Philip Henry Gosse.

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with the most charming regularity. It is the light re-
flected from the polished bottoms of these pits that im-
parts to the surface its sparkling brilliancy. At the bot-
tom of the spine there is a little depression, which fits a
tiny nipple on a wart-like prominence of the shell, as we
saw in Echinus; but a little way above this point there is
a singular projection or shoulder of the calcareous sub-
stance, which is set on at a very oblique angle with the
axis of the spine, reminding one, as we look at the spine
laterally, of the budding tines on the horn of a young
deer.

At first, perhaps, you are at a loss to know what pur-
pose this shoulder can serve; but by turning to the shell
and carefully observing the spines in their natural connec-
tion with it, you will observe that the obliquity of its
position accurately corresponds with the angle which the
individual spines form with the surface of the shell from
which they spring; and that the shoulder has its plane
exactly parallel with the latter, but raised a little way
above it. Now the entire shell, during life, was clothed
with a living vascular flesh, having a thickness exactly
corresponding to the distance of the shoulder from the
shell. This shoulder, then, was an attachment for the
muscular bands, whose office it was to move the spine to
and fro; the projection affording the muscles a much bet-



SEA-URCHINS AND SEA-CUCUMBERS 333

ter purchase, or power, than they could have had if they
had been inserted into the slender stem itself.

The tubercles on the shell show a structure which cor-
responds with this. They are very minute; but each of
them is regularly formed, and is crowned with its little
polished nipple, on which, as I have said, the spine
works, as by a ball-and-socket joint. These are arranged
with perfect regularity in quincunx, and by close exami-
nation you will see that each is enclosed in a little area
formed by a very low and narrow ridge of the shell, which
makes a network. On the lateral portions of the under
surface the meshes of this net are particularly conspicu-
ous, and we see that they constitute shallow hexagonal
cells, in the midst of which is seated the tubercle; yet not
in the exact centre either, but nearer the front than the
back of the area enclosed.

Now this elevated ridge affords, doubtless, the inser-
tion of the other end of the muscles that move the spine;
the ridge giving a better purchase than a flat surface, as
the keel on the breastbone of birds is deep in proportion
to the vigor of the muscles used for flight. And, surely,
the apparently trivial fact that the space behind the tu-
bercle is greater than that in front is not without signifi-
cance, since it implies a thicker muscle at that part, which
accords with the circumstance that such would be the in-
sertion of the muscle- band whose contraction produces the
outward stroke by which the sand is farced away from
the bed.

But what is the need of so much care being bestowed
upon the separate motion of these thousands of hair-like
spines, that each individual one should have a special
structure with special muscles for its individual move-



334 EVENINGS AT THE MICROSCOPE

ment? The hairs of our head we cannot move individ-
ually: why should the Heart-urchin move his? Truly,
these hairs are the feet with which he moves. The animal
inhabits the sand at the bottom of the sea in our shallow
bays, and burrows in it. By going carefully, with the
lens at your eye, over the shell, you perceive that the
spines, though all formed on a common model, differ con-
siderably in the detail of their form. I have shown you
what may be considered the average shape; but x in some,
especially the finer ones that clothe the sides, the club is
slender and pointed: in others, as in those behind the
mouth, which are the largest and coarsest of all,, the club
is dilated into a long flat spoon: while in the long, much-
bowed spines which densely crowd upon the back, the
form is almost uniformly taper throughout and pointed.
The animal sinks into the sand mouth downward. The
broad spoons behind the mouth come first into requisi-
tion, and scoop away the sand, each acting individually
and throwing it outward. Observe how beautifully they
are arranged for this purpose! diverging from the median
line with the curve backward and outward. Similar is
the arrangement of the slender side-spines; their curve is
still more backward, the tips arching uniformly outward.
They take, indeed, exactly the curve which the fore-paws
of a mole possess only in a retrograde direction, since
the Urchin sinks backward which has been shown to be
so effective for the excavating of the soil and the throw-
ing of it outward. Finally, the long spines on the back
are suited to reach the sand on each side, when the creat-
ure has descended to its depth, and by their motions work
it inward again, covering and concealing the industrious
and effective miner.



SEA-URCHINS AND SEA-CUCUMBERS 335

Thus we have another instance added to the ten thou-
sand times ten thousand of the wondrous wisdom of Grod
displayed in the least and most obscure things. "All thy
works shall praise thee, O Lord!" (Ps. cxlv. 10.)

There is an order of animals which naturalists put in
the same category as the Sea-urchins, but which an un-
scientific observer would regard as possessing little or no
affinity with them. Some are like long, soft, and fleshy
worms and others, which come the nearest to the creat-
ures we have been looking at, have still the lengthened
form, which, however, so closely resembles that of a warty
angled cucumber that the animals I allude to are famil-
iarly called Sea-cucumbers (Holothuriadce). The marine
zoologist frequently finds them beneath stones at extreme
low water, and larger forms as big in every direction as
a marketable cucumber are occasionally scraped from the
bottom of the deep sea by means of that useful instrument,
the dredge. If you drop one of them into sea-water you
will presently see from one extremity an exquisite array
unfold like a beautifully cut flower of many petals, or,
rather, a star of ramifying plumes. Soon it begins to
climb the walls of your aquarium, and then you catch
the first glimpse of its affinity to the Urchins; for the
short warts which run in longitudinal lines down the
body, corresponding to the angles, gradually lengthen
themselves, and are soon perceived to be sucking feet,
analogous in structure and in function to those with which
the Star-fish and Sea-urchin creep along.

But the relationship becomes more apparent still when
we find that the Cucumber has a skeleton of calcareous
substance deposited on exactly the same plan as in the
Urchin; viz., around insulated rounded cavities. It is



336 EVENINGS AT THE MICROSCOPE

true you may cut open the animal and find nothing at all
more solid than the somewhat tough and leathery skin;
but a calcareous skeleton is there notwithstanding, though
in truth only a rudimentary one. If we were to cut off a
considerable fragment of the skin, and spread it out to
dry upon- a plate of glass, and then cover it with Canada
balsam, we should find assisted by the translucency which
is communicated to the tissues by the balsam that the
skin is filled with scattered atoms of the calcareous struct-
ure, perfectly agreeing with that with which the solid
framework of the Urchin is built up, but minute and
isolated in the flesh instead of being united into one or
more masses of definite organic form.

But the atoms I speak of are still more perfectly seen
by dissolving the piece of skin in boiling potash, and
washing the sediment twice or thrice in pure water; this
may then be spread upon a glass slide, and covered with a
plate of thin glass, when it forms an interesting and per-
manent object for study. I have here a slide which is the
result of such treatment; to the naked eye it appears
sprinkled with the finest dust, but under magnifying
power it is seen to consist of numberless calcareous
bodies, of great beauty, and very free from extraneous
matter.

The elegance of the forms is remarkable, and also their
uniformity; for though there do occur here and there
among them plates of no regular shape, perforated with
large or small roundish orifices, yet the overwhelming ma-
jority are of one form, subject to slight modifications,
in shape and size.

Neglecting, then, the irregular pieces, we perceive that
the normal form is an oval of open work, built up by the



SEA-URCHINS AND SEA-CUCUMBERS

repetition of a single element. That element is a piece of
clear glassy material, highly refractive, of the shape of a
dumb-bell two globes united by a thick, short column.
The oval is constructed thus: suppose two dumb-bells to
be placed in contact, side by side, and soldered together,
there would be of course an oval aperture between their
columns. Then two other dumb-bells are united to these
in a similar manner, but one on each side, so that the
globes of each shall rest in the valley between the former
globes now united. These then are soldered fast in like
manner; and the result is that there are three oval aper-
tures. The next step is that on the top of the four united
globes two other dumb - bells
stand erect, and lean over
toward each other till their
upper globes come into con-
tact, their lower ones remain-
ing remote; these are soldered
to the mass and to each other,
at the points of contact, leav-
ing a fourth aperture. The same is repeated at the oppo-
site end by two other dumb-bells; and the structure is com-
plete as you see it. In almost all cases the two united
globes of these terminal elements are fused into one
globe, and in not a few instances the appearance is as if
these two dumb-bells were but one, bent over in a semi-
circular form; but still a good many specimens occur in
which the two dumb-bells can be quite distinguished from
each other. The calcareous matter that solders the ele-
ments together seems abundant, and has the appearance
that would be presented if they had been made of solid

glass, and united by glass in a state of fusion; the latter
15 SCIENCE 15




DUMB-BJCLLS IN HOLOTHURIA.



EVENINGS AT THE MICROSCOPE

having apparently run together, so as to smooth and round
angles and fill up chinks, even where, as is often the case,
the globes themselves have only mutually approximated,
and not come into actual contact.

The average dimensions of these oval aggregations may
be .004 inch in length, and a little more than .002 in
width ; but some specimens occur which are a little larger,
and others a little smaller than this; while the irregular
plates are sometimes three times the length.

Some of the more worm- like members of this class
have, however, a skeleton composed of pieces imbedded

in their skin, of even more re-
markable shapes than these. One
of these is the Chirodota violacea
a native of the southern coasts
of Europe. We have indeed a
British species of the same genus,
a specimen of which is in my pos-
session, but I have vainly exam-
ined the skin for any structure
analogous to this. 1 In the Medi-
terranean species the skin, especially of the belly-side, is
described as filled with plates exactly resembling broad
and thin wheels of glass, supported by four, five, or six
radiating spokes, and having the inner edge of the hoop
cut into teeth of excessive delicacy.

Another animal remarkable for its cuticular furniture
is the genus Synapla, which is very similar in form, and



1 The most careful and repeated search has not availed me to find in the
skin the least trace of calcareous atoms ; but this may possibly be because I had
unfortunately preserved my specimen in acetate of alumina and the acetic acid
has perhaps dissolved the lime.




WHEEL IN CHIRODOTA..



SEA-URCHINS AND SEA-CUCUMBERS 339

closely allied to the Chirodota. It is very common in the
Adriatic and Mediterranean seas, but has not yet been
taken on the British coasts. I would counsel you, how-
ever, to have your eyes open if you have the opportunity
of searching our coasts; for, as Miilier found one species,
the Synapta inhcerens, on the shores of Denmark, it is not
at all unlikely that we may possess either it or some other.
Should it ever come into your hands, slit open the skin of
the belly, where you will find, imbedded in little papillae
or warts, some highly curious spicula or calcareous forms.
Each consists of an oblong plate, perforated with large
holes in a regular manner, and having a projection on its
surface near one extremity, to which is jointed
a second piece, having the most singularly true
resemblance to an anchor. The flukes of this
anchor project from the skin, the shank stand-
ing obliquely upward from the plate, to which
it is articulated by a dilatation, where the ANCHOR-PLATE IN

SYNAPTA.

ring would be, which is cut into teeth.

Among the multitude of transparent creatures that swim
in the open sea, few are more interesting than those which
constitute the infant state of the very animals that we have
lately been examining the Sea-urchins and their allies.
It is a productive way of obtaining subjects for micro-
scopic research, to go out in a boat on a quiet summer's
day, especially in the afternoon, when the sun has been
shining, or when evening is waning into night, and with
a fine muslin net stretched over a brass ring at the end
of a pole skim the surface of the smooth sea. At inter-
vals you take in your net, and having a wide- mouthed
glass jar ready, nearly filled with sea-water, invert the
muslin in it, when your captives, small and great, float




340 EVENINGS AT THE MICROSCOPE

off into the receiver. After a few such essays, unless you
have very bad success indeed, you will see even with the
naked eye, but much more with a lens, that the water in
your jar is teeming with microscopic life; and though
many of your captives will not long survive the loss of
their freedom, still meanwhile you may secure many an
interesting object, and examine it, while yet the beauty
and freshness of life remain. And moreover, with care
and prudence, some selected subjects may be maintained
in vigor, at least long enough to afford you valuable in-
formation on the habits, economy, metamorphosis, and
development of animals of which even the scientific world
knows next to nothing.

I have just been so fortunate as to obtain in this way
the larval stage of one of our Sea-urchins, and have it
now in the thin glass trough which is on the stage of the
microscope. It is just visible to the unassisted sight as
a slowly moving point in the clear water, when the
vessel is held up to the light; but with the low power
which I am now using, it is distinctly made out in
all its parts, and is an object of singular elegance and
beauty.

It is, as you see, somewhat of the figure of a helmet;
the crest rising to a perpendicular point, which is rounded,
the visor or mask descending far down and ending in two
points, and a long ear hanging down on each side, so as
to reach the shoulders of the wearer. Of course such
comparisons are fanciful, but they assist one in intelligible
description.

Now, the entire helmet is composed of a gelatinous
flesh of the most perfect transparency, so that we can see
with absolute clearness everything that is within it. And



SEA-URCHINS AND SEA-CUCUMBERS 841

the first thing that strikes us is, that a framework or skel-
eton of extreme delicacy, composed of glassy rods, sup-
ports the whole structure. Look carefully at this and
mark its symmetry and elegance. There is, then, first,
a rod which passes through the crest perpendicularly, and
carries at its lower extremity a horizontal ring. To the
opposite sides of this ring are soldered two other very
slender rods, passing down nearly in a perpendicular direc-
tion, but a little diverging; and two other shorter rods
pass down from the front of the ring, parallel to these.
After a while each lateral pair of rods is united by a short
cross-piece, and the result is four lengthened rods, two
of which go down through the visor into the chin-points,
and two larger and stouter ones through the ears into the
shoulder- points. This, then, is the solid skeleton, the in-
terest of which is much enhanced, when we observe that
it is formed, on the common plan, out of perforated lime-
glass, the two ear-rods and the crest-rod being pierced with
a regular series of oval holes, and bearing on their edges
corresponding projecting points.

Now, to turn again to the gelatinous flesh. The inner
surface of the visor, or that which would be in contact
with the face of the wearer, supposing it to be a real
helmet, has a great squarish orifice with a thickened mar-
gin, which we see by its movements to be highly sensitive
and contractile. This square orifice is the mouth of the
larva, and it leads into a cavity in the upper part of the
visor, which is the gullet; and this in its turn terminates
in a narrowed extremity, which passes into the orifice of
a greater and higher cavity, the lip of which embraces it
just as the bung- hole of a barrel receives and embraces
the tube of a funnel. The latter cavity occupies the chief



342 EVENINGS AT THE MICROSCOPE

part of the volume of the helmet, the four rods diverging
to enclose it. It is the stomach.

It adds to the beauty of the little helmet- shaped creat-
ure, that while the greater portion of the substance is of
the most colorless transparency, the summit of the crest
and the tips of the shoulder-points are tinged with a lovely
rose- red. The whole exterior surface is, moreover, stud-
ded with those minute and glandular specks with which
every part of the adult Urchin is covered; and the light
is reflected from the various prominences with sparkling
brilliancy.

The little creature moves through the water with much
grace, and with a dignified deliberation; the crest being
always uppermost, and the perpendicular position invari-
ably maintained. It does not appear capable of resting,
its movements depending on incessantly vibrating cilia.
These organs we perceive densely clothing the long ear-
pieces, but more especially accumulated and more vigorous
in a thickened, fleshy band, which passes partly round the
whole helmet, at the origin of these pieces.

You do not discern the slightest resemblance of form
between this little slowly-swimming dome and the spined
and boxed Urchin which crawls over the rocks; and you
wonder by what steps the tiny atom of one- fortieth of an
inch in length is led to its adult stage. Fortunately I can
satisfy your curiosity on this point, not indeed from my
own observations, but by those of Professor Johann
Miiller, whose discoveries of the developments of these
and kindred animals are among the most interesting, be-
cause the most startling, of the marvels which modern zo-
ology has revealed to us. The whole process is full of
surprising details, to which the change of the caterpillar



SEA-URCHINS AND SEA-CUCUMBERS

to a chrysalis, and that of the chrysalis to a butterfly,
present no parallel, wonderful as those changes of form
appear and are. There we have but modifications of out-
ward form, produced by the successive moults or castings
of the external skin, and the gradual growth of the ani-
mal, which has from the first been present, though veiled.
But the construction of the Sea-urchin is by no means a
process of skin- casting, nor has it any recognized parallel
in the whole economy of natural history. It is a develop-
ment perfectly unique. I will endeavor
to make you acquainted with the results
arrived at from the researches of the em-
inent German zoologist to whom we are
indebted for almost all we know on the
matter.

Let me first premise that this beauti-
ful helmet- shaped creature is not the future
Urchin; and, strange to say, that only a
very small portion of the present structure,
namely, the stomach and gullet, will enter
into its composition. The helmet is a kind
of temporary nurse, within which the fu-
ture Urchin is to be formed, and by which LARVA OF SEA - URCHIN -
it is to be carried from place to place by its ciliary action,
while the young animal is gradually acquiring the power
of independent life, when the whole constitution of the
nurse wastes away and vanishes!

The first trace of the young Urchin is a filmy circular
plate, which is not symmetrical with the helmet, nor
formed even on the same plane, but appears obliquely
fixed on the exterior of the stomach, on one side, close
to the arch of transparent flesh which stretches from one




344 EVENINGS AT THE MICROSCOPE

of the points of the visor to one of the ear- points. Herr
Miiller compares the larva (which is not helmet- shaped in
every species) to a clock-case, of which the visor, with its
hanging gullet and mouth, forms the pendulum, and then
the newly- formed disk represents the face of the clock,
only it is put on the side instead of the front. Now this
tiny disk gradually grows into the form and assumes all
the organs of the Urchin, while the enveloping nurse,
flesh, rods, and all, wastes away to nothing.

The. disk, soon after its appearance, is seen to bear
prominences on its surface, in which is traced the figure
of a cinque-foil, the elements being five warts set sym-
metrically. These lengthen and grow into suckers, essen-
tially identical with those of the adult, but most dispro-
portionately large. In the five triangular interspaces
between these, little points and needles of solid calcare-
ous glass begin to form, very much like the crystals that
shoot across a drying drop of a solution of some salt;
these catch and unite, first into "f, and then into
|-| forms, and then into irregular networks. Meanwhile,
fleshy cylindrical columns spring up from the surface, one
in each of these interspaces, and presently develop, within
their substance, a similar framework of porous glass;
these soon manifest themselves to be the spines, and each
is seated on a little nucleus of network, on which it pos-
sesses the power of rotating.

At the same time pedicellarias begin to be formed;
and, what is specially marvellous, they are first seen, not
on the disk, which alone is to be the future Urchin, but
on the interior wall of the helmet, which is even now in
process of being dissipated, and even on the opposite side
to that which carries the disk. They commonly appear



SEA-URCHINS AND SEA-CUCUMBERS 345

four in number, arranged in two pairs; and one can see
in them they being, like the suckers, large out of all
proportion to the disk the stem, and the three-leaved
heads, which already exercise their characteristic snapping
movements.

The disk is meanwhile enlarging its area; and the
spines and suckers, gradually lengthening, at length push
themselves through the walls of the helmet; the hanging
points and crest of which are fast diminishing by a kind
of insensible absorption ; the ciliary movements become less
vigorous, and the mouth closes up.
But, correspondently, the Urchin is
beginning to acquire its own inde-
pendent power of locomotion; for
the suckers, now ever sprawling
about, are capable of adhering to
any foreign body with which they
come into contact, and of dragging
the whole structure about, by their
proper contractions. The cilia that (Development of Disk).
cover the thickened fringing band still exercise their
powers, and are the last to disappear.

When the disk has grown to such an extent as to
spread over about half of the larval stomach, very little
remains of the helmet, except the middle portions of the
glassy rods and the ciliary bands; all the rest of this ex-
quisitely modelled framework having vanished by insen-
sible degrees, no one knows how or where. The stomach
and gullet, indeed, are gradually sucked into the ever-
growing disk; but all the rest, flesh and rods, fringes,
bands, and cilia, waste away to nothing.

The mouth of the larva has no connection with the




YOUNG SEA-URCHIN



346 EVENINGS AT THE MICROSCOPE

mouth of the Urchin. The little isolated patches of glassy
network continue to spread through the flesh of the disk,
until the whole forms one uniform structure, and consti-
tutes a series of plates. The mouth is that spot in the
centre over which the calcareous frame is last extended;
and it is first distinguishable by the appearance of five
glassy points, which soon develop themselves into the five
converging jaws, which we see forming such a curious
apparatus on the inferior side of the Sea-urchin.

Actual observation has not traced the infant animal be-



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