Philip Henry Gosse.

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ject in different aspects, and under varying conditions of
light, position, etc.

1 "The Microscope," p. 553.


It is by making a thin transverse section of a spine -
cutting off a slice of it, to speak in homely phrase that
we shall demonstrate the structure, which is very beautiful.
This is an operation requiring much delicacy and practice,
and implements for the special purpose; and hence it is
best performed by professional persons, who prepare micro-
scopic objects for sale. You may see such a section, how-
ever, on this slide; but I do not know whether the spine
belongs to the species we are examining.

The whole central portion is formed of the sponge-like
calcareous matter, which, from the variously reflected and
refracted rays of light, appears nearly opaque, and of a
bluish color by transmitted light. This structure sends
forth radiating points (making longitudinal ridges, of
course, in the perfect spine); and it is the opacity of these
points (or ridges) which reach the circumference that gives
to the spine the appearance of being fluted. Indeed it
would be fluted if this were the entire structure; but the
open space left between these projecting radii is filled with
the solid glassy matter, having, as we see, a convex sur-
face. This, however, from its perfect transparency, is not
seen when we look at the side of the spine, the eye going
down to the bottom of the interspace. The spine is, in
fact, a fluted column of spongy glass, with the grooves
filled with solid glass.

We have not yet seen, however, the beautiful mech-
anism appropriated to the movement of these spines.
You can hardly see this to advantage in the living animal,
but here is the entire shelly box of a dead Echinus, on
which, while for the most part the surface is denuded
of spines, a few dozen remain sufficiently attached to show
what I wish to demonstrate, viz. , the mode of articulation.


You observe that the whole globose shell is covered with
tiny knobs, differing in size, and not set in very regular,
or at least not very obvious order, but showing a ten-
dency to run in lines from- pole to pole of the globe.
Giving attention to one of the larger of these knobs,
under a lens it is seen to be a hemi-
spherical eminence on the shell, the
very summit of which is crowned by
a tiny nipple of polished whiteness,
resembling ivory. Now if we care-
fully lift one of the still remaining
spines from its attachment, which in
its present dried state is so fragile
that the slightest touch is sufficient
for the purpose, we shall note that its
base rests on this tiny nipple ; and on


turning it up, and bringing the mag- (Segment of section.)
nifying power to bear upon its base, we see that this is
excavated with a hollow, whose dimensions exactly corre-
spond with those of the nipple. It is indeed a true "ball-
and-socket" joint, like that of the human hip or shoulder,
and is surrounded by a capsular ligament to keep it in
place, the muscles which sway the spine from side to side
and cause it to rotate being inserted outside the capsule.
Professor Edward Forbes calculates that upon a large
Echinus, such as this dried specimen of E. sphcera, there
are more than four thousand spines, every one of which
has the structure, the mechanism, and the movements that
we have been examining. Well may he say that 4t truly
the skill of the Great Architect of Nature is not less dis-
played in the construction of a Sea-urchin than in the
building up of a world I"


To return now to our .little E. miliaris, which has "been
all this time coursing round and round his saucer, wonder-
ing, perchance, at the narrowness and shallowness of the
White Sea in which he finds himself. Again we peer,
lens to eye, over the bristling surface, and discern, shoot-
ing up amid the spines, and almost as thickly crowded
as they, multitudes of the tiny organs which have caused
so much doubt and discussion among naturalists. Muller,
the great marine zoologist of Denmark, who first discov-
ered them, thought them parasitic animals, living pirati-
cally upon the unwilling Urchin, and accordingly gave
them generic and specific names. The term PcdiceUaria,
which he assigned to his supposed genus, is that by which
modern naturalists have agreed to call them still, though
the word is not now used in a generic sense, since it is
indubitably established that they are not independent ani-
mals, but essential parts of the Urchin itself. Miiller de-
scribed three distinct sorts, and I have added a fourth
to the number; they are named P. triphytta, tridens, globi-
fera, and stereophylla. They all agree in these particulars
that each has a long, slender, cylindrical, fleshy stem,
through the centre of which runs an axis or rod of cal-
careous substance; that the base of the stem rests on the
skin of the Urchin; that on the summit is placed a head
consisting of three pieces, which are capable of being
widely opened and of being closed together, at least at
their tips; that the edges of these pieces, which come into
mutual contact, are furnished with teeth, which lock into
each other; that the head-pieces (like the stem) consist of
calcareous centres, clothed with flesh; that, besides the
opening and shutting of the head, the stem can be swayed
from side to side; and that all these movements are spon-


taneous, and apparently voluntary. It appears that the
head-pieces close on any object presented to them, such as
the point of a needle, and hold with considerable force and
tenacity, so that the Pedicellaria may be drawn out of the
water without relaxing its grasp.

Looking at one of the first-named kind, the Pedicellaria
tripliylla, of this Echinus miliaris, we see that it consists
of three broad and thick sub- triangular pieces, jointed
into a head, set on a thickish stem of transparent gelat-
inous fibrous substance, through which a slender core oi
calcareous matter runs that looks fibrous and blue. The
three movable pieces or blades are convex externally, con-
cave internally; thin in substance, furnished along their
opposing or concave sides with two longitudinal ridges
or keels, each of which is cut into the most beautifully
fine teeth, so that the edge of each ridge looks like a
shark's tooth; the edges of the pieces are also similarly
toothed: these shut precisely into each other.

In the larger E. sphcera, the head- blades of this kind
have one stout central ridge, which is rounded and not
toothed. It forms the front of a great interior cavity,
which opens by two orifices on each side of the column.

The movable pieces enclose a skeleton of calcareous
substance, glassy, colorless, and brittle, in which, accord-
ing to the plan I have already described, are excavated
a multitude of oval cavities which form irregular rows; a
central line runs down each piece that is solid and free
from cavities. This calcareous skeleton is incased in a
gelatinous flesh, similar to and continuous with that of the

This is the smallest kind, the head being about -g^th of
an inch in height.


Considerable modifications are found to exist in the
details of each form, in the relative proportions which the
parts bear to each other, and so forth ; so that two forms,
which in their extreme conditions widely differ, mutually
approach, and appear to run into each other. This is the
case with the present, and with the form which I will now
show you.

P. tridens is much larger than any of the other forms,
the movable head being about -g^th of an
inch in length, and the whole organ
about -Jth of an inch. This may be con-
sidered as essentially P. triphylla, modi-
fied by the blades being greatly drawn
out in length, and at the same time ren-
dered quite slender, so that they may be
called pins; they meet only at the points,
where they often cross, the interspaces
of the basal parts being open. The inner
edges of these are notched with teeth as
in P. triphylla, of which those near the
tips are larger and cat into subordinate
teeth of exquisite minuteness.

We have here an opportunity of see-
ing that the oval or square markings,


TRIDENS. which are thickly placed throughout the

calcareous substance of the blades, are certainly cavities
in it; for in those examples in which the pins, which are
very brittle, are broken, the edge of the fracture is not
even, but jagged with holes exactly corresponding with
the marks in question; so that the structure is the same as
that of the spines and of all the other solid parts of the


"We will now examine some specimens of P. tridens,
treated with potash, which will enable us to see the cal-
careous support better. The head- blades expand at the
base into three- sided prisms or pyramids, each of the two
interior sides of which is indented with a large cavity,
leaving a projecting dividing ridge, armed with teeth some-
what remote from each other. The one exterior angle is
toothed in a corresponding manner, but the opposite angle
appears plain. The angle of one blade-base fits into the
cavity of its neighbor; and, so far as I have observed,
when the two edges thus overlap, it is the toothed one
that is on the outside. Looking from the circumference
toward the centre of the head, it is the left angle that is
toothed and external, the right being plain and sheathed.
This observation, however, applies only to E. miliaris;
for, in the corresponding organs of E. sphcera, both sides
of the trigonal base appear untoothed, except close to the
bottom, where a deep notch indents each margin.

Viewed from beneath, the head assumes an outline
which is rondo- triangular; but yet such that each side of
the triangle has a very obtuse projecting angle in the mid-
dle, where the blade-bases meet each other. They fit accu-
rately, and each has a deep oblong cavity in its bottom,
which does not, as I conceive, communicate with the

By selecting one of these heads, which has been di-
vested of its fleshy parts by immersion in caustic potash,
and then well cleansed by soaking in clean water, and
placing it under a low power of the microscope 100 ,di-
ameters, for example with a dark ground, and the light
of the lamp cast strongly upon it by means of the Lieber-
kuhn, or the side-condenser, we shall have an object of


most exquisite beauty. The material has all the transpar-
ency and sparkling brilliance of flint-glass, while the ele-
gantly-shaped pins, the perfect symmetry of the prismatic
bases, the arch which is lightly thrown across their cavity,
the minute teeth of the tips locking accurately into each
other, and the oval cavities in the whole structure set in
regular rows, and reflecting the light from thousands of
points, constitute a spectacle which cannot fail to elicit
your admiration.

P. globifera is formed on the same model as P. tri-
phylla, but is more globose, and each piece appears to have
a deep cleft at the point, which does not extend to the
interior side, where a thick ridge runs down from the
point to the base. At the summit of this ridge, in each
of the three divisions, there is set a strong acute spine,
directly horizontally inward, so that the three cross each
other when the blades close, which they do energetically
a formidable apparatus of prehension! The stem is
much more slender than in P. triphylla, and the height
of the head of one of average size is only ^d of an inch.
It is peculiar also in being slender throughout, and in
having the knobbed calcareous stalk extending up to the
head, which appears to work on it. In each of the other
sorts the stalk extends only through a part of the distance,
above which the investing fleshy neck becomes wider and

But the internal structure is not quite the same as in
the others. The main portion of the head is composed of
gelatinous flesh; the calcareous support being reduced to
that ridge which runs up the interior side of the blade.
This is somewhat bottle-shaped, with a bulbous base, and
a long slender neck, with two edges on the inner face,


which are armed with horizontal hooked spines, some of
which are double, and the whole terminates in a sort of
ring, formed by the last pair of spines, which unite into
the acute horizonal point that I have already mentioned.
The skeleton is filled with oval cavities like that of the

The fourth kind of Pedicellaria, which I call P. stereo-
phylla, is quite distinct from either of the others. It is
very minute, the head being only -g^th of an inch in
height. The head is a prolate solid spheroid, cut into
three segments, exactly as if an orange were divided by
three perpendicular incisions meeting at the centre. Thus
the blades meet accurately in every part when closed, but
expand to a horizontal condition. These are almost en-
tirely calcareous, being invested but thinly with the gelat-
inous flesh. They are filled with the usual oval cavities,
set in sub-parallel arched series.

The head is set on a hollow gelatinous neck nearly as
wide as itself, and thrown into numerous annular wrinkles;
its walls are comparatively thin, disclosing a wide cavity
apparently quite empty, as the blue calcareous stem ex-
tends only half-way from the base to the head.' At this
point the neck contracts rather abruptly, and continues to
the base, but just wide enough to invest the stem.

This sort is confined, so far as I have seen, to the
ovarian plates and their vicinity, where they are numerous.

Tims these tiny organs, so totally unlike anything with
which we may parallel them in other classes of animals,
do not merely afford us amusement, and delight us by
their elegance of shape and sparkling beauty, the variety
and singularity of their forms, the elaborateness of their
structure and the perfection of their mechanism, but ex-


cite our marvel as to what can be the object which they
subserve in the economy of the creature what purpose
can be fulfilled by so many hundreds of organs so singu-
lar, and scattered over the whole surface of the shelly

It is very difficult to answer this question. The only
organs with which they can be compared are the singular
44 birds' heads" in so many of the Polyzoa, which we
looked at some time ago. But, unfortunately, a like
mystery enshrouds the use of those processes, and the
only light that we have as yet upon either form is that
of dim conjecture. It has been supposed that, in both
cases, the function of the prehensile forceps is to seize
minute animalcules or floating atoms of food, and pass
them to the mouth: but the supposition is involved in
great difficulties; as the organs, however fitted for pre-
hension, seem peculiarly unsuited for transmitting objects;
besides that the great majority of them are placed very
remote from the mouth. I can only repeat the conjecture
which I have hazarded in the case of the Polyzoan "birds'
heads' ' ; viz. , that the Pedicettarice are intended to seize
minute animals, and to hold them till they die and de-
compose, as baits to attract clouds of Infusoria, which,
multiplying in the vicinity of the Urchin, may afford it
an abundant supply of food.

There is yet another series of organs which stretch out
from every part of the periphery of this living box;
scarcely less numerous than either the spines or Pedicel-
larice, but very different from both. They are what I
alluded to just now as the feet. Let us pay a moment's
attention to' their appearance and action, before we ex-
amine their structure.


327- -

We see, tlien, extending from various points of the
shelly case of the Urchin, and reaching to twice or thrice
the length of the longest spines, slender pellucid tubes,
slightly tapering toward their free extremity, which then
abruptly dilate into a hemispherical knob, with a flat end.
These very delicate organs are extended or contracted at
the will of the animal, turned in every direction, waved
hither and thither, and evidently have the faculty of ad-
hering very firmly by their dilated tips to any object to
which they are applied.

So much we can discern as we watch the creature dis-



porting in this vessel of water; but we will now endeavor
to learn a little more about its structure and economy.
Selecting for this purpose a sucker which is extended to
great length, I snip it across with a pair of sharp scissors,
as near the base as I can. Mark the result. The terminal
knob which was attached to the bottom of the saucer main-
tains its hold; but the tube" has suddenly shrunk up to a
sixth part of its former length, exchanging at the same
time its smooth slenderness and translucency for a corru-
gated semi-opacity. I push the knob aside with a needle's
point and thus destroy its adhesion; which done, I take


up the severed and shrunken sucker, and lay it in a little
sea- water in the live-box.

Under a power of 180 diameters we see that the tube
is composed of two series of muscular fibres, the one set
running lengthwise, the other transversely or annularly;
the former by their contraction diminishing the length of
the tube, the latter diminishing its calibre. The muscular
walls are covered with a transparent skin, studded with
round orange-colored spots, perhaps glandular, exactly
similar to those we saw on the exterior of the spines and

Now, to illustrate the action of these tubular feet, I
must again have recourse to the denuded shell of a pre-
served Echinus. Taking this globose empty box into your
hand, hold it up against the light, looking in at the large
orifice, which was once occupied by the mouth; you see
that the whole shell is pierced with minute holes pores,
which are arranged in ten longitudinal, or meridional lines,
associated so as to make five pairs of lines. Now, with
a lens, scrutinize more minutely a portion of any one of
these lines, and you discern that it is composed of a mul-
titude of pores, which have a peculiar order of arrange-
ment among themselves; that is to say, they form minor
rows which cross, obliquely or diagonally, the course of
the meridional line. These rows -are themselves double,
the pores running in pairs, not, however, with mathemat-
ical symmetry. In this species there are three pairs of
pores in each row, and so there are in the one which I
have here alive, but in other of our native species the
rows consist of five pairs.

These pores are intimately connected with the tubular
feet, each of which springs from a portion of the shell that


is perforated with, a pair of pores; so that the cavity of
every tube communicates with the interior of the shelly
box by two orifices.

Now on the interior side of these two pores that is,
within the cavity of the shell there is placed a little
membranous, or rather muscular, bladder, filled with a
fluid which is not materially different from sea-water.
There is a free communication between the bladder with-
in and the tube without the shell, by means of the pair
of pores through, which the fluid passes. By means of the
muscular fibres, which are under the control of the Ur-
chin's will, any portion of this double vessel can be con-
tracted to a certain extent. Suppose it is the interior
bladder; the effect of the contraction of its walls is to
diminish its capacity, and the contained fluid is forced
through the pores into the tube without. The longitudi-
nal fibres of this part being at the same moment relaxed,
the tube is lengthened because of the injected water. Sup-
pose, now, in turn, the fibres of the tube contract, while
those of the bladder relax; the fluid is driven back, the
bladder dilates, and the tube shortens, until, if the animal
so please, its swollen tip is brought close up to the pores.
By mechanism so beautiful and simple is the prolongation
or abbreviation of these organs effected.

We noticed, however, that the extremities of the tubes
had an adhesive power, which faculty it is that constitutes
them feet. They are prehensile, and thus they afford, as
we observed in the living Urchin, the means by which it
takes hold of even a smooth and vertical surface, as the
side of a glass tank, and drags up its body thereby.

Putting, now, the extremity of this cat- off tube under
graduated pressure, having first applied to it a drop of


caustic potash, we see that it carries a beautiful glassy
plate of extreme thinness, which lies free in the swollen
cavity of the termination of the tube. This plate is cir-
eular in form, apparently notched at the margin, and cut
with four or five (for the number varies) incisions, which
reach almost to the centre. The substance is formed of
the common clear brittle calcareous matter of the skele-
ton, hollowed into numberless cavities, according to the
general plan. The centre is perforated with a larger
round orifice. The appearance of marginal notching is
deceptive; and indicates a structure analogous to what we
see in the spine. The notched line indicates the extent
of the spongy structure; but beyond this the plate extends
into a perfectly circular smooth edge, but is constituted of
a layer of calcareous substance so thin that there is no
room for the ordinary cavities within it.

The round aperture in the centre plays an important
part in the function of the organ. The foot adheres on

the same principle as that by which
children take up large flat stones
with a piece of wetted leather, to
the middle of which a string is at-
tached. The boy drops his sucker
on the stone, and treads firmly on
it, to bring it into close contact with

SUCKER-PLATE OF URCHIN. the surf ace ; then he pulls at the
string perpendicularly, by which the central part of the
leather is lifted a little way from the stone, leaving a
vacuum there; since the contact of the edges with the
stone is so perfect that no air can find entrance between
them. Now the pressure of the atmosphere upon the
leather is so great that a considerable weight, perhaps


half a dozen pounds, may be lifted by the string before
the union yields.

Well, the very counterpart of this amusing operation
is repeated by the clever ''Urchin" whose performances
we are considering. The tube is his string; the dilated
end with the plate in it, his leather; his muscular power
acts like the other urchin's tread, to press the bottom of
the sucker against the surface of the rock. Then he pulls
the string; in other words, he drags inward the centre of
the muscular bottom of the sucker, which is, as it were,
sucked up into the central orifice of the plate. Thus a
vacuum is formed beneath the middle of the sucker, on
which the weight of the incumbent water and atmosphere
united presses with a force far more than sufficient to re-
sist the weight of his body, when he drags upon it, and,
as it were, warps himself up to the adhering point.

Here is in my cabinet a specimen of a Sea-urchin of
a less regular form: it is the Heart- urchin (Amphidotus
cordatus). Essentially, its structure agrees with that of
the more globular forms, but it is heart-shaped, and the
two orifices, instead of being at opposite poles, are sepa-
rated only by about one- third of the circumference. It
shows also singular impressed marks on its shell, as if
made by a seal on a plastic substance.

But what I chiefly wish to direct your attention to are
the spines. These differ much from the kindred organs
in Echinus, being far more numerous, very slender, curved,
thickening toward the tip, and lying down upon the shell
in the manner of hair, whence the species is sometimes
called the Hairy Sea- egg. The array of spines has a glit-
tering silky appearance in this dried state.

We will now put a few of them under a low power


of the microscope, using reflected light and a dark back-
ground. They thus present a very beautiful appearance;
elegantly- formed curved clubs, made of a substance which
seems to be between glass and ivory, having the whiten.ess
of the latter and the glittering brilliance of the former.
The entire surface appears to be exquisitely carved with
excessively minute oval pits, arranged in close- set lines,

Online LibraryPhilip Henry GosseEvenings at the microscope (Volume 1) → online text (page 22 of 32)