METHODS OF STUDY IN NATURAL HISTORY.

It may seem to some of my readers that I have wandered from my subject

and forgotten the title of these articles, which purport to be a series

of papers on "Methods of Study in Natural History." But some idea of

the progress of Natural History, of its growth as a science, of the

gradual evolving of general principles out of a chaotic mass of facts,

is a better aid to the student than direct instruction upon special

modes of investigation; and it is with the intention of presenting the

study of Natural History from this point of view that I have chosen my

title.

I have endeavored thus far to show how scientific facts have been

systematized so as to form a classification that daily grows more true

to Nature, in proportion as its errors are corrected by a more intimate

acquaintance with the facts; but I will now attempt a more difficult

task, and try to give some idea of the mental process by which facts

are transformed into scientific truth. I fear that the subject may seem

very dry to my readers, and I would again ask their indulgence for

details absolutely essential to my purpose, but which would indeed be

very wearisome, did they not lead us up to an intelligent and most

significant interpretation of their meaning.

I should be glad to remove the idea that science is the mere amassing

of facts. It is true that scientific results grow out of facts, but not

till they have been fertilized by thought The facts must be collected,

but their mere accumulation will never advance the sum of human

knowledge by one step;--it is the comparison of facts and their

transformation into ideas that lead to a deeper insight into the

significance of Nature. Stringing words together in incoherent

succession does not make an intelligible sentence; facts are the words

of God, and we may heap them together endlessly, but they will teach

us little or nothing till we place them in their true relations and

recognize the thought that binds them together as a consistent whole.

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.

The modern Crinoid without stem, or the Comatula, though agreeing with

the ancient in all the essential elements of structure, differs from it

in some specific features. It drops its stem when full-grown, though

the ab-oral region still remains the predominant part of the body and

retains its cup-like or calyx-like form. The Comatulae are not

abundant, and though represented by a number of Species, yet the type

as it exists at present is meagre in comparison to its richness in

former times. Indeed, this group of Echinoderms, which in the earliest

periods was the exponent of all its kind, has dwindled gradually, in

proportion as other representatives of the Class have come in, and

there exists only one species now, the Pentacrinus of the West Indies,

which retains its stem in its adult condition. It is a singular fact,

to which I have before alluded, and which would seem to have especial

reference to the maintenance of the same numeric proportions in all

times, that, while a Class is represented by few types, those types are

wonderfully rich and varied, but in proportion as other expressions of

the same structure are introduced, the first dwindle, and, if they do

not entirely disappear, become at least much less prominent than

before.

[Illustration: Ophiuran; showing one ray from the oral side.]

There remain only two other Orders to be considered, the Ophiurans and

the Holothurians. The Ophiurans approach the Crinoids more nearly than

any other group of Echinoderms, and in our classifications are placed

next above them. In them the ab-oral region, which has such a

remarkable predominance in the Crinoid, has become depressed; it no

longer extends into a stem, nor does it even rise into the calyx-like

or cup-like projection so characteristic of the Crinoids,--though,

when the animal is living, the ab-oral side of the disk is still quite

convex. The disk in the Ophiurans is small in comparison to the length

of the arms, and perfectly circular; it does not merge gradually in the

arms as in the Star-Fish, but the arms start abruptly from its

periphery. In these, as in the Crinoids, the interambulacral plates are

absent, and the interambulacral spaces are filled by an encroachment of

the ab-oral region upon them. There is an infinite variety and beauty

both of form and color in these Sea-Stars. The arms frequently measure

many times the diameter of the whole disk, and are so different in

size and ornamentation in the different Species that at first sight

one might take them for animals entirely distinct from each other. In

some the arms are comparatively short and quite simple,--in others

they are very long, and may be either stretched to their full length or

partly contracted to form a variety of graceful curves; in some they

are fringed all along the edges,--in others they are so ramified that

every arm seems like a little bush, as it were, and, intertwining with

each other, they make a thick network all around the animal. In the

geological succession, these Ophiurans follow the Crinoids, being

introduced at about the Carboniferous period, and perhaps earlier.

They have had their representatives in all succeeding times, and are

still very numerous in the present epoch.

To show the correspondence of the Holothurians with the typical formula

of the whole class of Echinoderms, I will return to the Sea-Urchins,

since they are more nearly allied with that Order than with any of the

other groups. We have seen that the Sea-Urchins approach most nearly to

the sphere, and that in them the oral region and the sides predominate

so greatly over the ab-oral region that the latter is reduced to a

small area on the summit of the sphere. In order to transform the

Sea-Urchin into a Holothurian, we have only to stretch it out from end

to end till it becomes a cylinder, with the oral region or mouth at

one extremity, and the ab-oral region, which in the Holothurian is

reduced to its minimum, at the other. The zones of the Sea-Urchin now

extend as parallel rows on the Holothurian, running from one end to the

other of the long cylindrical body. On account of their form, some of

them have been taken for Worms, and so classified by naturalists; but

as soon as their true structure was understood, which agrees in every

respect with that of the other Echinoderms, and has no affinity

whatever with the articulated structure of the Worms, they found their

true place in our classifications.

[Illustration: Holothurian.]

The natural attitude of these animals is different from that of the

other Echinoderms: they lie on one side, and move with the oral

opening forward, and this has been one cause of the mistakes as to

their true nature. But when we would compare animals, we should place

them, not in the attitude which is natural to them in their native

element, but in what I would call their normal position,--that is, such

a position as brings the corresponding parts in all into the same

relation. For instance, the natural attitude of the Crinoid is with

the ab-oral region downward, attached to a stem, and the oral region or

mouth upward; the Ophiuran turns its oral region, along which all the

suckers or ambulacra are arranged, toward the surface along which it

moves; the Star-Fish does the same; the Sea-Urchin also has its oral

opening downward; but the Holothurian moves on one side, mouth

foremost, as represented in the adjoining wood-cut, dragging itself

onward, like all the rest, by means of its rows of suckers. If, now, we

compare these animals in the various attitudes natural to them, we may

fail to recognize the identity of parts, or, at least, it will not

strike us at once. But if we place them all--Holothurian, Sea-Urchin,

Star-Fish, Ophiuran, and Crinoid--with the oral or mouth side

downward, for instance, we shall see immediately that the small area at

the opposite end of the Holothurian corresponds to the area on the top

of the Sea-Urchin; that the upper side of the Star-Fish is the same

region enlarged; that, in the Ophiuran, that region makes one side of

the small circular disk; while in the Crinoid it is enlarged and

extended to make the calyx-like projection and stem. In the same way,

if we place them in the same attitude, we shall see that the long,

straight rows of suckers along the length of the Holothurian, and the

arching zones of suckers on the spherical body of the Sea-Urchin, and

the furrows with the suckers protruding from them along the arms of

the Star-Fish and Ophiuran, and the radiating series of pores from the

oral opening in the Crinoid are one and the same thing in all, only

altered somewhat in their relative proportion and extent. Around the

oral opening of the Holothurian there are appendages capable of the

most extraordinary changes, which seem at first to be peculiar to these

animals, and to have no affinity with any corresponding feature in the

same Class. But a closer investigation has shown them to be only

modifications of the locomotive suckers of the Star-Fish and

Sea-Urchin, but ramifying to such an extent as to assume the form of

branching feelers. The little tufts projecting from the oral side in

the Sea-Urchins, described as gills, are another form of the same kind

of appendage.

The Holothurians have not the hard, brittle surface of the other

Echinoderms; on the contrary, their envelope is tough and leathery,

capable of great contraction and dilatation. No idea can be formed of

the beauty of these animals either from dried specimens or from those

preserved in alcohol. Of course, in either case, they lose their color,

become shrunken, and the movable appendages about the mouth shrivel up.

One who had seen the Holothurian only as preserved in museums would be

amazed at the spectacle of the living animal, especially if his first

introduction should be to one of the deep, rich crimson-colored

species, such as are found in quantities in the Bay of Fundy. I have

seen such an animal, when first thrown into a tank of sea-water, remain

for a while closely contracted, looking like a soft crimson ball.

Slowly, almost imperceptibly, as it becomes accustomed to its new

position, it begins to elongate; the fringes creep softly out,

spreading gradually all their ramifications, till one end of the animal

seems crowned with feathery, crimson sea-weeds of the most delicate

tracery. It is much to be regretted that these lower marine animals

are not better known. The plumage of the tropical birds, the down on

the most brilliant butterfly's wing, are not more beautiful in coloring

than the hues of many Radiates, and there is no grace of motion

surpassing the movements of some of them in their native element. The

habit of keeping marine animals in tanks is happily growing constantly

more popular, and before long the beauty of these inhabitants of the

ocean will be as familiar to us as that of Birds and Insects. Many of

the most beautiful among them are, however, difficult to obtain, and

not easily kept alive in confinement, so that they are not often seen

in aquariums.

Having thus endeavored to sketch each different kind of Echinoderm, let

us try to forget them all in their individuality, and think only of the

structural formula that applies equally to each. In all, the body has

three distinct regions, the oral, the ab-oral, and the sides; but by

giving a predominance to one or other of these regions, a variety of

outlines characteristic of the different groups is produced. In all,

the parts radiate from the oral opening, and join in the ab-oral

region. In all, this radiation is accompanied by rows of suckers

following the line of the diverging rays. It is always the same

structure, but, endowed with the freedom of life, it is never

monotonous, notwithstanding its absolute permanence. In short, drop

off the stem of the Crinoid, and depress its calyx to form a flat disk,

and we have an Ophiuran; expand that disk, and let it merge gradually

in the arms, and we have a Star-Fish; draw up the rays of the

Star-Fish, and unite them at the tips so as to form a spherical

outline, and we have a Sea-Urchin; stretch out the Sea-Urchin to form

a cylinder, and we have a Holothurian.

And now let me ask,--Is it my ingenuity that has imposed upon these

structures the conclusion I have drawn from them?--have I so combined

them in my thought that they have become to me a plastic form, out of

which I draw a Crinoid, an Ophiuran, a Star-Fish, a Sea-Urchin, or a

Holothurian at will? or is this structural idea inherent in them all,

so that every observer who has a true insight into their organization

must find it written there? Had our scientific results anything to do

with our invention, every naturalist's conclusions would be colored

by his individual opinions; but when we find all naturalists

converging more and more towards each other, arriving, as their

knowledge increases, at exactly the same views, then we must believe

that these structures are the Creative Ideas in living reality. In

other words, so far as there is truth in them, our systems are what

they are, not because Aristotle, Linnaeus, Cuvier, or all the men who

ever studied Nature, have so thought and so expressed their thought,

but because God so thought and so expressed His thought in material

forms when He laid the plan of Creation, and when man himself existed

only in the intellectual conception of his Maker.