Chapter I: Front Matter (1)

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Transcriber's notes:

(1) Numbers following letters (without space) like C2 were originally
printed in subscript. Letter subscripts are preceded by an
underscore, like C_n.

(2) Characters following a carat (^) were printed in superscript.

(3) Side-notes were relocated to function as titles of their respective
paragraphs.

(4) Macrons and breves above letters and dots below letters were not
inserted.

(5) [root] stands for the root symbol; [alpha], [beta], etc. for greek
letters.

(6) The letter 'Eth' is replaced by 'd'.

(7) The following typographical errors have been corrected:

ARTICLE ECHTERNACH: "The origin of this festival is uncertain, but
it dates at least from the 13th century and was probably instituted
during an outbreak of cholera." 'an' amended from 'on'.

ARTICLE ECONOMIC ENTOMOLOGY: "Among the Coleoptera or beetles there
is a group of world-wide pests, the Elateridae or click beetles,
the adults of the various 'wireworms.'" 'beetles' amended from
'bettles'.

ARTICLE EDUCATION: "... laid down that the application of a
candidate might in no circumstances be rejected on any religious
ground, nor on the ground of social antecedents or the like." 'be'
amended from 'he'.

ARTICLE EDUCATION: "The public assistance afforded to secondary
education in Wales under the Intermediate Act is supplemented by
the grants of the Board of Education, and the Board's revised
Secondary School Regulations were applied to Wales in 1908."
'education' amended from 'educaton'.

ENCYCLOPAEDIA BRITANNICA

A DICTIONARY OF ARTS, SCIENCES, LITERATURE
AND GENERAL INFORMATION

ELEVENTH EDITION

VOLUME VIII, SLICE X

Echinoderma to Edward

ARTICLES IN THIS SLICE:

ECHINODERMA EDESSA (Macedonia)
ECHINUS EDESSA (Mesopotamia)
ECHIUROIDEA EDFU
ECHMIADZIN EDGAR (king of the English)
ECHO EDGAR (son of Edward)
ECHTERNACH EDGECUMBE
ECHUCA EDGE HILL
ECIJA EDGEWORTH, MARIA
ECK, JOHANN MAIER EDGEWORTH, RICHARD LOVELL
ECKERMANN, JOHANN PETER EDGEWORTH DE FIRMONT, HENRY ESSEX
ECKERNFORDE EDGREN-LEFFLER, ANNE CHARLOTTE
ECKERSBERG, KRISTOFFER EDHEM PASHA
ECKHART, JOHANNES EDICT
ECKHEL, JOSEPH HILARIUS EDINBURGH
ECKMUHL EDINBURGHSHIRE
ECLECTICISM EDISON, THOMAS ALVA
ECLIPSE EDMONTON (Alberta, Canada)
ECLIPTIC EDMONTON (England)
ECLOGITE EDMUND, SAINT
ECLOGUE EDMUND (king of East Anglia)
ECONOMIC ENTOMOLOGY EDMUND I.
ECONOMICS EDMUND (Ironside)
ECONOMY (Pennsylvania, U.S.A.) EDMUND (king of Sicily)
ECONOMY EDMUNDS, GEORGE FRANKLIN
ECSTASY EDOM
ECTOSPORA EDRED
ECUADOR EDRIC, STREONA
ECZEMA EDUCATION
EDAM EDWARD (The Elder)
EDDA EDWARD (The Martyr)
EDDIUS EDWARD (The Confessor)
EDELINCK, GERARD EDWARD I.
EDELWEISS EDWARD II.
EDEN, SIR ASHLEY EDWARD III.
EDEN EDWARD IV.
EDENBRIDGE EDWARD V.
EDEN HALL, LUCK OF EDWARD VI.
EDENKOBEN EDWARD VII.
EDENTATA EDWARD (prince of Wales)
EDENTON

ECHINODERMA.[1] The [Greek: echinoderma], or "urchin-skinned" animals, have long been a favourite subject of study with the collectors of sea-animals or of fossils, since the lime deposited in their skins forms hard tests or shells readily preserved in the cabinet. These were described during the 18th and first half of the 19th centuries by many eminent naturalists, such as J.T. Klein, J.H. Linck, C. Linnaeus, N.G. Leske, J.S. Miller, L. v. Buch, E. Desor and L. Agassiz; but it was the researches of Johannes Muller (1840-1850) that formed the groundwork of scientific conceptions of the group, proving it one of the great phyla of the animal kingdom. The anatomists and embryologists of the next quarter of a century confirmed rather than expanded the views of Muller. Thus, about 1875, the distinction of Echinoderms from such radiate animals as jelly-fish and corals (see COELENTERA), by their possession of a body-cavity ("coelom") distinct from the gut, was fully realized; while their severance from the worms (especially Gephyrea), with which some Echinoderrns were long confused, had been necessitated by the recognition in all of a radial symmetry, impressed on the original bilateral symmetry of the larva through the growth of a special division of the coelom, known as the "hydrocoel," and giving rise to a set of water-bearing canals--the water-vascular or ambulacral system. There was also sufficient comprehension of the differences between the main classes of Echinoderms--the sea-urchins or Echinoidea, the starfish or Asteroidea, the brittle-stars and their allies known as Ophiuroidea, the worm-like Holothurians, the feather-stars and sea-lilies called Crinoidea, with their extinct relatives the sac-like Cystidea, the bud-formed Blastoidea, and the flattened Edrioasteroidea--while within the larger of these classes, such as Echinoidea and Crinoidea, fair working classifications had been established. But the study that should elucidate the fundamental similarities or homologies between the several classes, and should suggest the relations of the Echinoderma to other phyla, had scarcely begun. Indeed, the time was not ripe for such discussions, still less for the tracing of lines of descent and their embodiment in a genealogical classification. Since then exploring expeditions have made known a host of new genera, often exhibiting unfamiliar types of structure.

Among these the abyssal starfish and holothurians described by W.P.
Sladen and H. Theel respectively, in the Report of the "Challenger"
Expedition, are most notable. The sea-urchins, ophiuroids and crinoids
also have yielded many important novelties to A. Agassiz
("Challenger," "Blake," and "Albatross" Expeditions), T. Lyman
("Challenger"), Sladen ("Astrophiura," _Ann. Mag. Nat. Hist._, 1879),
F.J. Bell (numerous papers in _Ann. Mag. Nat. Hist._ and in _Proc.
Zool. Soc._), E. Perrier ("Travailleur" and "Talisman," Cape Horn and
Monaco Expeditions), P.H. Carpenter "Challenger" Reports), and others.
The anatomical researches of these authors, as well as those of S.
Loven ("On Pourtalesia" and "Echinologica," published by the Swedish
Academy of Science), H. Ludwig (_Morphologische Studien_, Leipzig,
1877-1882), O. Hamann (_Histologie der Echinodermen_, Jena,
1883-1889), L. Cuenot ("Etudes morphologiques," _Arch. Biol._, 1891,
and papers therein referred to), P.M. Duncan ("Revision of the
Echinoidea," _Journ. Linn. Soc._, 1890), H. Prouho ("Sur Dorocidaris,"
_Arch. Zool. Exper._, 1888), and many more, need only be mentioned to
recall the great advance that has been made. In physiology may be
instanced W.B. Carpenter's proof of the nervous nature of the
chambered organ and axial cords of crinoids (_Proc. Roy. Soc._, 1884),
the researches of H. Durham (_Quart. Journ. Micr. Sci._, 1891) and
others into the wandering cells of the body-cavity, and the study of
the deposition of the skeletal substance ("stereom") by Theel (in
_Festskrift for Lilljeborg_, 1896). Knowledge of the development has
been enormously extended by numerous embryologists, e.g. Ludwig (_op.
cit._), E.W. MacBride ("Asterina gibbosa," _Quart. Journ. Micr. Sci._,
1896), H. Bury (_Quart. Journ. Micr. Sci._, 1889, 1895), Seeliger (on
"Antedon," _Zool. Jahrb._, 1893), S. Goto ("Asterias pallida," _Journ.
Coll. Sci. Japan_, 1896), C. Grave ("Ophiura," _Mem. Johns Hopkins
Univ._, 1899), Theel ("Echinocyamus," _Nov. Act. Soc. Sci. Upsala_,
1892), R. Semon ("Synapta," _Jena. Zeitschr._, 1888), and Loven (_opp.
citt._); and though the theories based thereon may have been fantastic
and contradictory, we are now near the time when the results can be
co-ordinated and some agreement reached. But the scattered details of
comparative anatomy are capable of manifold arrangement, while the
palimpsest of individual development is not merely fragmentary, but
often has the fragments misplaced. The morphologist may propose
classifications, and the embryologist may erect genealogical trees,
but all schemes which do not agree with the direct evidence of fossils
must be abandoned; and it is this evidence, above all, that gained
enormously in volume and in value during the last quarter of the 19th
century. The Silurian crinoids and cystids of Sweden have been
illustrated in N.P. Angelin's _Iconographia crinoideorum_ (1878); the
Palaeozoic crinoids and cystids of Bohemia are dealt with in J.
Barrande's _Systeme silurien_ (1887 and 1899); P.H. Carpenter
published important papers on fossil crinoids in the _Journal_ of the
Geological Society, on Cystidea in that of the Linnean Society, 1891,
and, together with R. Etheridge, jun., compiled the large _Catalogue
of Blastoidea in the British Museum_, 1886; O. Jaekel, in addition to
valuable studies on crinoids and cystids appearing in the
_Zeitschrift_ of the German Geological Society, has published the
first volume of _Die Stammesgeschichte der Pelmatozoen_ (Berlin,
1899), a richly suggestive work; the Mesozoic Echinoderms of France,
Switzerland and Portugal have been made known by P. de Loriol, G.H.
Cotteau, J. Lambert, V. Gauthier and others (see _Paleontologie
francaise_, _Mem. Soc. paleontol. de la Suisse_, _Trabalhos Comm.
Geol. Portugal_, &c.); a beautiful and interesting Devonian fauna from
Bundenbach has been described by O. Follmann, Jaekel, and especially
B. Sturtz (see _Verhandl. nat. Vereins preuss. Rheinlande, Palaont.
Abhandl._, and _Palaeontographica_); while the multitude of North
American palaeozoic crinoids has been attacked by C. Wachsmuth and F.
Springer in the _Proceedings_ (1879, 1881, 1885, 1886), of the
Philadelphia Academy and the _Memoirs_ (1897) of the Harvard Museum.

The vast mass of material made known by these and many other distinguished writers has to be included in our classification, and that classification itself must be controlled by the story it reveals. Thus it is that a change, characteristic of modern systematic zoology, is affecting the subdivisions of the classes. It is not long since the main lines of division corresponded roughly to gaps in geological history: the orders were Palaeocrinoidea and Neocrinoidea, Palechinoidea and Euechinoidea, Palaeasteroidea and Euasteroidea, and so forth. Or divisions were based upon certain modifications of structure which, as we now see, affected assemblages of diverse affinity: thus both Blastoidea and Euechinoidea were divided into Regularia and Irregularia; the Holothuroidea into Pneumophora and Apneumona; and Crinoids were discussed under the heads "stalked" and "unstalked." The barriers between these groups may be regarded as horizontal planes cutting across the branches of the ascending tree of life at levels determined chiefly by our ignorance; as knowledge increases, and as the conception of a genealogical classification gains acceptance, they are being replaced by vertical partitions which separate branch from branch. The changes may be appreciated by comparing the systematic synopses at the end of this article with the classification adopted in 1877 in the 9th edition of the _Ency. Brit._ (vol. vii.), or in any zoological text-book contemporary therewith. In the present stage of our knowledge these minor divisions are the really important ones. For, whereas to one brilliant suggestion of far-reaching homology another can always be opposed, by the detailed comparison of individual growth-stages in carefully selected series of fossils, and by the minute application to these of the principle that individual history repeats race history, it actually is possible to unfold lines of descent that do not admit of doubt. The gradual linking up of these will manifest the true genealogy of each class, and reconstruct its ancestral forms by proof instead of conjecture. The problem of the interrelations of the classes will thus be reduced to its simplest terms, and even questions as to the nature of the primitive Echinoderm and its affinity to the ancestors of other phyla may become more than exercises for the ingenuity of youth. Work has been and is being done by the laborious methods here alluded to, and though the diversity of opinion as to the broader groupings of classification is still restricted only by the number of writers, we can point to an ever-increasing body of assured knowledge on which all are agreed. Unfortunately such allusion to these disconnected certainties as alone might be introduced here would be too brief for comprehension, and we are forced to select a few of the broader hypotheses for a treatment that may seem dogmatic and prejudiced.

_Calycinal Theory._--The theory which had most influence on the
conceptions of Echinoderms in the two concluding decades of the 19th
century was that of Loven, elaborated by P.H. Carpenter, Sladen and
others. This, which may be called the _calycinal_ theory, will be
appreciated by comparing the structure of a simple crinoid with that
of some other types. A crinoid reduced to its simplest elements
consists of three principal portions--(i.) a theca or test enclosing
the viscera; (ii.) five arms stretching upwards or outwards from the
theca, sometimes single, sometimes branching; (iii.) a stem stretching
downwards from the theca and attaching it to the sea-floor (see fig.
1). That part of the theca below the origins of the free arms is
called the "dorsal cup"; the ventral part above the origins of the
arms, serving as cover to the cup, is known as the "tegmen." All these
parts are supported by plates or ossicles of crystalline carbonate of
lime. The cup, in its simplest form, consists of two circlets of five
plates. Each plate of the upper circlet supports an arm, and is called
a "radial"; the plates of the lower circlet, the "basals," rest on the
stem and alternate with those of the upper circlet, i.e. are
interradial in position. Some crinoids have yet another circlet below
these, the constituent plates of which are called "infrabasals," and
are situated radially. The tegmen in most primitive forms, as well as
in the embryonic stages of the living _Antedon_ (fig. 2), consists of
five large triangular plates, alternating with the radials, and called
"orals," because they roof over the mouth. In addition to these three
or four circlets of plates, two other elements were once supposed
essential to the ideal crinoid: the dorso-central and the oro-central.
The former term was applied to a flattened plate observed in the
embryonic stage of a single genus (_Antedon_) at that end of the stem
attached to the sea-floor, and comparable to the foot of a wine-glass
(fig. 2). In some crinoids which have no trace of a stem (e.g.
_Marsupites_) a pentagonal plate is found at the bottom of the cup,
where the stem would naturally have arisen ("centrale" in fig. 1); and
since it was believed that the stem always grew by addition of
ossicles immediately below the infrabasals, it was inferred that this
pentagonal plate was the centro-dorsal in its primitive position, as
though the wine-glass had been evolved from a tumbler by pulling the
bottom out to form the foot. The oro-central was, it must be admitted,
a theoretical conception due to a desire for symmetry, and was not
confirmed by anything better than some erroneous observations on
certain fossils, which were supposed to show a plate at the oral pole
between the five orals; but this plate, so far as it exists at all, is
now known to be nothing but an oral shifted in position. The theory
was that all the plates just described, and more particularly those of
the cup, which were termed "the calycinal system," could be traced,
not merely in all crinoids, but in all Echinoderms, whether fixed
forms such as cystids and blastoids, or free forms such as ophiuroids
and echinoids, even--with the eye of faith--in holothurians. It was
admitted that these elements might atrophy, or be displaced, or be
otherwise obscured; but their complete and symmetrical disposition was
regarded as typical and original. Thus the genera exhibiting it were
regarded as primitive, and those orders and classes in which it was
least obscured were supposed to approach most nearly the ancestral
Echinoderm. Every one knows that an "apical system," composed of two
circlets known as "genitals" or basals and "oculars" or radials,
occurs round the aboral pole of echinoids (fig. 3, A), and that a few
genera (e.g. _Salenia_, fig. 3, B) possess a sub-central plate (the
"suranal"), which might be identified with the centro-dorsal. It is
also the case that many asterids (fig. 3, D) and ophiurids (fig. 3, C)
have a similar arrangement of plates on the dorsal (i.e. aboral)
surface of the disk. Accepting the homology of these apical systems
with the calycinal system, the theory would regard the aboral pole of
a sea-urchin or starfish as corresponding in everything, except its
relations to the sea-floor, with the aboral pole of a fixed
echinoderm.

The theory has been vigorously opposed, notably by Semon (_op. cit._),
who saw in the holothurians a nearer approach to the ancestral form
than was furnished by any calyculate echinoderm, and by the Sarasins,
who derived the echinoids from the holothurians through forms with
flexible tests (_Echinothuridae_, which, however, are now known to be
specialized in this respect). The support that appeared to be given to
the theory by the presence of supposed calycinal plates in the embryo
of echinoids and asteroids has been, in the opinion of many,
undermined by E.W. MacBride (_op. cit._), who has insisted that in the
fixed stage of the developing starfish, _Asterina_, the relations of
these plates to the stem are quite different from those which they
bear in the developing and adult crinoid. But, however correct the
observations and the homologies of MacBride may be, they do not, as
Bury (_op. cit._) has well pointed out, afford sufficient grounds for
his inference that the abactinal (i.e. aboral) poles of starfish and
crinoids are not comparable with one another, and that all conclusions
based on the supposed homology of the dorso-central of echinoids and
asteroids with that of crinoids are incorrect. Bury himself, however,
has inflicted a severe blow on the theory by his proof that the
so-called oculars of Echinoidea, which were supposed to represent the
radials, are homologous with the "terminals" (i.e. the plates at the
tips of the rays) in Asteroidea and Ophiuroidea, and therefore not
homologous with the radially disposed plates often seen around the
aboral pole of those animals. For, if these radial constituents of the
supposed apical system in an ophiurid have really some other origin,
why can we not say the same of the supposed basals? Indeed, Bury is
constrained to admit that the view of Semon and others may be correct,
and that these so-called calycinal systems may not be heirlooms from a
calyculate ancestor, but may have been independently developed in the
various classes owing to the action of similar causes. That this view
must be correct is urged by students of fossils. Palaeontology lends
no support to the idea that the dorso-central is a primitive element;
it exists in none of the early echinoids, and the suranal of
_Saleniidae_ arises from the minor plates around the anus. There is no
reason to suppose that the central apical plate of certain
free-swimming crinoids has any more to do with the distal foot-plate
of the larval _Antedon_ stem than has the so-called centro-dorsal of
_Antedon_ itself, which is nothing but the compressed proximal end of
the stem. As for the supposed basals of Echinoidea, Asteroidea and
Ophiuroidea, they are scarcely to be distinguished among the ten or
more small plates that surround the anus of _Bothriocidaris_, which is
the oldest and probably the most ancestral of fossil sea-urchins (fig.
5). A calycinal system may be quite apparent in the later Ophiuroidea
and in a few Asteroidea, but there is no trace of it in the older
Palaeozoic types, unless we are to transfer the appellation to the
terminals. Those plates are perhaps constant throughout sea-urchins
and starfish (though it would puzzle any one to detect them in certain
Silurian echinoids), and they may be traced in some of the fixed
echinoderms; but there is no proof that they represent the radials of
a simple crinoid, and there are certainly many cystids in which no
such plates existed. Loven and M. Neumayr adduced the Triassic
sea-urchin _Tiarechinus_, in which the apical system forms half of the
test, as an argument for the origin of Echinoidea from an ancestor in
which the apical system was of great importance; but a genus appearing
so late in time, in an isolated sea, under conditions that dwarfed the
other echinoid dwellers therein, cannot seriously be thought to
elucidate the origin of pre-Silurian Echinoidea, and the recent
discovery of an intermediate form suggests that we have here nothing
but degenerate descendants of a well-known Palaeozoic family
(_Lepidocentridae_). But to pursue the tale of isolated instances
would be wearisome. The calycinal theory is not merely an assertion of
certain homologies, a few of which might be disputed without affecting
the rest: it governs our whole conception of the echinoderms, because
it implies their descent from a calyculate ancestor--not a
"crinoid-phantom," that bogey of the Sarasins, but a form with
definite plates subject to a quinqueradiate arrangement, with which
its internal organs must likewise have been correlated. To this
ingenious and plausible theory the revelations of the rocks are more
and more believed to be opposed.

T, The five interradial tentacles.
M, The water-pore, leading by the stone-canal stc to the water-ring,
from which hangs a Polian vesicle pb.
oc, Supposed otocysts.
m, Longitudinal muscles.
sk, Calcareous spicules.
st, Stomach.
(After Semon.)]

_Pentactaea Theory._--In opposition to the calycinal theory has been
the _Pentactaea_ theory of R. Semon. There have always been many
zoologists prepared to ascribe an ancestral character to the
holothurians. The absence of an apical system of plates; the fact that
radial symmetry has not affected the generative organs, as it has in
all other recent classes; the well-developed muscles of the body-wall,
supposed to be directly inherited from some worm-like ancestor; the
presence on the inner walls of the body in the family _Synaptidae_ of
ciliated funnels, which have been rashly compared to the excretory
organs (nephridia) of many worms; the outgrowth from the rectum in
other genera of caeca (Cuvierian organs and respiratory trees), which
recall the anal glands of the Gephyrean worms; the absence of podia
(tube-feet) in many genera, and even of the radial water-vessels in
_Synaptidae_; the absence of that peculiar structure known in other
echinoderms by the names "axial organ," "ovoid gland," &c.; the
simpler form of the larva--all these features have, for good reason or
bad, been regarded as primitive. Some of the more striking of these
features are confined to _Synaptidae_; in that family too the absence
of the radial water-vessels from the adult is correlated with
continuity of the circular muscle-layer, while the gut runs almost
straight from the anterior mouth to the posterior anus. Early in the
life-history of _Synapta_ occurs a stage with five tentacles around
the mouth, and into these pass canals from the water-ring, the radial
canals to the body-wall making a subsequent, and only temporary,
appearance (fig. 4). Semon called this stage the _Pentactula_, and
supposed that, in its early history, the class had passed through a
similar stage, which he called the _Pentactaea_, and regarded as the
ancestor of all Echinoderms. It has since been proved that the five
tentacles with their canals are interradial, so that one can scarcely
look on the _Pentactula_ as a primitive stage, while the apparent
simplicity of the _Synaptidae_, at least as compared with other
holothurians, is now believed to be the result of regressive changes.
The _Pentactaea_, at all events as it sprang from the brain of Semon,
must pass to the limbo of mythological ancestors.

_Pelmatozoic Theory._--The rejection of the calycinal and _Pentactaea_
theories need not scatter our conceptions of Echinoderm structure back
into the chaos from which they seemed to have emerged. The idea of a
calyculate ancestor, though by no means connoting fixation, turned
men's minds in the direction of the fixed forms, simply because in
them the calyx was best developed. The _Pentactaea_ again suggested a
search for some primitive type in which quinqueradiate symmetry was
exhibited in circumoral appendages, but had not affected the nervous,
water-vascular, muscular or skeletal systems to any great extent, and
the generative organs not at all. Study of the earliest larval stages
has always led to the conclusion that the Echinoderms must have
descended from some freely-moving form with a bilateral symmetry, and,
connecting this with the ideas just mentioned, we reach the conception
that this supposed bilateral ancestor (or _Dipleurula_) may have
become fixed, and may have gradually acquired a radial symmetry in
consequence of its sedentary mode of life. The different extent of
quinqueradiate symmetry in the different classes would thus depend on
the period at which they diverged from the sedentary stock. The
tracing of this history, and the explanation of the general characters
of Echinoderms and of the differentiating features of the classes in
accordance therewith, constitutes the _Pelmatozoic_ theory.

The word "Pelmatozoa" literally means "stalked animals," but the name
is now used to denote all Cystidea, Blastoidea, Crinoidea and
Edrioasteroidea, as opposed to the other classes, which may be called
Eleutherozoa. Many Pelmatozoa have, it is true, no stalk, while some
are freely-moving, but all agree in the possession of certain
characters obviously connected with a fixed mode of life. Thus, the
mouth is central and turned away from the sea-floor; the animal does
not seize its food by tentacles, limbs or jaws, neither does it move
in search of it, but a series of ciliated grooves which radiate from
the mouth sweep along currents of water, in the eddies of which minute
food-particles are caught up and carried down into the gullet; the
undigested food is driven out through an anus which is on the upper or
oral side of the theca, but as far distant as practicable from the
mouth and ciliated grooves. Such characters are found in any
primitive, sedentary group. More peculiarly Echinoderm features, in
which the Pelmatozoan nature is manifest, are the enclosing of the
viscera in a calcified and plated theca, for protection against those
enemies from which a fixed animal cannot flee; the development, at the
aboral pole of this theca, of a motor nerve-centre giving off branches
to the stroma connecting the various plates of the theca and of its
brachial, anal, and columnar extensions, and thus co-ordinating the
movements of the whole skeleton; the absence of suckers from the
podia, which, when present, are respiratory, not locomotor, in
function. There are other features of most, if not all, Pelmatozoa
that appear to be due to a fixed existence; but those are also found
in the Eleutherozoa. The Pelmatozoic theory thus regards the
Pelmatozoa as the more ancestral forms, and the Pelmatozoan stage as
one that must have been passed through by all Echinoderms during their
evolution from the _Dipleurula_. It might be possible to prove the
origin of all classes from Pelmatozoa, without thereby explaining the
origin of such fundamental features as radial symmetry, the
developmental metamorphosis, and the torsion that affects both gut and
body-cavities during that process; but the acceptance of a
_Dipleurula_ as the common ancestor necessitates an explanation of
these features. Such explanation is an integral part of the
Pelmatozoic theory, but is provided by no other.

The evidence for the Pelmatozoic theory is supplied by palaeontology,
embryology, the comparative anatomy of the classes, and a
consideration of other phyla. Palaeontology, so far as it goes, is a
sure guide, but some of the oldest fossiliferous rocks yield remains
of distinctly differentiated crinoids, asteroids and echinoids, so
that the problem is not solved merely by collecting fossils. Two lines
of argument appear fruitful. First, a comparison of the relative
numbers of the representatives of the various classes at different
epochs; according to this they may be placed in the following order,
with the oldest first: Cystidea, Crinoidea, Blastoidea, Asteroidea,
Ophiuroidea, Echinoidea. As for Holothuroidea, the fossil evidence
allows us to say no more than that the class existed in early
Carboniferous times, if not before. The second method is to work out
by slow and sure steps the lines of descent of the different families,
orders, and classes, and so either to arrive at the ancestral form of
each class, or to plot out the curve of evolution, which may then
legitimately be projected into "the dark backward and abysm of time."
In this way the many highly modified orders of Cystidea may be traced
back to a simple, many-plated ancestor with little or no radiate
symmetry (see below). All the complicated structures of Blastoidea are
evolved from a fairly simple type, which in its turn is linked on to
one of the cystid orders. That the crinoids are all deducible from
some such simple form as that above described under the head
"calycinal theory," is now generally admitted. Although, in the
extreme correlation of the radial food-grooves, nerves, water-vessels,
and so forth, with a radiate symmetry of the theca, such a type
differs from the Cystidea, while in the possession of jointed
processes from the radial plates, bearing the grooves and the various
body-systems outwards from the theca, it differs from all other
Echinoderms, nevertheless ancient forms are known which, if they are
not themselves the actual links, suggest how the crinoid type may have
been evolved from some of the more regular cystids. The fourth class
of Pelmatozoa--the Edrioasteroidea--differs from the others in the
structure of its ambulacra. As in all Pelmatozoa these seem to have
borne ciliated food-grooves protected by movable covering-plates (fig.
11). Beneath each food-groove was a radial water-vessel and probably a
nerve and blood-vessel, all which structures passed either between
certain regularly arranged thecal plates, or along a furrow floored by
those plates, which were then in two alternating series. The important
and distinctive feature is the presence of pores between the
flooring-plates, on either side of the groove; and these, we cannot
doubt, served for the passage of podia. Thus in a highly developed
edrioasteroid, such as _Edrioaster_ itself (fig. 11), there was a true
ambulacrum, apparently constructed like that of a starfish, but
differing in the possession of a ciliated food-groove protected by
covering-plates. The simpler forms of Edrioasteroidea, with their more
sac-like body and undifferentiated plates, may well have been derived
from early Cystidea of yet simpler structure, and there seems no
reason to follow Jaekel in regarding the class as itself the more
primitive. Turning to fossil Asteroidea, we find the earlier ophiurids
scarcely distinguishable from the asterids, while in the alternation
of the ambulacrals, which undoubtedly correspond to the
flooring-plates of _Edrioaster_, both groups approach the Pelmatozoan
type. These facts have been expressed by Sturtz in his names
Encrinasteriae and Ophio-encrinasteriae. There is no difficulty in
deducing the highly differentiated asterids and ophiurids of a later
day from these simpler types. The evolution of the modern Echinoidea
from their Palaeozoic ancestors is also well understood, but in this
case the ancestral form to which the palaeontologist is led does not
at first sight present many resemblances to the Pelmatozoa. It is,
however, characterized by simplicity of structure, and a short
description of it will serve to clear the problem from unnecessary
difficulties. _Bothriocidaris_ (fig. 5), a small echinoid from the
Ordovician rocks of Esthonia, is in essential structure just the form
demanded by comparative palaeontology to make a starting-point. It is
spheroidal, with the mouth and anus at opposite poles; there are five
ambulacra, and the ambulacral plates are large, simple and
alternating, each being pierced by two podial pores which lie in a
small oval depression; the ambulacrals next the mouth form a closed
ring of ten plates; the interambulacrals lie in single columns between
the ambulacra, and are separated from the mouth-area by the proximal
ambulacrals just mentioned, and sometimes by the second set of
ambulacrals also; the ambulacra end in the five oculars or terminals,
which meet in a ring around the anal area and have no podial pores,
but one of them serves as a madreporite; within this ring is a
star-shaped area filled with minute irregular plates, none of which
can safely be selected as the homologues of the so-called basals or
genitals of later forms; within the ring of ambulacrals around the
mouth are five somewhat pointed plates, which Jaekel regards as teeth,
but which can scarcely be homologous with the interradially placed
teeth of later echinoids, since they are radial in position; small
spines are present, especially around the podial pores. The position
of the pores near the centre of the ambulacrals in _Bothriocidaris_
need not be regarded as primitive, since other early Palaeozoic
genera, not to mention the young of living forms, show that the podia
originally passed out between the plates, and were only gradually
surrounded by their substance; thus the original structure of the
echinoid ambulacra differed from that of the early asteroid in the
position of the radial vessels and nerves, which here lie beneath the
plates instead of outside them. To this point we shall recur;
palaeontology, though it suggests a clue, does not furnish an actual
link either between Echinoidea and Asteroidea, or between those
classes and Pelmatozoa.

The argument from embryology leads further back. First, as already
mentioned, it outlines the general features of the _Dipleurula_;
secondly, it indicates the way in which this free-moving form became
fixed, and how its internal organs were modified in consequence; but
when we seek, thirdly, for light on the relations of the classes, we
find the features of the adult coming in so rapidly that such
intermediate stages as may have existed are either squeezed out or
profoundly modified. The difficulty of rearing the larvae in an
aquarium towards the close of the metamorphosis may account for the
slight information available concerning the stages that immediately
follow the embryonic. Another difficulty is due to the fact that the
types studied, and especially the crinoid _Antedon_, are highly
specialized, so that some of the embryonic features are not really
primitive as regards the class, but only as regards each particular
genus. Thus inferences from embryonic development need to be checked
by palaeontology, and supplemented by comparison of the anatomy of
other living genera.

Minute anatomical research has also aided to establish the Pelmatozoic
theory by the gradual recognition in other classes of features
formerly supposed to be confined to Pelmatozoa. Thus the elements of
the Pelmatozoan ventral groove are now detected in so different a
structure as the echinoid ambulacrum, while an aboral nervous system,
the diminished representative of that in crinoids, has been traced in
all Eleutherozoa except Holothurians. The broader theories of modern
zoology might seem to have little bearing on the Echinoderma, for it
is not long since the study of these animals was compared to a
landlocked sea undisturbed by such storms as rage around the origin of
the Vertebrata. This, however, is no more the case. The conception of
the _Dipleurula_ derives its chief weight from the fact that it is
comparable to the early larval forms of other primitive coelomate
animals, such as _Balanoglossus_, _Phoronis_, Chaetognatha,
Brachiopoda and Bryozoa. So too the explanation of radial symmetry and
torsion of organs as due to a Pelmatozoic mode of life finds
confirmation in many other phyla. Instead of discussing all these
questions separately, with the details necessary for an adequate
presentation of the argument, we shall now sketch the history of the
Echinoderms in accordance with the Pelmatozoic theory. Such a sketch
must pass lightly over debatable ground, and must consist largely of
suggestions still in need of confirmation; but if it serves as a frame
into which more precise and more detailed statements may be fitted as
they come to the ken of the reader, its object will be attained.

_Evolution of the Echinoderms._--It is reasonable to suppose that the Coelomata--animals in which the body-cavity is divided into a gut passing from mouth to anus and a hollow (coelom) surrounding it--were derived from the simpler Coelentera, in which the primitive body-cavity (archenteron) is not so divided, and has only one aperture serving as both mouth and anus. We may, with Sedgwick, suppose the coelom to have originated by the enlargement and separation of pouches that pressed outwards from the archenteron into the thickened body-wall (such structures as the genital pouches of some Coelentera, not yet shut off from the rest of the cavity), and they would probably have been four in number and radially disposed about the central cavity. The evolution of this cavity into a gut is foreshadowed in some Coelentera by the elliptical shape of the aperture, and by the development at its ends of a ciliated channel along which food is swept; we have only to suppose the approximation of the sides of the ellipse and their eventual fusion, to complete the transformation of the radially symmetrical Coelenterate into a bilaterally symmetrical Coelomate with mouth and anus at opposite ends of the long axis. We further suppose that of the four coelomic pouches one was in front of the mouth, one behind the anus, and one on each side. Such an animal, if it ever existed, probably lived near the surface of the sea, and even here it may have changed its medusoid mode of locomotion for one in the direction of its mouth. Thus the bilateral symmetry would have been accentuated, and the organism shaped more definitely into three segments, namely (1) a preoral segment or lobe, containing the anterior coelomic cavity; (2) a middle segment, containing the gut, and the two middle coelomic cavities; (3) a posterior segment, containing the posterior coelomic cavity, which, however, owing to the backward prolongation of the anus, became divided into two--a right and left posterior coelom. Each of these cavities presumably excreted waste products to the exterior by a pore. There was probably a nervous area, with a tuft of cilia, at the anterior end; while, at all events in forms that remained pelagic, the ciliated nervous tracts of the rest of the body may be supposed to have become arranged in bands around the body-segments. Such a form as this is roughly represented to-day by the _Actinotrocha_ larva of _Phoronis_, the importance of which has been brought out by Masterman. But only slight modifications are required to produce the _Tornaria_ larva of the Enteropneusta and other larvae, including the special type that is inferred from the _Dipleurula_ larval stages of recent forms to have characterized the ancestor of the Echinoderms. We cannot enter here into all the details of comparison between these larval forms; amid much that is hypothetical a few homologies are widely accepted, and the preceding account will show the kind of relation that the Echinoderms bear to other animals, including what are now usually regarded as the ancestors of the Chordata (to which back-boned animals belong), as well as the nature of the evidence that their study has been, or may be, made to yield. How the hypothetical _Dipleurula_ became an Echinoderm, and how the primitive Echinoderms diverged in structure so as to form the various classes, are questions to which an answer is attempted in the following paragraphs:--

Confining our attention to that form of Dipleurula (fig. 6) which, it
is supposed, gave rise to the Echinoderma, we infer from embryological
data that its special features were as follow:--The anterior coelomic
cavity was wholly or partially divided, and from each half a duct led
to the exterior, opening at a pore near the middle line of the back.
The middle cavities were smaller, and the ducts from them came to
unite with those from the anterior cavities, and no longer opened
directly to the exterior; whether these cavities were already
specialized as water-sacs cannot be asserted, but they certainly had
become so at a slightly later stage. The posterior cavities were the
largest, but what had become of their original opening to the exterior
is uncertain. The genital products were derived from the lining of the
coelomic cavities, but it would not be safe to say that any particular
region was as yet specialized for generation. The epithelium of the
outer surface was probably ciliated, and a portion of it in the
preoral lobe differentiated as a sense-organ, with longer cilia and
underlying nerve-centre, from which two nerves ran back below the
ventral surface. Into the space between the walls of the coelom and
the outer body-wall, originally filled with jelly, definite cells now
wandered, chiefly derived from the coelomic walls. Some of these cells
produced muscles and connective tissue; others absorbed and removed
waste products, iron salts, calcium carbonate and the like, and so
were ready to be utilized for the deposition of pigment or of skeletal
substance. In some of these respects the _Dipleurula_ may have
diverged from the ancestor of Enteropneusta and of other animals, but
it could not as yet have been recognized as echinodermal by a
zoologist, for it presented none of the structural peculiarities of
the modern adult echinoderm.

Now ensued the great event that originated the phylum--the discovery
of the sea-floor. This being apprehended by the sensory anterior end,
it was by that end that the _Dipleurula_ attached itself; not,
however, by the pole, since that would have interfered at once with
the sensory organ, but a little to one side, the right side being the
one chosen for a reason we cannot now fathom; it may be that fixation
was facilitated by the presence of the pore on that side, and by the
utilization of the excretion from it as a cement. The first result was
that which is always seen to follow in such cases--the passage of the
mouth towards the upper surface (fig. 7). As it passed up along the
left side, the gut caught hold of the left water-sac and pulled it
upwards, curving it in the process; this being attached to the left
duct from the anterior body-cavity, this structure with its water-pore
was also pulled up, and the pore came to lie between mouth and anus.
The forward portion of the anterior coelom shared in the constriction
and elongation of the preoral lobe; but its hinder portion was dragged
up along with the water-pore and formed a canal lying along the outer
wall (the parietal canal). As the gut coiled, it pressed inwards the
middle of the left posterior coelom of the _Dipleurula_, and drew the
whole towards the mouth, while the corresponding cavity on the right
was pressed down by the stomach towards the fixed end of the animal
and became involved in the elongation of that region. These changes,
which may still be traced in the development of _Antedon_, resulted in
the primitive Pelmatozoon (fig. 7), represented in the rocks by such a
genus as _Aritocystis_ (fig. 8). The pear-shaped body is encased in a
theca formed by a number of polygonal plates, and is attached by its
narrow end. On the broad upper surface are four openings, that nearest
the centre being the mouth, which is slit-like, and that nearest the
periphery being the anus. The two other openings are minute, and
placed between those two; one close to the mouth is almost certainly
the water-pore, while that nearer the anus is regarded as a genital
aperture. Which of the coelomic cavities this last is connected with
is uncertain, for there is considerable doubt as to the origin of the
genital glands in the embryonic development of recent echinoderms. It
seems clear, however, that there was but a single duct and a single
bunch of reproductive cells, as in the holothurians, though perhaps
bifurcate, as in some of those animals. The line between mouth and
anus, along which these openings are situate, corresponds with the
plane of union between the two horns of the curved left posterior
coelom, the united walls of which form the "dorsal mesentery." Since
this must have, on our theory, enclosed the parietal canal from the
anterior coelom, it is possible that the genital products were
developed from the lining cells of that cavity, and that the genital
pore was nothing but its original pore not yet united with that from
the water-sac. The concrescence of these pores can be traced in other
cystids; but as the genital organs became affected by radial symmetry
the original function of the duct was lost, and the reproductive
elements escaped to the exterior in another way. _Aristocystis_ may
have had ciliated food-grooves leading to its mouth, but these have
left no traces on the structure of the test. Traces, however, are
perceptible in genera believed to be descended from such a simple
type, and the majority may be grouped under two heads. One group
includes those in which the grooves wander outwards from the mouth
over the thecal plates, which gradually become arranged regularly on
either side of the grooves, while further extensions ascend from the
grooves on small jointed processes called "brachioles" (fig. 9). In
the other group the grooves do not tend so much to stretch over the
theca as to be raised away from it on relatively larger brachioles,
arising close around the mouth (fig. 10).

These two types are, in the main, correlated with two gradual
differentiations in the minute structure of the thecal plates.
Originally the calcareous substance of the plates (stereom) was
pierced by irregular canals, more or less vertical, and containing
strands of the soft tissue (stroma) that deposited the stereom, as
well as spaces filled with fluid. In the former group (fig. 9) these
canals became connected in pairs (diplopores) still perpendicular to
the surface, and this structure, combined with that of the grooves,
characterizes the order--Diploporita. In the latter group (fig. 10)
the canals, that is to say, the stroma-strands, came to lie parallel
to the surface and to cross the sutures between the plates, which were
thus more flexibly and more strongly united: since the canals
crossing each suture naturally occupy a rhombic area, the order is
called Rhombifera. At first the grooves were three, one proceeding
from each end of the mouth-slit, and the third in a direction opposed
to the anus; with reference to the Pelmatozoan structure, the anal
side may be termed posterior, and this groove anterior. Eventually
each lateral groove forked, so that there were five grooves. These
gradually impressed themselves on the theca and influenced the
arrangement of the internal organs: it is fairly safe to assume that
nerves, blood-vessels and branches from the water-sac stretched out
along with these grooves, each system starting from a ring around the
gullet. At last a quinqueradiate symmetry influenced the plates of the
theca, partly through the development of a plate at the end of each
groove (terminal), partly through plates at the aboral pole of the
theca (basals and infrabasals) arising in response to mechanical
pressure, but soon intimately connected with the cords of an aboral
nervous system. Before the latter plates arose, the stem had developed
by the elongation and constriction of the fixed end of the theca, the
gradual regularization of the plates involved, and their coalescence
into rings. The crinoid type was differentiated by the extension of
the food-grooves and associated organs along radial outgrowths from
the theca itself. These constituted the arms (brachia), and five
definite radial plates of the theca were specialized for their
support. These radials may be homologous with the terminals already
mentioned, but this is neither necessary nor certain. In this
development of brachial extensions of the theca the genital organs
were involved, and their ripe products formed at the ends of the
brachia or in the branches therefrom. The remains of the original
genital gland within the theca became the "axial organ" surrounded by
the "axial sinus" derived from the anterior coelom, and this again by
structures derived from the right posterior coelom, which, as
explained above, had been depressed to the aboral pole. These last
structures formed a nervous sheath around the axial sinus with its
blood-vessels, and became divided into five lobes correlated with the
five basals (the "chambered organ") and forming the aboral
nerve-centre. Before these changes were complete the Holothurioidea
must have diverged, by the assumption of a crawling existence. Thus in
them the mouth and anus reverted to opposite poles, and only the
torsion of the gut and coelom, and the radial extensions of the
nervous, water-vascular and blood-vascular systems, testified to their
Pelmatozoan ancestry. The ciliated grooves, no longer needed for the
collection of food, closed over, and are still traceable as ciliated
canals overlying the radial nerves. At the same time the thecal plates
degenerated into spicules. The Edrioasteroidea followed a different
line from that of the cystids above mentioned and their descendants.
The theca became sessile, and in its later developments much flattened
(fig. 11). Mouth, water-pore and anus remained as in _Aristocystis_,
but the five ciliated grooves radiated from the mouth between the
thecal plates rather than over them, and were, as usual, protected by
covering-plates. The important feature was the extension of radial
canals from the water-sac along these grooves, with branches passing
between the flooring-plates of the grooves (fig. 12, A). The
resemblance of the flooring-plates to the ambulacral ossicles of a
starfish is so exact that one can explain it only by supposing similar
relations of the water-canals and their branches (podia). On the
thinly plated under surface of well-preserved specimens of
_Edrioaster_ are seen five interradial swellings (fig. 11, B). These
are likely to have been produced by the ripe genital glands, which may
have extruded their products directly through the membranous
integument of the under side. No other way out for them is apparent,
and it is clear that _Edrioaster_ was not permanently and solidly
fixed to the sea-floor.

Now comes a great change, unfortunately difficult to follow whether in
the fossils or in the modern embryos. We suppose some such form as
_Edrioaster_, which appears to have lived near the shore, to have been
repeatedly overturned by waves. Those that were able to accommodate
themselves to this topsy-turvy existence, by taking food in directly
through the mouth, survived, and their podia gradually specialized as
sucking feet. Such a form as this, when once its covering-plates had
atrophied, would be a starfish without more ado (fig. 12, B); but the
sea-urchins present a more difficult problem, on which
_Bothriocidaris_ sheds no light. An Upper Silurian echinoid, however,
_Palaeodiscus_, is believed by W.J. Sollas and W.K. Spencer to have
had in its ambulacra an inner as well as an outer series of plates. If
this be correct, the only change from _Edrioaster_, as regards the
ambulacra, was that in _Palaeodiscus_ the covering-plates could no
longer open, but closed permanently over the whole groove, while the
podia issued through slits between them. In more typical echinoids the
covering-plates alone remained to form the ordinary ambulacral plates,
while the flooring-plates disappeared, the canals and other organs
remaining as before. In any case we have to admit a closure of the
integument over the ciliated groove (fig. 12, D, e) just as in
holothurians, since this is necessitated by anatomical evidence. The
genital organs in both Asteroidea and Echinoidea would retain the
interradial position they first assumed in _Edrioaster_; and in
Echinoidea their primitive temporary openings to the exterior were
converted into definite pores, correlated with five interradially
placed plates at the aboral pole. The anus also naturally moved to
this superior and aboral position. In the Echinoidea the water-canals
and associated structures, ending in the terminal plates, stretched
right up to these genital plates; but in the Asteroidea they never
reached the aboral surface, so that the terminals have always been
separated from the aboral pole by a number of plates.

_Analysis of Echinoderm Characters._--Regarding the Echinoderms as a whole in the light of the foregoing account, we may give the following analytic summary of the characters that distinguish them from other coelomate animals:--

They live in salt or brackish water; a primitive bilateral symmetry is
still manifest in the right and left divisions of the coelom; the
middle coelomic cavities are primitively transformed into two
hydrocoels communicating with the exterior indirectly through a duct
or ducts of the anterior coelom; stereom, composed of crystalline
carbonate of lime, is, with few exceptions, deposited by special
amoebocytes in the meshes of a mesodermal stroma, chiefly in the
integument; reproductive cells are derived from the endothelium,
apparently of the anterior coelom; total segmentation of the ovum
produces a coeloblastula and gastrula by invagination; mesenchyme is
formed in the segmentation cavity by migration of cells, chiefly from
the hypoblast. Known Echinoderms show the following features, imagined
to be due to an ancestral pelmatozoic stage:--Increase in the coelomic
cavities of the left side, and atrophy of those on the right; the
dextral coil of the gut, recognizable in all classes, though often
obscured; an incomplete secondary bilateralism about the plane
including the main axis and the water-pore or its successor, the
madreporite, often obscured by one or other of various tertiary
bilateralisms; the change of the hydrocoel into a circumoral, arcuate
or ring canal; development through a free-swimming, bilaterally
symmetrical, ciliated larva, of which in many cases only a portion is
transformed into the adult Echinoderm (where care of the brood has
secondarily arisen, this larva is not developed). All living, and most
extinct, Echinoderms show the following features, almost certainly due
to an ancestral pelmatozoic stage:--An incomplete radial symmetry, of
which five is usually the dominant number, is superimposed on the
secondary bilateralism, owing to the outgrowth from the mouth region
of one unpaired and two paired ciliated grooves; these have a floor of
nervous epithelium, and are accompanied by subjacent radial canals
from the water-ring, giving off lateral podia and thus forming
ambulacra, and by a perihaemal system of canals apparently growing out
from coelomic cavities. All living Echinoderms have a lacunar, haemal
system of diverse origin; this, the ambulacral system, and the
coelomic cavities, contain a fluid holding albumen in solution and
carrying numerous amoebocytes, which are developed in special
lymph-glands and are capable of wandering through all tissues. The
Echinoderms may be divided into seven classes, whose probable
relations are thus indicated:--

/ Cystidea------Edrioasteroidea
| | |\
PELMATOZOA < | | \ Holothurioidea \
| | | |
| Blastoidea | > ELEUTHEROZOA
\ Crinoidea | |
Stelliformia |
Echinoidea /

Brief systematic accounts of these classes follow:--