Atlantic Monthly, Vol. 10, No. 57, July, 1862

V >> Various >> Atlantic Monthly, Vol. 10, No. 57, July, 1862

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I have spoken of the plans that lie at the foundation of all the
variety of the Animal Kingdom as so many structural ideas which must
have had an intellectual existence in the Creative Conception
independently of any special material expression of them. Difficult
though it be to present these plans as pure abstract formulae, distinct
from the animals that represent them, I would nevertheless attempt to
do it, in order to show how the countless forms of animal life have
been generalized into the few grand, but simple intellectual
conceptions on which all the past populations of the earth as well as
the present creation are founded. In such attempts to divest the
thought of its material expression, especially when that expression is
multiplied in such thousand-fold variety of form and color, our
familiarity with living animals is almost an obstacle to our success.
For I shall hardly be able to allude to the formula of the Radiates,
for instance,--the abstract idea that includes all the structural
possibilities of that division of the Animal Kingdom,--without
recalling to my readers a Polyp or a Jelly-Fish, a Sea-Urchin or a
Star-Fish. Neither can I present the structural elements of the Mollusk
plan, without reminding them of an Oyster or a Clam, a Snail or a
Cuttle-Fish,--or of the Articulate plan, without calling up at once the
form of a Worm, a Lobster, or an Insect,--or of the Vertebrate plan,
without giving it the special character of Fish, Reptile, Bird, or
Mammal. Yet I insist that all living beings are but the different modes
of expressing these formulae, and that all animals have, within the
limits of their own branch of the Animal Kingdom, the same structural
elements, though each branch is entirely distinct. If this be true,
and if these organic formulae have the precision of mathematical
formulae, with which I have compared them, they should be susceptible
of the same tests.

The mathematician proves the identity of propositions that have the
same mathematical value and significance by their convertibility. If
they have the same mathematical quantities, it must be possible to
transform them, one into another, without changing anything that is
essential in either. The problem before us is of the same character.
If, for instance, all Radiates, be they Sea-Anemones, Jelly-Fishes,
Star-Fishes, or Sea-Urchins, are only various modes of expressing the
same organic formula, each having the sum of all its structural
elements, it should be possible to demonstrate that they are
reciprocally convertible. This is actually the case, and I hope to be
able to convince my readers that it is no fanciful theory, but may be
demonstrated as clearly as the problems of the geometer. The
naturalist has his mathematics, as well as the geometer and the
astronomer; and if the mathematics of the Animal Kingdom have a greater
flexibility than those of the positive sciences, and are therefore not
so easily resolved into their invariable elements, it is because they
have the freedom and pliability of life, and evade our efforts to bring
all their external variety within the limits of the same structural
law which nevertheless controls and includes them all.

I wish that I could take as the illustration of this statement animals
with whose structure the least scientific of my readers might be
presumed to be familiar; but such a comparison of the Vertebrates,
showing the identity and relation of structural elements throughout
the Branch, or even in any one of its Classes, would be too extensive
and complicated, and I must resort to the Radiates,--that branch of the
Animal Kingdom which, though less generally known, has the simplest
structural elements.

I will take, then, for the further illustration of my subject, the
Radiates, and especially the class of Echinoderms, Star-Fishes,
Sea-Urchins, and the like, both in the fossil and the living types; and
though some special description of these animals is absolutely
essential, I will beg my readers to remember that the general idea,
and not its special manifestations, is the thing I am aiming at, and
that, if we analyze the special parts characteristic of these
different groups, it is only that we may resolve them back again into
the structural plan that includes them all.

I have already in a previous article named the different Orders of this
Class in their relative rank, and have compared the standing of the
living ones, according to the greater or less complication of their
structure, with the succession of the fossil ones. Of the five Orders,
Beches-de-Mer, Sea-Urchins, Star-Fishes, Ophiurans, and
Crinoids,--or, to name them all according to their scientific
nomenclature, Holothurians, Echinoids, Asteroids, Ophiurans, and
Crinoids,--the last-named are lowest in structure and earliest in time.
Cuvier was the first naturalist who detected the true nature of the
Crinoids, and placed them where they belong in the classification of
the Animal Kingdom. They had been observed before, and long and
laborious investigations had been undertaken upon them, but they were
especially baffling to the student, because they were known only in the
fossil condition from incomplete specimens; and though they still have
their representatives among the type of Echinoderms as it exists at
present, yet, partly owing to the rarity of the living specimens and
partly to the imperfect condition of the fossil ones, the relation
between them was not recognized. The errors about them certainly did
not arise from any want of interest in the subject among naturalists,
for no less than three hundred and eighty different authors have
published their investigations upon the Crinoids, and the books that
have been printed about these animals, many of which were written long
before their animal nature was suspected, would furnish a library in
themselves. The ancients knew little about them. The only one to be
found in the European seas resembles the Star-Fish closely, and they
called it Asterias; but even Aristotle was ignorant of its true
structural relations, and alludes only to its motion and general
appearance. Some account of the gradual steps by which naturalists have
deciphered the true nature of these lowest Echinoderms and their
history in past times may not be without interest, and is very
instructive as showing bow such problems may be solved.

In the sixteenth century some stones were found bearing the impression
of a star on their surface. They received the name of Trochites, and
gave rise to much discussion. Naturalists puzzled their brains about
them, called them star-shaped crystals, aquatic plants, corals; and to
these last Linnaeus himself, the great authority of the time on all
such questions, referred them. Beside these stony stars, which were
found in great quantities when attention was once called to them,
impressions of a peculiar kind had been observed in the rocks,
resembling flowers on long stems, and called "stone lilies" naturally
enough, for their long, graceful stems, terminating either in a
branching crown or a closer cup, recall the lily tribe among flowers.
The long stems of these seeming lilies are divided transversely at
regular intervals;--the stem is easily broken at any of these natural
divisions, and on each such fragment is stamped a star-like impression
resembling those found upon the loose stones or Trochites.

About a century ago, Guettard the naturalist described a curious
specimen from Porto Rico, so similar to these fossil lilies of the
rocks that he believed they must have some relation to each other. He
did not detect its animal nature, but from its long stem and branching
crown he called it a marine palm. Thus far neither the true nature of
the living specimen, nor of the Trochites, nor of the fossil lilies
was understood, but it was nevertheless an important step to have found
that there was a relation between them. A century passed away, and
Guettard's specimen, preserved at the Jardin des Plantes, waited with
Sphinx-like patience for the man who should solve its riddle.

Cuvier, who held the key to so many of the secrets of Nature, detected
at last its true structure; he pronounced it to be a Star-Fish with a
stem, and at once the three series of facts respecting the Trochites,
the fossil lilies, and Guettard's marine palm assumed their true
relation to each other. The Troehites were recognized as simply the
broken portions of the stem of some of these old fossil Crinoids, and
the Crinoids themselves were seen to be the ancient representatives of
the present Comatulae and Star-Fishes with stems. So is it often with
the study of Nature; many scattered links are collected before the man
comes who sees the connection between them and speaks the word that
reconstructs the broken chain.

I will begin my comparison of all Echinoderms with an analysis of the
Star-Fishes and Sea-Urchins, because I think I can best show the
identity of parts between them, notwithstanding the difference in
their external form; the Sea-Urchins having always a spherical body,
while the Star-Fishes are always star-shaped, though in some the star
is only hinted at, sketched out, as it were, in a simply pentagonal
outline, while in others the indentations between the rays are very
deep, and the rays themselves so intricate in their ramifications as to
be broken up into a complete net-work of branches. But under all this
variety of outline, our problem remains always the same: to build with
the same number of pieces a star and a sphere, having the liberty,
however, of cutting the pieces differently and changing their relative
proportions. Let us take first the Sea-Urchin and examine in detail
all parts of its external structure. I shall say nothing of the
internal structure of any of these animals, because it does not affect
the comparison of their different forms and the external arrangement of
parts, which is the subject of the present article.

On the lower side is the mouth, and we may call that side and all the
parts that radiate from it the oral region. On the upper side is a
small area to which the parts converge, and which, from its position
just opposite the so-called mouth or oral opening, we may call the
_ab-oral region_. I prefer these more general terms, because, if
we speak of the mouth, we are at once reminded of the mouth in the
higher animals, and in this sense the word, as applied to the aperture
through which the Sea-Urchins receive their food, is a misnomer. Very
naturally the habit has become prevalent of naming the different parts
of animals from their function, and not from their structure; and in
all animals the aperture through which food enters the body is called
the mouth, though there is not the least structural relation between
the organs so designated, except within the limits of each different
branch or division. To speak of these opposite regions in the
Sea-Urchin as the upper and lower sides would equally mislead us,
since, as we have seen, there is, properly speaking, no above and
below, no right and left sides, no front and hind extremities in these
animals, all parts being evenly distributed around a vertical axis. I
will, therefore, although it has been my wish to avoid technicalities
as much as possible in these papers, make use of the unfamiliar terms
oral and ab-oral regions, to indicate the mouth with the parts
diverging from it and the opposite area towards which all these parts
converge. [Footnote: When reference is made to the whole structure,
including the internal organs as well as the solid parts of the
surface, the terms _actinal_ and _ab-actinal_ are preferable
to oral and ab-oral.]

[Illustration: Sea-Urchin seen from the oral side, showing the zones
with the spines and suckers; for the ab-oral side, on the summit of
which the zones unite, see February Number, p. 216.]

The whole surface of the animal is divided by zones,--ten in number,
five broader ones alternating with five narrower ones. The five broad
zones are composed of large plates on which are the most prominent
spines, attached to tubercles that remain on the surface even when the
spines drop off after death, and mark the places where the spines have
been. The five small zones are perforated with regular rows of holes,
and through these perforations pass the suckers or water-tubes which
are their locomotive appendages. For this reason these narrower zones
are called the _ambulacra_, while the broader zones intervening
between them and supporting the spines are called the
_interambulacra_. Motion, however, is not the only function of
these suckers; they are subservient also to respiration and
circulation, taking in water, which is conveyed through them into
various parts of the body.

[Illustration: Portion of Sea-Urchin representing one narrow zone with
a part of the broad zones on either side and the ab-oral area on the
summit.]

The oral aperture is occupied by five plates, which may be called jaws,
remembering always that here again this word signifies the function,
and not the structure usually associated with the presence of jaws in
the higher animals; and each of these jaws or plates terminates in a
tooth. Even the mode of eating in these animals is controlled by their
radiate structure; for these jaws, evenly distributed about the
circular oral aperture, open to receive the prey and then are brought
together to crush it, the points meeting in the centre, thus working
concentrically, instead of moving up and down or from right to left,
as in other animals. From the oral opening the ten zones diverge,
spreading over the whole surface, like the ribs on a melon, and
converging in the opposite direction till they meet in the small space
which we have called the ab-oral region opposite the starting-point.

Here the broad zones terminate in five large plates differing somewhat
from those that form the zones in other parts of the body, and called
ovarian plates, because the eggs pass out through certain openings in
them; while the five narrow zones terminate in five small plates on
each of which is an eye, making thus five eyes alternating with five
ovarian plates. The centre of this area containing the ovarian plates
and the visual plates is filled up with small movable plates closing
the space between them. I should add that one of the five ovarian
plates is larger than the other four, and has a peculiar structure,
long a puzzle to naturalists. It is perforated with minute holes,
forming an exceedingly delicate sieve, and this is actually the purpose
it serves. It is, as it were, a filter, and opens into a canal which
conducts water through the interior of the body; closed by this sieve
on the outside, all the water that passes into it is purified from all
foreign substances that might be injurious to the animal, and is thus
fitted to pass into the water-system, from which arise the main
branches leading to the minute suckers which project through the holes
in the narrow zones of plates.

[Illustration: Star-Fish from the ab-oral side.]

Now in order to transform theoretically our Sea-Urchin into a
Star-Fish, what have we to do? Let the reader imagine for a moment that
the small ab-oral area closing the space between the ovarian plates and
the eye-plates is elastic and may be stretched out indefinitely; then
split the five broad zones along the centre and draw them down to the
same level with the mouth, carrying the ovarian plates between them.
We have then a star, just as, dividing, for instance, the peel of an
orange into five compartments, leaving them, of course, united at the
base, then stripping it off and spreading it out flat, we should have a
five-rayed star.

[Illustration: One arm of Star-Fish from the oral side.]

But in thus dividing the broad zones of the Sea-Urchins, we leave the
narrow zones in their original relation to them, except that every
narrow zone, instead of being placed between two broad zones, has now
one-half of each of the zones with which it alternated in the
Sea-Urchin on either side of it and lies between them. The adjoining
wood-cut represents a single ray of a Star-Fish, drawn from what we
call its lower side or the oral side. Along the centre of every such
ray, diverging from the central opening or the mouth, we have a
furrow, corresponding exactly to the narrower zones of the Sea-Urchin.
It is composed of comparatively small perforated plates through which
pass the suckers or locomotive appendages. On either side of the
furrows are other plates corresponding to the plates of the broad zones
in the Sea-Urchin. Where shall we look for the five eyes? Of course, at
the tip of every ray; exactly where they were when the rays were drawn
up to form the summit of a sphere, so that the eyes, which are now at
their extremities, were clustered together at their point of meeting.
Where shall we look for the ovarian plates? At each angle of the five
rays, because, when the broad zones of which they formed the summit
were divided, they followed the split, and now occupy the place which,
though it seems so different on the surface of the Star-Fish, is
nevertheless, relatively to the rest of the body, the same as they
occupied in the Sea-Urchin. Assuming, as we premised, that the central
area of the ab-oral region, forming the space between the plates at the
summit of the zones in the Sea-Urchin, is elastic, it has stretched
with the spreading out of the zones, following the indentation between
the rays, and now forms the whole upper surface of the body. All the
internal organs of the animal lie between the oral and ab-oral
regions, just as they did in the Sea-Urchin, only that in the Star-
Fish these regions are coequal in extent, while in the Sea-Urchin the
ab-oral region is very contracted, and the oral region with the parts
belonging to it occupies the greater part of its surface.

Such being the identity of parts between a Star-Fish and a Sea-Urchin,
let us see now how the Star-Fish may be transformed into the
Pedunculated Crinoid, the earliest representative of its Class, or
into a Comatula, one of the free animals that represent the Crinoids in
our day.

[Illustration: Crinoid with branching crown; oral side turned upward.]

We have seen that in the Sea-Urchins the ab-oral region is very
contracted, the oral region and the parts radiating from it and forming
the sides being the predominant features in the structure; and we
shall find, as we proceed in our comparison, that the different
proportion of these three parts, the oral and ab-oral regions and the
sides, determines the different outlines of the various Orders in this
Class. In the Sea-Urchin the oral region and the sides are predominant,
while the ab-oral region is very small. In the Star-Fish, the oral and
ab-oral regions are brought into equal relations, neither
preponderating over the other, and the sides are compressed, so that,
seen in profile, the outline of the Star-Fish is that of a slightly
convex disk, instead of a sphere, as in the Sea-Urchin. But when we
come to the Crinoids, we find that the great preponderance of the
ab-oral region determines all that peculiarity of form that
distinguishes them from the other Echinoderms, while the oral region is
comparatively insignificant. The ab-oral region in the Crinoid rises
to form a sort of cup-like or calyx-like projection. The plates forming
it, which in the Star-Fish or the Sea-Urchin are movable, are soldered
together so as to be perfectly immovable in the Crinoid. Let this
seeming calyx be now prolonged into a stem, and we see at once how
striking is the resemblance to a flower; turn it downwards, an attitude
which is natural to these Crinoids, and the likeness to a drooping
lily is still more remarkable The oral region, with the radiating
ambulacra, is now limited to the small flat area opposite the juncture
of the stem with the calyx; and whether it stretches out to form long
arms, or is more compact, so as to close the calyx like a cup, it
seems in either case to form a flower-like crown. In these groups of
Echinoderms the interambulacral plates are absent; there are no rows
of plates of a different kind alternating with the ambulacral ones, as
in the Sea-Urchins and the Star-Fishes, but the ab-oral region closes
immediately upon the ambulacra.

It seems a contradiction to say, that, though these Crinoids were the
only representatives of their Class in the early geological ages,
while it includes five Orders at the present time, Echinoderms were as
numerous and various then as now. But, paradoxical as it may seem, this
is nevertheless true, not only for this Class, but for many others in
the Animal Kingdom. The same numerical proportions, the same richness
and vividness of conception were manifested in the early creation as
now; and though many of the groups were wanting that are most prominent
in modern geological periods, those that existed were expressed in such
endless variety that the Animal Kingdom seems to have been as full
then as it is to-day. The Class of the Echinoderms is one of the most
remarkable instances of this. In the Silurian period, the Crinoids
stood alone; there were neither Ophiurans, Asteroids, Echinoids, nor
Holothurians; and yet in one single locality, Lockport, in the State
of New York, over an area of not more than a few square miles, where
the Silurian deposits have been carefully examined, there have been
found more different Species of Echinoderms than are living now along
our whole Atlantic coast from Maine to Florida.

There is nothing more striking in these early populations of the earth
than the richness of the types. It would seem as if, before the world
was prepared for the manifold existences that find their home here now,
when organic life was limited by the absence of many of the present
physical conditions, the whole wealth of the Creative Thought lavished
itself upon the forms already introduced upon the globe. After thirty
years' study of the fossil Crinoids, I am every day astonished by some
new evidence of the ingenuity, the invention, the skill, if I may so
speak, shown in varying this single pattern of animal life. When one
has become, by long study of Nature, in some sense intimate with the
animal creation, it is impossible not to recognize in it the immediate
action of thought, and even to specialize the intellectual faculties
it reveals. It speaks of an infinite power of combination and analysis,
of reminiscence and prophecy, of that which has been in eternal harmony
with that which is to be; and while we stand in reverence before the
grandeur of the Creative Conception as a whole, there breaks from it
such lightness of fancy, such richness of invention, such variety and
vividness of color, nay, even the ripple of mirthfulness,--for Nature
has its humorous side also,--that we lose our grasp of its completeness
in wonder at its details, and our sense of its unity is clouded by its
marvellous fertility. There may seem to be an irreverence in thus
characterizing the Creative Thought by epithets which we derive from
the exercise of our own mental faculties; but it is nevertheless true,
that, the nearer we come to Nature, the more does it seem to us that
all our intellectual endowments are merely the echo of the Almighty
Mind, and that the eternal archetypes of all manifestations of thought
in man are found in the Creation of which he is the crowning work.

In no group of the Animal Kingdom is the fertility of invention more
striking than in the Crinoids. They seem like the productions of one
who handles his work with an infinite ease and delight, taking pleasure
in presenting the same thought under a thousand different aspects.
Some new cut of the plates, some slight change in their relative
position is constantly varying their outlines, from a close cup to an
open crown, from the long pear-shaped oval of the calyx in some to its
circular or square or pentagonal form in others. An angle that is
simple in one projects by a fold of the surface and becomes a fluted
column in another; a plate that was smooth but now has here a
symmetrical figure upon it drawn in beaded lines; the stem which is
perfectly unbroken in one, except by the transverse divisions common to
them all, in the next puts out feathery plumes at every such transverse
break. In some the plates of the stem are all rigid and firmly soldered
together; in others they are articulated upon each other in such a
manner as to give it the greatest flexibility, and allow the seeming
flower to wave and bend upon its stalk. It would require an endless
number of illustrations to give even a faint idea of the variety of
these fossil Crinoids. There is no change that the fancy can suggest
within the limits of the same structure that does not find expression
among them. Since I have become intimate with their wonderful
complications, I have sometimes amused myself with anticipating some
new variation of the theme, by the introduction of some undescribed
structural complication, and then seeking for it among the specimens
at my command, and I have never failed to find it in one or other of
these ever-changing forms.

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