissors as near the base as I can reach. I put it,
with as little delay as possible, into the live-box, and ex-
amine it with a high power, say 600 diameters. Look at
it. You s^e the ciliary currents very distinctly ; and if you
move the stage so as to bring the basal portion into view,
you may discern even the cilia themselves, very numerous
and short, quivering with a rapid movement. The cur-
rents are not longitudinal, but transverse, and somewhat
peculiar. The floating atoms which come within their
vortex are drawn in at right angles to the axis of the
spine, and are presently hurled away in the same plane;



SEA-URCHINS AND SEA-CUCUMBERS 315

forming a circle, whose plane is perpendicular to the direc-
tion of the spine. The surface upon which these cilia are
set is a transparent gelatinous skin, of extreme tenuity,
stretched tightly over the solid portion, which it com-
pletely covers, and studded with minute oval orange-
colored grains.

The substance of which the spines are composed is
best seen by crushing a few of these organs into frag-
ments. We now see a texture beautifully delicate; they
are formed of calcareous substance as transparent as glass,
and reflecting the light like that material; hard but very
brittle; clear and solid, with a fibrous appearance in some
parts, but in others excavated into innumerable smooth
rounded cavities which join each other in all possible ways.
It is to this structure that the spine owes its strength, its
lightness, and its brittleness.

This arrangement of the calcareous deposit in a sort of
glass full of minute inter-communicating hollows, is very
peculiar, but it is invariably found in the solid parts of
this class of animals; so that the experienced naturalist,
on being presented with the minutest fragment of solid
substance, would, by testing it with his microscope, be
able at once to affirm with certainty whether it had be-
longed to an Echinoderm or not. And this uniformity
obtains in all the divers forms which the animals assume,
and in all the various organs which are strengthened by
calcareous deposits Crinoid, Brittle-star, Five-finger, Ur-
chin, Sea-gherkin, or Synapta; ray, plate, spine, sucker-
disk, lantern, pedicellaria, dumb-bell, wheel, or skin-
anchorwhenever we find calcareous matter, we invariably
find it honeycombed, and eroded, as it were, in this re-
markable fashion.



316 EVENINGS AT THE MICROSCOPE

Dr. Carpenter has described this texture so well that I
shall not apologize for quoting his words to you, especially
as you will have an opportunity here of testing their cor-
rectness, by personal observation. "It is," he remarks,
"in the structure of that calcareous skeleton, which prob-
ably exists, under some form or other, in every member of
this class, that the microscopist finds most to interest him.
This attains its highest development in the Echinida, in
which it forms a box-like shell, or 'test,' composed of
^numerous polygonal plates jointed to each other with great
exactness, and beset on its external surface with 'spines,'
which may have the form of prickles of no great length, or
may be stout, club-shaped bodies, or, again, may be very
long and slender rods. The intimate structure of the shell
is everywhere the same: for it is composed of a network,
which consists of carbonate of lime, with a very small
quantity of animal matter as a basis, and which extends
in every direction (i.e. in thickness, as well as in length
and breadth), its areolse or interspaces freely communi-
cating with each other. These 'areolae,' and the solid
structure which surrounds them, may bear an extremely
variable proportion one to the other; so that, in two
masses of equal size, the one or the other may greatly
predominate; and the texture may have either a remark-
able lightness and porosity, if the network be a very open
one, or may possess a considerable degree of compactness
if the solid portion be strengthened. Grenerally speaking,
the different layers of this network, which are connected
together by pillars that pass from one to the other in a
direction perpendicular to their plane, are so arranged that
the perforations in one shall correspond to the intermediate
solid structure in the next, and their transparency is such



SEA-URCHINS AND SEA-CUCUMBERS 317

that when we are examining a section thin enough to con-
tain two or three such layers, it is easy, by properly 'fo-
cusing' the microscope, to bring either one of them into
distinct view. From this very simple but very beautiful
arrangement, it comes to pass that the plates of which the
entire 'test' is made up possess a very considerable degree
of strength, notwithstanding that their porousness is such
that if a portion of a fractured edge, or any other part from
which the investing membrane has been removed, be laid
upon fluid of almost any description, this will be rapidly
sucked up into its substance." '

To return, however, to our spine. When we look at it
laterally, the appearance is such that we cannot but firmly
believe that it is grooved throughout with straight and
deep longitudinal furrows. But if we break off the same
spine transversely, and so exhibit it that the broken end
shall be presented to the eye, we perceive that there are
no grooves; but that the points in the circumference,
which seemed to be the summits of the ridges, which are
very narrow, are really lower than the intermediate spaces,
which we supposed to be the grooves, and that the surface
of these spaces is really convex in a slight degree.

The explanation of these contradictory appearances is
easily given. Meanwhile, however, they read an impor-
tant lesson to the inexperienced microscopist, not to decide
too hastily on the character of a surface or a structure,
from one aspect merely. So many are the chances of illu-
sion, that the student should always seek to view his sub-



Online LibraryPhilip Henry GosseEvenings at the microscope (Volume 1) → online text (page 21 of 32)