Sea-Urchins (Sea-Hedgehogs).
'Truly the skill of the Great Architect of Nature is not less displayed
in the construction of the Sea-Urchin than in the building up of a
world.'--P. FORBES.
[Illustration:
1 THE APLYSIA or SEA-HARE
2 PURPLE-TIPPED SEA-URCHIN
3 Spine of PURPLE-TIPPED SEA-URCHIN
4, 5 Suckers of PURPLE-TIPPED SEA-URCHIN
6 COMMON SUN-STAR]
XIX.
Sea-Urchins are frequently taken in dredging. Several common species,
usually of a small size, are often found among the rocks situated
between tide marks.
Into the aquarium no specimens larger than from one to two inches in
diameter should be introduced, and even these require to be closely
watched, for if afflicted with a fatal illness, I know of no animal
whose remains sooner taint the water. Almost before life is extinct,
the Urchin throws out a light-coloured nauseous fluid, that speedily
poisons the surrounding water, and, of course, causes the destruction
of any inhabitants of the tank who may neither have the sense nor
opportunity to inhale copious draughts of fresh air. As a rule, if the
suckers are motionless, or if on touching the animal it is found not to
be adherent to any object, transfer it at once to your 'infirmary' for
further observation.
I have always found small specimens to be much more lively and
walkative, (if I may be allowed the expression) than their more
corpulent brethren.
The inflexible, mail-like crust, or shell, as it is commonly called,
of the Echinus is perhaps one of the most marvellous objects on which
the eye can rest. Although at first sight it appears to be a solid
calcareous box, it is in reality composed of several hundred pentagonal
plates,[17] of various sizes, so closely dove-tailed together that
their marks of junction are scarcely perceptible. Upon a superficial
examination we are apt (most erroneously) to consider this wonderful
piece of work to be more elaborate than the wants of the animal demand.
The fact of the Lobster or Crab throwing off its entire shell at
certain seasons, to admit of the increased growth of the animal is a
truly marvellous phenomenon, still, it would more excite our wonder
were we to find that, instead of being cast away at all, the hard,
inelastic envelope which surrounds the bodies of crustaceans was made
to swell or expand proportionately with the soft parts of the animal!
Now, the mosaic-like shell of the Sea-Urchin, though built up, as
before stated, of several hundred pieces, is by a beautiful process
slowly and imperceptibly enlarged correspondingly with the growth of
the animal.
[17] In a specimen that I examined, and then carefully took to pieces,
there were exactly 1780 plates.
The gradual enlargement of the Echinus shell takes place in the
following manner:--
Over the entire surface of the globular shell, spines, and joints of
the living Urchin, there exists a delicate membrane that insinuates
itself between the pentagonal plates above mentioned, and continually
deposits around the edges a certain portion of calcareous matter
(carbonate of lime). The same process being also carried on by the
fleshy covering that surrounds the spines, &c., it must be evident that
so long as the vital power of the animal exists, each plate and spine,
still keeping to its original form, must be daily and hourly augmented
in size until the Sea-Egg has attained its full and mature dimensions.
As to how the spines retain their relative position in each plate, as
the latter gradually becomes enlarged, I cannot positively state; but
may be permitted to mention, that, judging from carefully prepared
sections of the plates when submitted to the microscope, each spine
appeared to my eye to be by some singular process urged along in a kind
of groove to its proper place.
The hedgehog-like spines that surround the globose body of the
Sea-Urchin are all moveable at the will of the animal,--each prickle
being connected by a ball-and-socket joint to a pearly tubercle, which
acts as the 'socket' on which the 'ball' of the spine revolves. If
the spine be removed, a comparatively smooth surface will be left, on
which are various sized tubercles systematically arranged. Situated at
regular intervals between the tubercles are ten broad bands, disposed
in pairs, and containing many hundreds of very minute perforations, or
ambulacral orifices, as they are generally termed by naturalists.
Through these apertures issue numerous sucker-like feet, closely
resembling those of the Star-fish, but endowed with far greater powers
of contraction and extension.
The number of suckers is very great. In an Urchin measuring exactly
three inches in diameter, by aid of a hand lens, I counted no less than
3300 pores in the ten avenues. Now, these pores are always situated in
pairs, and as each sucker occupies a pair of pores, it will give 1650
as the total amount of suckers.
There is no doubt that it is almost entirely by means of these curious
organs that the Sea-Urchin is enabled to move about from place to
place, although no less an authority than Professor Agassiz asserts
to the contrary. 'How, in fact,' says this author, 'could these small
tentacula, situated as they generally are in that part of the body
which is never brought into contact with the ground when the animal
moves, and overhung by calcareous solid spines--how, I ask, could
these flexible tubes be used as organs of motion? It is an undeniable
fact, and I have often observed it myself, that _it is with their
spines the Echini move themselves, seize their_ _prey, and bring it
to their mouths_ by turning the rays of their lower edge in different
directions. But the correction of an error respecting the functions
of the ambulacral tubes does not solve the problem relating to their
nature and use. This problem we are yet unable to solve, as we know
nothing more respecting them than that they are connected with the
aquiferous system.'
Many other writers, among whom is Professor Forbes (from whose work
on Star-fishes I have transferred the foregoing extract), assert, in
opposition to the great Swiss naturalist, that the Echinidæ move by
the joint action of their suckers and spines. 'The argument,' says the
great British naturalist, 'against the suckers being organs of motion,
founded on their position above as well as below, would equally apply
to the spines, to which organs Professor Agassiz has attributed all
progressive powers in these animals.'
The fact is now so well established, that it is scarcely necessary
for the writer to state, that from personal observation he can fully
confirm the evidence of Professor Forbes relative to the functions
of the suckers of the Sea-Urchins. But although that talented author
entertained no doubt as to the organs in question being powerful
locomotive agents, he evidently seems to have felt himself unable to
suggest any purpose they could possibly serve when situated on the back
or upper part of the animal.
My own experience incontestibly proves that the suckers in question
are used for precisely the same purpose as those situated in any
other part of the body. I am enabled to state, from having repeatedly
witnessed the phenomenon, that _the Echinus can walk about with equal
facility while lying on its back as in its more natural position_. The
advantage of this power to the animal under certain circumstances will
be apparent upon a little reflection.
With regard to the spines, I fancy their purpose is almost solely to
assist the Urchin to burrow in the sand, and to protect it from the
attacks of its enemies. It may be, however, that at particular times
they serve as aids to locomotion, but that their assistance can be, and
is, often dispensed with entirely by the animal, I can most positively
assert.
My experiments were always conducted in glass vases, up the smooth,
polished sides of which my specimens frequently advanced. Upon reaching
the surface of the water, I have seen an Urchin roll completely round
and move along on its back, then after a time change its position, and
travel round the circumference of the vessel _while attached by its
side_, the body of the animal being sometimes inverted.
At such times as these it must be quite evident that the spines would
be totally useless, and that by the suckers alone did the animal
perform its interesting movements.
According to a certain writer, there are some foreign species of the
Echini remarkable for possessing spines, which act both as offensive
and defensive weapons. 'On one occasion' (this writer says) 'when
searching for a fish in the crevice of a coral rock, I felt a severe
pain in my hand, and upon withdrawing it, found my fingers covered
with slender spines, evidently those of the Echinus, of a grey colour,
elegantly banded with black.
'They projected from my fingers like well-planted arrows from a target,
and their points being barbed could not be removed, but remained
for some weeks imbedded as black specks in the skin. Its concealed
situation did not permit me to examine this particular Echinus. In some
experiments I approached the spines with so much caution, that had they
been the most finely pointed needles in a fixed state no injury could
have been received from them, yet their points were always stuck into
my hand rapidly and severely.'
In addition to those above described, the Sea-Urchin is provided with
other organs, in shape somewhat resembling minute pincers, supported on
fleshy stems, which always keep up an incessant motion when the animal
is in a healthy condition. They are scattered in great numbers over
the surface of the body, among the spines, and around the mouth of the
Urchin.
The use of these singular objects--by naturalists termed
Pedicellariæ--is totally unknown. Some writers think they are an
integral part of the Echinus, others describe them as distinct and
parasitic animals. There is good reason to believe that the former will
eventually be proved to be the correct explanation of the matter.
Its masticatory apparatus is not the least wonderful portion of the
Sea-Urchin. The teeth, five in number, which may frequently be seen
protruding from the mouth, are of extreme hardness, and of seemingly
disproportionate length. They are not fixed in sockets as ours are,
or they would be speedily worn away by their action on the shelled
mollusca upon which the animal feeds, but fresh substance is added
to each tooth as fast as it is worn away by use, as in the case of
many gnawing animals. 'In order to allow of such an arrangement, as
well as to provide for the movements of the teeth, jaws are provided,
which are situated in the interior of the shell, and these jaws, from
their great complexity and unique structure, form perhaps the most
admirable masticating instrument met with in the animal kingdom. The
entire apparatus removed from the shell consists of the following
parts. There are five long teeth, each of which is enclosed in a
triangular bony piece, that for the sake of brevity we will call jaws.
The five jaws are united together by various muscles, so as to form a
pentagonal pyramid, having its apex in contact with the oval orifice
of the shell, while its base is connected with several bony levers by
means of numerous muscles provided for the movements of the whole.
When the five jaws are fixed together in their natural position, they
form a five-sided conical mass, aptly enough compared by Aristotle
to a lantern, and not unfrequently described by modern writers under
the name of "the lantern of Aristotle." The whole of this complicated
machinery is suspended by muscles from a frame-work fixed in the
interior of the shell, and may often be picked up upon the beach, or
still better exposed _in situ_ in a dead Echinus, by those who would
examine closely this wonderful piece of mechanism.'[18]
[18] The 'Aquarian Naturalist,' p. 224.
I have made two careful drawings of the jaws and teeth of the Echinus.
No. 1 represents, as it were, the 'elevation' of the pentagonal pyramid
above described, while No. 2 constitutes the 'plan' of the same object.
The _Echinus sphæra_, or common Egg-Urchin, may often be seen forming
a curious ornament in the drawing-rooms of the "West End," and also in
the dwellings of the poorer classes, who, according to some authors,
boil it like eggs, and so eat it. Hence its popular title. Among the
ancients the Echinidæ were accounted a favourite dish. 'They were
dressed with vinegar, honied wine or mead, parsley and mint. They were
the first dish in the famous supper of Lentulus when he was made Flamen
Martialis. By some of the concomitant dishes they seemed designed as a
whet for the second course to the holy personages, priests and vestals,
invited on the occasion.'
The illustration on Plate 10 was drawn from a living specimen, and
gives a somewhat unusual representation of a Sea-Urchin. In general the
spines alone are shown, but I have endeavoured to give the uninitiated
reader some faint notion of the appearance which the _suckers_ present
when extended from the surface of the shell.
The young Urchin sat very quietly while I was engaged in taking his
portrait, but continually extended crowds of his slender tubular legs
in all directions, as above indicated, much to my gratification and
apparently to his own.
In preparing a Sea-Urchin for a chimney ornament, the most important
point is to remove the spines so as to let the tubercles remain entire.
In performing this operation some little experience is necessary.
Several times I attempted the process by aid of a pen-knife and a pair
of pliers, but not with a satisfactory result. Having mentioned my
difficulty to a friend, he laughingly asked me if I had ever heard of a
certain pilgrim who, for some peccadillo he had committed, was doomed
to perform penance by walking to Loretto's shrine with peas in his
shoes? Of course I was acquainted with the story, but could not see
what it had to do with Sea-Urchins, and told my brother naturalist so.
Still smiling, he said, 'Do you remember the relief that was said to be
afforded to the humorous rascal, both mentally and bodily, by _boiling
his peas_?' Yes. 'Well, then,' was the reply, 'do you boil your
Sea-Eggs, and you will find your troubles speedily cease.' I did as I
was directed, and found the advice of great service; for, after being
an hour or two in the 'pot,' the spines of the Urchin may be totally
rubbed off by aid of a nail brush, or some such instrument. Moreover,
the colour of the shell is improved, and the dental apparatus may be
drawn out entire, with the greatest ease.
I may here take opportunity to mention, that the student who may think
proper to act upon the hint above given, should not boil the Urchin too
long, or the fleshy parts will become dissolved, and the entire shell
fall into a multitude of fragments.
This unfortunate result actually happened on one occasion to a genial,
clever friend of mine, much to his chagrin and my malicious delight.
There are several other species of Sea-Urchins whose forms are
tolerably well distinguished by their popular appellations. Thus we
have the 'Silky Spined Urchin;' the 'Green Pea-Urchin,'--the latter
is the commonest and prettiest of all its kindred, its back being
covered with a kind of powdery green, as is seen on the elytra of
many beetles; the 'Cake-Urchin,' which from its flattened form may be
regarded as a link between the Sea-Urchins and the true Star-fishes;
the 'Purple Heart-Urchin,' and the pretty 'Rosy Heart-Urchin,'
appropriately named from the brilliant crimson hue that its body
presents during life.