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ECHINODERMA

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Originally appearing in Volume V08, Page 881 of the 1911 Encyclopedia Britannica.

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ECHINODERMA .1 The ix vaeppa, or " urchin-skinned " animals, have See also:

long been a favourite subject of study with the collectors of See also:

sea-animals or of fossils, since the See also:

lime deposited in their skins forms hard tests or shells readily preserved in the See also:

CABINET

cabinet . These were described during the 18th and first See also:

HALF

half of the 19th centuries by many eminent naturalists, such as J . T . See also:

KLEIN, JULIUS LEOPOLD (1810–1876)

Klein, J . H . Linck, C . See also:

LINNAEUS

Linnaeus, N . G . Leske, J . S . See also:

Miller, L. v . See also:

Buch, E .

See also:

BARRANDE, JOACHIM (1799-1883)

Barrande's Systeme silurien (1887 and 1899); P . H . Carpenter published important papers on fossil crinoids in the See also:

Journal of the See also:

GEOLOGICAL

Geological Society, on Cystidea in that of the Linnean Society, 1891, and, together with R . See also:

Etheridge, jun., compiled the large See also:

CATALOGUE (a Fr. adaptation of the Gr. KaraXoyos, a register, from Karalyety, to enrol or pick out)

Catalogue of Blastoidea in the See also:

BRITISH

British Museum, 1886; O . Jaekel, in addition to valuable studies on crinoids and cystids appearing in the Zeitschrift of the See also:

German Geological Society, has published the first volume of See also:

Die Stammesgeschichte der Pelmatozoen (See also:

Berlin, 1899), a richly suggestive See also:

WORK

work; the Mesozoic Echinoderms of See also:

France, See also:

SWITZERLAND

Switzerland and See also:

PORTUGAL

Portugal have been made known by P. de Loriol, G . H . Cotteau, J . See also:

Lambert, V . Gauthier and others (see Paleontologie rancaise, Mem . Soc. paleontol. de la Suisse, Trabalhos See also:

COMM

Comm . Geol . Portugal, &c.) ; a beautiful and interesting Devonian See also:

FAUNA

fauna from Bundenbach has been described by O .

Follmann, Jaekel, and especially B . Stiirtz (see Verhandl. nat . Vereins preuss . Rheinlande, Paldont . Abhandl., and Palaeontographica) ; while the multitude of See also:

North See also:

AMERICAN

American palaeozoic crinoids has been attacked by C . See also:

WACHSMUTH, CHARLES (1829-1896)

Wachsmuth and F . See also:

Springer in the Proceedings (1879, 1881, 1885, 1886), of the See also:

PHILADELPHIA

Philadelphia Academy and the See also:

MEMOIRS

Memoirs (1897) of the Harvard Museum . The vast See also:

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 See also:

story it revea' Thus it is that a See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change, characteristic of See also:

MODERN

modern systematic See also:

ZOOLOGY (from Gr. Nov, a living thing, and ?byos, theory)

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 See also:

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 See also:

groups may be regarded as See also:

HORIZONTAL

horizontal planes cutting across the branches of the ascending See also:

tree of See also:

LIFE

life at levels determined chiefly by our See also:

IGNORANCE (Lat. ignorantia, from ignorare, not to know); want of knowledge, a state of mind which in law has important consequences. A well-known legal maxim runs: ignorantia juris non excusat ("ignorance of the law does not excuse")

ignorance; as knowledge increases, and as the conception of a genealogical classification gains See also:

ACCEPTANCE (Lat. acceptare, frequentative form of accipere, to receive)

acceptance, they are being replaced by See also:

VERTICAL (from Lat. vertex, highest point)

vertical partitions which See also:

SEPARATE

separate See also:

BRANCH (from the Fr. branche, late Lat. branca, an animal's paw)

branch from branch . The changes may be appreciated by comparing the systematic synopses at the end of this See also:

ARTICLE (from Lat. articulus, a joint)

article with the classification adopted in 1877 in the 9th edition of the Ency . But .

(vol. vii.), or in any zoological See also:

TEXT (Lat. textum, woven fabric, from texere, to weave)

text-See also:

BOOK

book contemporary therewith . In the See also:

PRESENT

present See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

stage of our knowledge these See also:

minor divisions are the really important ones . For, whereas to one brilliant See also:

SUGGESTION

suggestion of far-reaching homology another can always be opposed, by the detailed comparison of individual growth-stages in carefully selected See also:

SERIES (a Latin word from serere, to join)

series of fossils, and by the See also:

minute application to these of the principle that individual history repeats See also:

RACE

race history, it actually is possible to unfold lines of descent that do not admit of doubt . The See also:

GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

gradual linking up of these will See also:

MANIFEST (Lat. manifestus, clear, open to view)

manifest the true See also:

GENEALOGY (from the Gr. yivos, family, and Vryos, theory)

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 See also:

PRIMITIVE

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 See also:

OPINION (Lat. opinio, from opinari, to think)

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 brieffor 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 See also:

INFLUENCE (Late Lat. influentia, from influere, to flow in)

influence on the conceptions of Echinoderms in the two concluding decades of the 19th century was that of Lovett, 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 See also:

SIMPLE

simple crinoid with that of some other types . A crinoid reduced to its simplest elements consists of three See also:

PRINCIPAL

principal portions—(l.) a theca or test enclosing the viscera; (ii.) five arms stretching upwards or outwards from the theca, sometimes single, sometimes branching; (iii.) a See also:

STEM (O. Eng. staefn, stemn, cf. Du. stain, Ger. Stamm, &c., probably related to " staff ")

stem stretching downwards from the theca and attaching it to the sea-See also:

FLOOR (from O. Eng. flor, a word common to many Teutonic languages, cf. Dutch vloer, and Ger. Flur, a field, in the feminine, and a floor, masculine)

floor (see fig .

I) . That See also:

PART

part of the theca below the origins of the See also:

FREE

free arms is called the " dorsal See also:

CUP (in O.E. cuppe; generally taken to be from Late Lat. cuppa, a variant of Lat. cupa, a cask, cf. Gr. KuaeXXov)

cup "; the ventral part above the origins of the arms, serving as See also:

COVER (from the Fr. convert, from couvrir, to cover, Lat. cooperire)

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 See also:

FORM (Lat. forma)

form, consists of two circlets of five plates . Each See also:

PLATE

plate of the upper circlet supports an See also:

ARM (a common Teutonic word; the Indo-European root is ar, to join or fit; cf. the Lat. armus, shoulder, and the plural word arma, weapons, Gr. apphs, joint, and the reduplicated apapilKSV, to join)

arm, and is called a " radial " the plates of the See also:

LOWER

lower circlet, the " basals," See also:

REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

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, alter- nating with the radials, and called " orals," because they roof over the mouth . In FIG . 2.-An See also:

early addition to these three or four circlets of stage in the develop-plates, two other elements were once sup- ment of Antedon, show-posed essential to the ideal crinoid: the See also:

ing the See also:

FOOT

foot-plate or dorso-central and the oro-central . The dorso-central " fp at former See also:

TERM

term was applied to a flattened plate the end of the stem observed in the embryonic stage of a single See also:

col . Some of the genus (Antedon) at that end of the stem thecal plates, infra-attached to the sea-floor, and comparable to basals I B, basals B, the foot of a See also:

WINE (Lat. vinum, Gr. oivos)

wine-See also:

glass (fig . 2) .

In some and orals 0 are formcrinoids which have no trace of a stern ing around the body-(e.g . Marsupites) a pentagonal plate is cavities r.pc and l.pc; found at the bbttom of the cup, where the p is the water-See also:

PORE

pore. stem would naturally have arisen (" cen- (After Seeliger.) trale " in fig . I) ; and since it was believed that the stem always See also:

GREW, NEHEMIAH (1641-1712)

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 See also:

TUMBLER

tumbler by pulling the bottom out to form the foot . The orocentral was, it must be admitted, a theoretical conception due to a See also:

DESIRE

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 See also:

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 *ere 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 See also:

eye of faith—in holothurians . It was admitted that these elements might See also:

ATROPHY (Gr. It- priv., rpo4, nourishment)

atrophy, or be displaced, or be otherwise obscured ; but their See also:

COMPLETE

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 See also:

ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round the aboral pole of echinoids (fig . 3, A), and C .D 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 See also:

case that many asterids (fig .

3, D) and ophiurids (fig . 3, C) have a similar arrangement of plates on the dorsal (i.e. aboral) See also:

SURFACE

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 See also:

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 See also:

BLOW, JOHN (1648-1708)

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 sup-posed 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 See also:

HEIR (Lat. heres, from a root meaning to grasp, seen in heres or erus, master of a house, Gr. xeip, hand, Sans. hat ana, hand)

heir-looms from a calyculate ancestor, but may have been independently See also:

DEVELOPED

developed in the various classes owing to the See also:

ACTION

action of similar causes . That this view must be correct is urged by students of fossils .

Palaeontology lends no support to the See also:

IDEA (Gr. Ibia, connected with i&eiv, to see; cf. Lat. species from specere, to look at)

idea that the dorsocentral is a primitive See also:

ELEMENT (Lat. elementum)

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 See also:

REASON (Lat. ratio, through French raison)

reason to suppose that the central apical plate of certain free-See also:

SWIMMING (from " swim," A.S. swimman, the root being common in Teutonic languages)

swimming crinoids has any more to do with the distal foot-plate of the larval Antedon stem than has the so-called centrodorsal 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 See also:

OLDEST

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 See also:

TRANSFER (from Lat. transferre, to bear across, carry over)

transfer the appellation to the terminals . Those plates are perhaps See also:

CONSTANT, BENJAMIN (1845-1902)

constant throughout sea-urchins and starfish (though it would See also:

PUZZLE

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 . Lovell and M . See also:

NEUMAYR, MELCHIOR (1845-1890)

Neumayr adduced the Triassic sea-urchin Tiarechinus, in which the apical system forms half of the test, as an See also:

ARGUMENT

argument for the origin of Echinoidea from an ancestor in which the apical system was of great importance; but a genus appearing so See also:

LATE

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 See also:

RECENT

recent See also:

DISCOVERY

discovery of an inter-mediate form suggests that we have here nothing but degenerate descendants of a well-known Palaeozoic See also:

FAMILY

family (Lepidocentridae) . But to pursue the See also:

TALE (O.Eng. talu, number, account, story; the word is common to many Teutonic languages; cf. Ger. Zahl, number, Erzahlung, narrative, Du. taal, speech, language)

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 " crinoidphantom," that bogey of the Sarasins, but a form with definite plates subject to a quinqueradiate arrangement, with which its See also:

INTERNAL

internal See also:

ORGANS

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 . 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 See also:

CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character to the holothurians . The See also:

ABSENCE (Lat. absentia)

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-See also:

wall, sup-posed 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 See also:

RESPIRATORY

respiratory trees), which recall the anal glands of the Gephyrean worms; the absence of podia (See also:

TUBE (Lat. tuba)

tube-feet) in many genera, and even of the radial water-vessels in Synaptidae; the absence of that See also:

PECULIAR

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 See also:

GOOD, JOHN MASON (1764-1827)

good reason or See also:

BAD, BCD

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-See also:

RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

ring, the radial canals to the body-wall making a subsequent, and only temporary, See also:

APPEARANCE (from Lat. apparere, to appear)

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 k, T, The five inter-radial tentacles . M, The water-pore, leading by the See also:

stone-See also:

canal stc to the water-ring, from which hangs a Polian vesicle pb . oc, Supposed otocysts . m, See also:

LONGITUDINAL

Longitudinal muscles . sk, Calcareous spicules. st, See also:

STOMACH (Gr. vrbsaxos from arbga, a mouth)

Stomach .

(After Semon.) changes . The Pentactaea, at all events as it sprang from the See also:

BRAIN (A.S. braegen)

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 See also:

CHAOS

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 See also:

SEARCH, or VISIT AND SEARCH

search for some primitive type in which quinqueradiate symmetry was exhibited in circumoral appendages, but had not affected the nervous, water-vascular, See also:

MUSCULAR

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 See also:

period at which they diverged from the sedentary stock . The tracing of this history, and the explanation of the See also:

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 See also:

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 See also:

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 Pe atozoan nature is manifest, are the enclosing of the viscera in a ca fled and plated theca, for See also:

PROTECTION

protection against those enemies from which a fixed animal cannot flee; the development, at the aboral pole of this theca, of a motor See also:

NERVE (Lat. nervus, Gr. vevpov, a bowstring)

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 See also:

SKELETON

skeleton; the absence of suckers from the podia, which, when present, are respiratory, not locomotor, in See also:

FUNCTION

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 See also:

EVOLUTION

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 See also:

METAMORPHOSIS

metamorphosis, and the torsion that affects both gut and body-cavities during that See also:

PROCESS

process; but the acceptance of a Dipleurula as the See also:

COMMON

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, See also:

EMBRYOLOGY

embryology, the comparative anatomy of the classes, and a See also:

CONSIDERATION (from Lat. considerare, to look at closely, examine, generally taken to be from con-, and the base seen in sidus, sideris, a star, the word being supposed to be originally an astrological or astronomical term)

consideration of other phyla . Palaeontology, so far as it goes, is a sure See also:

GUIDE (in Mid. Eng. gyde, from the Fr. guide; the earlier French form was guie, English " guy," the d was due to the Italian form guida; the ultimate origin is probably Teutonic, the word being connected with the base seen in O. Eng. witan, to know)

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 See also:

COLLECTING

collecting fossils . Two lines of argument appear fruitful . First, a comparison of the relative See also:

numbers of the representatives of the various classes at different epochs; according to this they may be placed in the following See also:

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 See also:

plot out the See also:

CURVE (Lat. curvus, bent)

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 See also:

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 See also:

ancient forms are known which, if theyare not themselves the actual links, suggest how the crinoid type may have been evolved from some of the more See also:

REGULAR

regular cystids . The See also:

FOURTH

fourth class of Pelmatozoa—the Edrioasteroidea—differs from the others in the structure of its ambulacra . As in all Pelmatozoa these seem to have See also:

BORNE, KARL LUDWIG (1786–1837)

borne ciliated food-grooves protected by movable covering-plates (fig. zt) . Beneath each food-groove was a radial water-See also:

VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

vessel and probably a nerve and See also:

BLOOD

blood-vessel, all which structures passed either between certain regularly arranged theca) 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. t t), 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 See also:

ALTERNATION (from Lat. alternare, to do by turns)

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 See also:

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 See also:

SHORT, FRANCIS JOB (1857– )

short description of it will serve to clear the problem from unnecessary difficulties . Bothriocidaris (fig . 5), a small echinoid anus . A B 5.-Bothriocidaris globulus . A, from the side; B, the plates around the aboral pole . (After Jaekel.) The short spines which were attached to the tubercles are not See also:

DRAWN

drawn . from the Ordovician rocks of See also:

ESTHONIA (Ger. Ehstland and Ehshland, Esthonian Eestimae and Meie-maa, also Viroma and Rahvama; Lettish Iggaun Senna)

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 See also:

lie in a small See also:

OVAL (Lat. ovum, egg)

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-See also:

AREA

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 See also:

STAR

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 See also:

TEETH (O.E. Eel); plural of tooth, O.E. top)

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 See also:

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 See also:

CLUE

clue, does not furnish an actual See also:

LINK

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 See also:

LIGHT

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 See also:

AQUARIUM (plural aquaria)

aquarium towards the See also:

close of the metamorphosis may See also:

account for the slight See also:

INFORMATION (from Lat. informare, to give shape or form to, to represent, describe)

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 See also:

RESEARCH (O. Fr. recerche, from recercher, re- and cercer, mod. chercher, to search; Late Lat. circare, to go round in a circle, to explore)

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 See also:

VERTEBRATA

Vertebrata . This, however, is no more the case .. The conception of the Dipleurula derives its See also:

CHIEF (from Fr. chef, head, Lat. caput)

chief See also:

WEIGHT

weight from the fact that it is comparable to the early larval forms of other primitive coelomate animals, such as See also:

BALANOGLOSSUS

Balanoglossus, Phoronis, See also:

CHAETOGNATHA

Chaetognatha, See also:

BRACHIOPODA

Brachiopoda and Bryozoa . So too the explanation of radial symmetry and torsion of organs as due to a Pelmatozoic mode of life finds See also:

CONFIRMATION (Lat. confirmatio, from confirmare, to establish, make firm)

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, See also:

SKETCH (directly adapted from Dutch schets, which was taken from Ital. schizzo, a rough draft, Lat. schedium, something hastily made, Gr. oxE&os, sudden, off-hand, axeb6v, near by; Ger. Skizze and Fr. esquisse are from the same source)

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 See also:

FRAME

frame into which more precise and more detailed statements may be 'fitted as they come to the See also:

ken of the reader, its See also:

OBJECT

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 See also:

APERTURE (from Lat. aperire, to open)

aperture serving as both mouth and anus, We may, with See also:

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 See also:

ELLIPSE (adapted from Gr. EXkei 'tc, a deficiency, iXXeirely, to fall behind)

ellipse and their eventual See also:

FUSION

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 See also:

AXIS (Lat. for " axle ")

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 (I) a preoral segment or See also:

LOBE

lobe, containing the anterior coelomic cavity; (2) a See also:

MIDDLE

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 See also:

LEFT

left posterior coelom . Each of these cavities presumably excreted See also:

WASTE (O. Fr. wast, guast, gast, gaste; Lat. vastus, vast, desolate)

waste products to the exterior by a pore . There was probably a nervous area, with a tuft of See also:

CILIA (plural of Lat. cilium, eyelash)

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 characterizedthe 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 See also:

KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

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 . See also:

ROW, JOHN (c. 1525—1580)

Row 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 See also:

ANSWER (derived from and, against, and the same root as swear)

answer is attempted in the following paragraphs : — Confining our See also:

ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

attention to that form of Dipleurula (fig . 6) which, it is supposed, gave rise to the Echinoderma, we infer from embryo- lefl h drocoel .

Irtghe 1 /epos& coelom . anterior coelom mouth . !poste coelom, logical 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 See also:

LINE

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 coelom