THE QASTRAEA-THKORYj ETC. 223

mucous membranes in general. The word " squamous"sufficiently clearly indicates the general character of anepithelium made up of flattened cells which overlap, as" tesselated" equally clearly signifies an epithelium of flat-tened cells fitting into each other at their edges. Theseare general distinctions. Such special forms as the sinuouscells of the commencing lymphatics or the jagged cellsof the epidermis do not need any distinctive general ap-pellation.

We perhaps do want easy terms which shall denotewhether the epithelium in any spot consists of several layers,or of one pronounced layer only. The latter might be calledmonoderic (Sepog = btp/xa), the former polyderic.

Epithelium itself would simply mean cells lining a cavityor coating a free surface.

The GASTRAEA-THEORY, the PHYLOGENETIC CLASSIFICATIONof the ANIMAL KINGDOM and the HOMOLOGY of theGERM-LAMELLJE. By ERNST HAECKEL. (Translated byE. PERCEVAL W R I G H T , M.D., F.L.S., Sec. R.I.A.",Professor of Botany, Trin. Coll., Dublin. With PI. VII.)

{Continued from p. 165.)

5.—THE^SYSTEMATIC SIGNIFICATION OF THE GASTRAEATHEORY.

T H E following conclusions relating to the natural systemof the animal kingdom, or, what is the same thing, to itsgenealogical tree, result from the foregoing discussions,which 1 have already explained, partly in the ' Biology ofthe Calcareous Sponges' and partly in the fourth edition ofthe ' Naturliche Schopfungsgeschichte' (in the eighteenthlecture). The whole animal kingdom divides into twolarge principal groups, the gastrula forming the separatingboundary line between them; on the one side the stem-group of the primary animals {Protozoa); on the other,the six higher stem-groups which we oppose to the othersas animals with germ-lamellce (Metazoa or Blastozoa).In the primary animals (the Protozoa) the entire bodyconsists either (1) of a simple cytode (Monera, Monotha-lamia), or (2) of an aggregate of cytodes (Polythalamia), or(3) of a simple cell (Amoebae, unicellular Gregarinse, Infu-

224 ERNST HAECKEL.

soria), or (4) of an aggregate of simple, similar cells (poly-cellulav Gregarinse, Synamceba), or, lastly (5), those wherethe cells of the body may even be differentiated to a slightextent, but which still form, no germ-lamellfe, and encloseno true intestinal cavities. The individuality of the Protozoaalways remains fixed at a very low point; that is, they eitherform a morphon of the first order, a simple plastid (a cytodeor a cell), or they form, at most, a morphon of the secondorder, an " organ" in a purely morphological sense, anidorgan (see the doctrine of individuality in the ' Biology ofthe Calcareous Sponges,' p. 103, &c.). But the Protozoanever raise themselves to the importance of a morphon ofthe third or fourth order, a Person or a Stock (in the sensedefined in the passage quoted). Just as a true intestine (thefirst and oldest organ of the germ-lamellar animals) is want-ing in the Protozoa, so are absent also all the differentiatedsystems or organs which we find in. the former. TheProtozoa have no nervous system, muscular system, vascularsystem, dermal system, &c. They also want the differen-tiated tissues.

On the important grounds which I have fully developedin the second volume of the ' General Morphology; and inmy ' Monograph of Monera,' it seems to be a real advantage,especially towards the comprehension of general biology, toseparate a large portion of the so-called Protozoa from theanimal kingdom, and to relegate them, to the neutral kingdomof Protista, intermediate between the animal and vegetablekingdoms. To this would belong part of the Monera, theAmoeboida, and Flagellata, in addition to the Catallacta, theLabyrinthulea, the Myxomyceta, and the entire class, so richin forms, of Rhizcpoda,, with all its different divisions;Acyttaria, Radiolaria, &c. All these Protista are to be re-garded as independent organic stems or phyla, which do notstand in any kind of genealogical connection with the animalkingdom,and consequently do not belong to its natural system.On the other hand, there are very simple organisms whicheither belong to the actual stem-forms of the animal kingdom,and form the true root of the animal genealogical tree, orrepresent independent offshoots from that root, as well asthose very simple organisms which display an undoubtedlyanimal character (as the Infusoria), which are to be separatedfrom these neutral primary forms or Protista as true primaryanimals or PROTOZOA. These Monera and Amoebae shouldbe regarded as true primary animals, representing theoldest stem-forms of the animal kingdom, and I have classedtbese in the fourth edition of the Schopfungsgeschichtc

THE GASTRAEA-THEORY, ETC. 225

as egg-animals (Ovularia), because they possess a shape cor-responding to the simplest (nucleus-containing) egg-cell orthe egg-cytode (without nucleus). With these must also bereckoned the planula representing animal forms (Planeeada),and, finally, the Gregarinje, the Acinetee, and the true ciliatedInfusoria (Ciliata).

The -second main division of the animal kingdom is com-posed of the six higher stem-groups, which are all derivedfrom the common stem-form of the Gastraea. We class themtogether as germ-lamellar animals, METAZOA (or Blastozoa),or animals with an intestine (Gastrozoa). In all these animals,from the sponges up to the Vertebrata, the body alwaysoriginally develops itself from two primary germ-lamellee, theanimal exoderm, and the vegetative endoderm. The latteralways encloses a true intestinal cavity with a mouth-openiug.1

Therefore the body has the form-value of a morphon of thethird order, a true person, or is composed of several persons,and is then an individual form of the fourth order, astock ('Biology of the Calcareoxis Sponges,' p. 103, &c).All these germ-lamellar animals possess at least two differentsystems of organs, namely, the dermal system (the coveringof the outer germ-lamellse with its derivatives) and theintestinal system (the intestinal outfolding of the innergerm-lamella with its derivatives).

In further classifying the Metazoa, we may, in the firstplace, advantageously make use of three different principles ofdivision—1. The want or possession of the ccelom. 2. Thedifferent number of the secondary germ-lamellse. 3. Theradial or bilateral fundamental form.

If we would attach a principal importance to the ccelomand the vascular or blood system depending upon it, then themain division Metazoa divides next into two distinct groups ;on the one side the lower germ-lamellar animals withoutcoelom or hjemolymph; Zoophyta and Acoelomi (Plathelmin-thes); on the other the higher Metazoa with coelom and hcemo-lymph; the Coelomati and the four highest groups of animalsspringing from these—Echinodermata, Arthropoda, Mollusca,and Vertebrata (vide the ' Biology of Calcareous Sponges,'pp. 467, 468). We could adopt for these two groups theoriginal terms, in their strictest sense, of Aristotle, Anaema

1 The few animals among the Blastozoa which are without an intestine,the Cestoda and Acanthocephala, eamiot be considered here as an exception,as they have apparently lost the intestine in consequence of their parasjtichabits, and originally sprung from worms provided with an intestine. Thisfollows unquestionably, from their comparative anatomy and ontogenesis.—'•Fide ' General Morphology,' vol. ii, p. lxxx.

226 ERNST HA.ECKEL.

and Ensema (but in any case not with the expressed limits oftheir author). Anaema or true " bloodless" Metazoa are theZoophyta and Plathelminthes (Accelomi). Ensema or true" blood animals" are, on the other hand, the Ccelomati(worms with blood and ccelom), and the four highest animalraces arising from these. The former could be defined asAntemaria and the latter as'Hmmataria.

The attempt to employ the number and differentiation ofthe constituent germ-lamellse, as the fundamental principleof division for the main groups of the animal kingdom, hapvery recently been twice carried out in different ways byGustav Jaeger and E. Kay Lankester. The first givesin his suggestive 'Manual of General Zoology' (1871) aspecial chapter on the " Principles of the Layers and of theGroups of Layers: Stratography of the Animal Body."Jaeger separates here—1. Two-layered animals (" the lowestmulticellular.animals"). 2. Three-layered animals (Coelen-terata). 3. Five-layered animals (Enterata or animals withintestines; our Bilateria, the five higher groups of animals).Praiseworthy as the attempt is, to apply " stratography" inthis manner to animal morphology, it must yet be regardedas misleading in details. This becomes at once apparentby comparing Jaeger's explanation (especially §§ 55, 67) withour explanation in the present essay, Avhich has the Gastraea-theory fpr its basis. Just as little can I concur in details withthe attempt of E. R. Lankester (loc. cit., p. 325). He dividesthe animal kingdom into—1. Homoblastica, without dif-ferentiated germ-lamellee (Protozoa). 2. Diploblastica (withtwo germ-lamellee (Ccelenterata). 3. Tripoblastica, withthree germ-lamellse (the five higher groups, our Bilateria).

In our own opinion, if a man wished to characterise in thisway the main groups of the animal kingdom by the number ofthe germ-lamellae, he would do much better to separate theminto the following four or five sections:—1. Ablasteria: Animalswithout germ-lamellae (Protozoa). 2. Diblasteria: Animalswith two permanent germ-lamellae (Gastraeada, SpongicR, andthe lowest Acalephm). 3. Triblasteria: Animals with threegerm-lamellae (the bulk of the Acalephm—Hydromedusa,Ctenophorce, Corals). 4. Tetrablasteria: Animals with fourgerm-lamellse (cuticular nervous and muscular layers, andintestinal muscular and glandular layers). The Bilateria, orthe five higher groups of animals collectively. Among theselast the Accelomi (the worms without body-cavity or blood,the Plathelminthes) would represent the lower condition ofdevelopment, from which the Coelomati (the worms withbody-cavity and blood) have subsequently developed them-

THE GASTRAEA-THEORY, ETC. 2 2 7

selves by the shrinking apart of the two muscular layers.The four highest groups of animals, the Echinodermata,Arthropoda, Mollusca, and Vertebrata, are diverging descend-ants of the four different forms of Ccelomati. I t is not diffi-cult to derive these four typical phyla from the commonroot-group of the worms. Their comparative anatomy andontogenesis still shows us, even now, that they have nearrelatives among the Ccelomati. The Annelida lead to theArthropoda and Echinodermata, the Bryozoa (?) to the Mol-lusca, the Tunicata (Ascidia) up to the Vertebrata (videLecture 18 in the ' Natiiiiiche Schopfungsgeschichtc). Ifwe wish to regard the coelom (which has originated by sepa-ration of the animal and vegetative muscular layer) and thecells which belong to it (coelom-epithelia, lymph-cells, blood-cells) in Jaeger's sense as representatives of a special fifthlayer, an intermediate fifth germ-lamella, we should have torefer the Acoelomi only (Plathelminth.es), and perhaps aportion of the Acalephse, to the Tetrablasteria. On the otherhand, all the animals provided with a ccelom (the Ccelomatiand the four highest groups of animals) would form a specialfifth main group: Pentablasteria, with five germ-lamellae orprincipal layers of tissues : — 1 . Cuticular nervous layer. 2.Cuticular muscular layer. 3. Coelom. layer, or lymphlayer, vascular layer in a modified sense. 4. Intestinalfibrous layer. 5. Intestinal glandular layer.

An arrangement of these five principal groups of the animalkingdom, with their known and generally accepted " types,"would yield the following results :

1 Ablasteria . . . 1 Protozoa . Protozoa Protozoa.o TV,, , • 0 C Gastraeada. ")2 Drblastena. . . 2 (Spongi£e . £ Zoophyte-3 Triblasteria . . 3 Acalephse . J4 Tetrablasteria. . 4 Acoelomi . ") -rr

fCoelomati . j Vermes j>Metazoa.Mollusca . A

5 Pentablasteria . 5 -{ Ecliinodermata fmI Arthropoda C iJ'Pozoa

L Vertebrata. JHowever attractive it may appear to us from a phylogenetic

point of view, to employ the number and differentiation of thegerm-lamellse in this manner as a basis for the classificationof the animal kingdom, yet on a closer examination importantobstacles present themselves, which do not justify the strict1

carrying out of this principle of division. Independently ofthe fact that we do not yet know the ontogenesis of manyanimals (especially of the lower orders) at all sufficiently,there are intermediate transitional forms between the five

228 EHNST HAECKEL.

groups mentioned, which admit of no sharp division, and,moreover, cases occur in the lower phyla of the Melazoa, inwhich nearly related forms of one stock must be placedin different groups of Blasteria. Although most of theAcalephce (Hydromedusse, Ctenophora, Corals) are probablyblasteria, yet Diblasteria are among their lower forms(Hydra), and probably many Tetrablasteria are among theirhigher forms. Among the Accelomi (Plathelminthes) pro-bably many Triblasteria, or even Diblasteria, may be foundamong the predominating Tetrablasteria forms j and so inother cases.

On these and other grounds it appears muclu»»i* preferableto employ only characters drawn from the phylogenesis of theMetazoa as the leading principle for their further division, inwhich the stereometric (radial or bilateral) essential form ofthe parts of the body plays a decisive part. The furtherdevelopment of the gastrula here appears next denned.Following this I have already arrived at the opinion (in the' Biology of Calcareous Sponges') that the descendants ofthe Gastraea, as the common root-form of all the Metazoa,subsequently divided into two branches, the Protascus, whichis to be regarded as the root-form of all the Zoophyta, and theProthelmis, which is to be regarded as the common root-formof all the five higher groups of animals. The division ofthese two principal branches is quite mechanically dependenton the two different modes of life to which the descendents ofthe monaxial (neither "radiate" nor "bilateral") Gastraea firstadapted themselves. The one group resigned the freelymoving habits of the swimming Gastraea, attached itself bythe pole of the axis of its body opposite to its mouth, andthen developed eo ipso further into the so-called "radiate type"(Zoophyta). The other group of the descendant of theGastraea retained the power of moving freely from place toplace, proceeded from the swimming method of moving tocreeping on the sea-bottom, and developed eo ipso into theso-called " bilateral type " (the five higher groups of animals,Vermes and Typozoa). I therefore regard only on the oneside the fixed habits of life in the root-form of the Zoophyta(Protascus) as the mechanical " acting cause " of their radiatetype, or, more correctly expressed, of their actinote (regularlypyramidal) essential form; and, on the other side, the creep-ing habits of life in the root-form of the worms (Prothelmis)as the mechanical causa efficiens of its bilateral type, or, morecorrectly expressed, of its dipleural (amphithect-pyramidal)fundamental form. This has been inherited from the wormsby the four highest stein-gioups of animals.

THE GASTRAEA-THEORY, ETC. 229

On the ground of this phylogenetic consideration we canclass together the whole of the originally bilateral descendantsof the Gastraca (the successors of Prothelmis) in a naturalmain group, which we will briefly designate Bilateria orSphenota(" wedge-animals," on account of their wedge-shapedessential form in the sense of Bronn). This group includesall the worms and the four highest groups of animals derivedfrom them ; the Mollusca, Echiuodermata, Arthropoda, andVeitebrata.1

6. SIGNIFICATION OF THE GASTKAEA-THEOHY IS RESPECTTO THIS HOMOLOGY OF TYPES.

By comparing the gcrm-lamcllce in the different groups ofanimals we arc led to ihe important question, how far theorgans and systems of organs in general are capable of amorphological comparison in the seven phyla of the animalkingdom, and how far a true homology in the strictest sense(i. e. homophyly) is to be carried out between them? Thosewho maintain Baer's and Cuviev's doctrine of types in itsoriginal rigid sense, and consider all ihc types of the animalkingdom as perfectly separated morphological units, mustnaturally answer this question generally in the negative.Those, on the other hand, who regard the theory of types inthe light of the theory of descent, and those who admit themodification of it, which we have attempted here by theGastraea-theory, as well as the generalisation of the gertn-lamellae theory which depends upon it, must, to a certainextent, agree to sucli a morphological comparison. In fact,Gegenbaur2 has recently expressed himself in this sense,and Kowalevsky 3 also in his latest work.

Although this question about the homologies of the groupsof animals is extremely important and interesting for com-parative anatomy and phylogenesis, yet its positive solutionseems difficult and entangled in the present imperfect

1 In all the Ve'rtebrata, Annulusa, and Mollusca, tlic dip'cural or bilateralessential form is just as undisputed as in the Venues. But the root-formof the Ecbinodennata possesses also the same fundamental furm. Accordingto our theory of Ecliinodermsita wo consider as such the arlicuhtted worin-person which has stiil preserved most of its independence in the " Ann" ofthe Asterida. The radiate form of the developed specimens of Echiuoder-mata (star-shaped Coimi, composed of five or more Persons), therefore formsjust as little of an objection as the radiate form of specimens of the Synas-cidiau stock (Botiyllus).2 Gegenbaur, 'J3iundzii<jc cler vergl. Anatomic,' ed. 3, p. S2.3 Kowalevsky, ' Embryologisclie Stuclien an Wiirmern und Arthropoden,1S71, conclusion.

230 ERNST HAECKEL.

condition of morphology. I therefore lay no more stress onthe following explanation than that of a provisional attempt.The phylon of the Protozoa is naturally entirely excludedfrom this consideration, as, according to our previously ex-pressed opinion, no animal of this root-group rises to theformation of germ-lameUte, and therefore, the organs de-veloped from the latter are also completely absent in theProtozoa. We therefore, for instance, consider any morpho-logical comparison of any part of the body of an infusoriumwith au apparently representative (and physiologically,perhaps, equally important, and therefore analogous) portionof a germ-lamellar animal as quite inadmissible. As I havealready shown in an essay " O n the Morphology of theInfusoria," the intestine of the Ciliata can, for instance, belooked upon as such and compared with the intestine of theMetazoa. But in a morphological aspect these parts cannotgenerally be compared at all. The intestine of the Ciliata isbut a portion of a single highly differentiated cell; the in-testine of the Metazoa is a cavity enclosed by the many-celledinner germ-lamellae. Homologies can only exist betweenthe six stem-groups of the Metazoa, which are all derivedfrom the Gastraea.

As the most certain and universal homology which isapplicable throughout the whole series of Metazoa (from thesponges to the vertebrates), we may take the comparison ofthose organs which are already differentiated in the simplestMetazoa (the Gastraeada and the lowest sponges), and whichpersist in them throughout their lives in their simplest con-dition ; that is, firstly, the primitive intestinal canal with itsepithelium (the intestinal glandular layer, the entoderm ofthe gastvula) ; and, secondly, the most superficial coveringof the body (the cuticular layer or the epidermis, the exo-derm of the gastmla). With reference to this latter, it isexpressly to be noticed that, indeed, the originally completehomology of the epidermis in the six phyla of the Metazoamay be unsatisfactory and frequently disturbed, in conse-quence of earlier commenced cuticular processes, by whichthe original outer epidermis layer is changed or stripped offinto a transitory embryonal covering (as in Hydra, Kleinen-berg), but that none the less the epidermis constantly retainsat least a layer of cells, and serves as a foundation for theothers, consequently the epidermis, as a whole, and as aderivative of the simple exoderm of the gastrula, is homo-logous in all the six stem-groups of the Metazoa.1

1 The formation of many embryonal coverings, which arise ontogeneticallyfrom the uppermost germ lamella (the horny layer), is perhaps to be explained

THE GAST11AI2A-THE0K.V, ETC. 231

The question of the homology of the central nervous systemis much more difficult. This has, doubtless, arisen from theexodeimin all six stem-groups of the Metazoa, but the centralnervous system of the zoophytes has certainly arisen inde-pendently of that of the worms, and is in no respect to becompared to it. On the other hand, the simplest form of thecentral nervous system, which is found in the worms, espe-cially the simple pair of ganglia lying over the oesophagus, theso-called upper pair of ganglia or primitive brain, is to heregarded as homologous, firstly, in all classes of the group ofworms, and, secondly, is to be compared also to the corre-sponding parts in the Mollusca and Arthropoda, as well as tothe original medullary tube of the "Vertebrata (from which thebrain of the latter is only the furthest differentiated division1).This original central organ has been lost in the Echinodermata,and their cesophageal ring is only a secondary commissurebetween the five radial nervous threads, which appear in theAsterida in their most original form. Each of these five radialthreads of the Echinodermata is homologous to the jointed ven-tral cord of the Annelida and Arthropoda. It is necessary toaccept the correctness of my theory of the origin of theEchinodermata for the conception of this apparently para-doxical comparison, according to which the root-form of thephylon of the form of the Asterida is to be regarded as astem composed of five-jointed worms united into a star-shape.This theory has, indeed, been rejected by Claus, Leuckart,Semper, and others, but without their putting any other

phylogenetically by moultings (or "Mauserungen") which the ancestors ofthe organism in question have suffered in earlier periods of the earth'shistory. So is, especially, to be explained the larval'form of many of thohigher Crustacea, which originates within the egg-shell, and is itself fre-quently changed, upon repeated moultings of the root-form of the Crustacea,the Nauplius, and other old root-forms which have arisen from this. (Com-pare the statements and explanations relating to this in the detailed worksof Fritz Midler, Edouard von Beneden, A. Dolirn, &c.) This is, perhaps,also the explanation of the so-called Amniou in many animals. On the otherhand, the amnion of the vertebrata is certainly of a different origin. As forthe special homology of this amnion in Vertebrata and Arthropoda, as main-tained by Ivowalevsky and others, it is already contradicted, independentlyof other reasons, by the fact that the amnion only occurs in the three higherclasses of Vertebrata (Amniota). This has, therefore, apparently firstdeveloped itself here, during the origination of the root-form of the Amniof.aJrom the Amphibia, and is entirely unconnected with the amnion of theArthropoda. The latter is only analogous (and komomorphous) to thoformer, but not truly homologous (homophylous).

1 The spinal marrow of the Vertebrata, and the ventral nervous cord olthe Annulosa, are of course not analogous from this point of view, and thesecan just as little be compared as the sympathetic marginal cord of the formerand the ventral nervous cord of the latter.

&32 ERNST HAECK.EL.

natural theory in its place, and without their having madeany attempt to explain the origin of the Echinodermata. Onthe other hand, my theory, which fully explains its origin,has received the full sanction of two zoologists of the first rankupon whose judgment I lay the greatest weight, Gegenbaurand M. Sars (senior), the last recognised as one of the natu-ralists most thoroughly acquainted with the Echinodermata.1

The organs of sense of the different groups of animals arefor the most part (perhaps entirely, with the exception of theskin as the organ of touch) not homologous; moreover, thehomology is often not to be proved even within one of thesegroups, or is even positively negatived within a given class, as,for instance, in the organs of hearing of different insects. Allpoint to these as of polyphyletic origin, and as havingoriginated at different times from different portions of theupper germ-lamellse. This manifoldly different and inde-pendent origin of the organs of sense is also very well con-ceivable phylogenetically.

The primordial kidneys have probably also originated fromthe upper germ-lameUte, and these organs are probably homo-logous in all the Bilateria (in all the members of the fivehigher animal groups). The simplest form would be repre-sented by the so-called " excretory organs " or " water-vascular system " of the Plathelminthes, which are originallynothing more than strongly developed tube-shaped dermalglands (like the sweat-glands). Comparative anatomy willperhaps later be in a position to prove that these primarykidneys of the unarticulated Plathelminthes, which reappearin each metamer of the articulated Vermes as so-calledlooped canals or segmental organs, have given rise both tothe kidneys of the Mollusca and to the primary kidneys ofthe Vertebrata.3 Gegenbaur has already proved the homo-

1 The origin of the central nervous system from the original outer layer ofthe body of the animal, the horny layer, is one of the most striking examplesof the value of the phylogenetic view, and its signification for the compre-hension of the ontogenetic process. Hitherto this origination of the " in-ternal" nervous system from the outer germ-lamellse has been almostuniversally considered wonderful and paradoxical. But as soon as the pro-blem is thus stated : " How can the nervous system generally have originatedat first (phyletically) ?" only the one answer, after ripe reflection, will begiven to it: " From the most superficial parts of the body, which were con-stantly in communication with the outer world." Only from this constantcommunication could the first "sensation" develop itself. The nervoussystem has then withdrawn itself secondarily into the protected interior ofthe body, " separated from the horny layer." I do not consider the idea ofa special "nervous layer," which many embryologists separate from thecuticular sensitive layer, t.o be confirmed.

3 In Amphioxus the broad canal discovered by von Kathke, and more fully

THE GASTRAEA-tfHEORY, ETC. 233

logy of the " shell-glands " of the lower Crustacea among theArthropoda (and the " green glands " of the Decapoda) withthe primary kidneys of the Verm.es. The Tracheata havequite lost this excretory organ, and the Malpighian tubesof the intestinal canal have taken its place. If we regardthe primary kidneys as originally (phylogenetically) inthis manner separated skin-glands, it also explains theiroriginally superficial position in the vertebrate embryo.They are here undoubtedly derived from the upper gerna-lamellce, either directly from the horny layer or indirectlyfrom cells of the " axial cord," which, have passed from thehorny layer into the dermal fibrous layer.

The dermal muscular layer, or the dermal fibrous layer(the " flesh-layer " of Baer, the dermal layers and primaryvertebrate layers of Remak), is, as a whole, in its originalsimple commencement, probably homologous in all the sixbranches of the Metazoa, or certainly, at least, in the fivephylse of the Bilateria. It has probably originated in theVermes, as well as in the Zoophyta (Hydra, &c), from theupper germ-lamellee, and has been inherited from the Vermesby the four higher groups of animals. The corium and themuscular dermal sheath are to be regarded as the two earliestproducts of its subdivision; both are perhaps of the sameorigin, and therefore homologous within the five higherphylse (the Bilateria). The muscles of the trunk of theV"ertebrata also proceed from this layer.

On the other hand, the skeleton system in the differentgroups of animals is not homologous. Both the internalskeleton formations of the Zoophytes, as well as those of theEchinodermata and the Vertebrata, are entirely differentformations, peculiar to each phylon, although, all threeappear to originate from the dermal fibrous layer.

The external skeleton of the Vermes and. Arthropoda,which is only a chitinised differentiation of the epidermis(the so-called hypodermis or chitinogen membrane), as wellas the calcareous shells of the Mollusca (also exudations from

described by J. Miiller, which runs on each side in tlie folds of the skin ofthe ventral surface (immediately at the outer surface of the sexual glands),and which opens externally behind on both sides of tlie Poms abdominalis,is perhaps to be considered as a homologue, or as a rudiment of the originalprimary kidneys. (A second further opening in the mouth-cavity is pro-blematical.) If the comparison of this dermal canal of Amphioxus (fig. 40,PI. I, of J. Miiller's work) with the primary kidneys of the Vertebrata,nnd with the similar excretory organs of the Vermes, were correct, this

234 EllNSX HAECKE'L;

the epidermis), do not come under consideration here"at all.

The ccelom or the body-cavity, the original " pleuro-peri-toneal cavity," which is entirely absent in the Protozoa,' Zoo-phyta and Acoelomi (Plathelminthes), is certainly homolo-gous in the Ccelomata, and in the four higher stem-groups ofanimals. It originates everywhere as a slit between the twomuscular layers, and has apparently descended from theCoelomati, the worms Avith blood, to the four higher groupsof animals. However, this homology is not to be establishedby comparison with the cavity of segmentation, from whichKowalevsky makes the coelom proceed (comp. above, p. 165).Thecoelom is originally filled with a fluid, which, on accountof its varying characters, can be defined as hfemolymph orhcemochyle. But in the higher worms this nutritive fluid isalready differentiated into two different constituents, into thecolourless chyle or lymph which fills the body-cavity, andinto the coloured blood, which circulates in the closed vas-cular system. This differentiation also recurs in theVertebrata.

The intestinal muscular layer, or the intestinal fibrous layer(the " vascular layer" of Baer, the intestinal fibrous layerand middle layer of Remak), appears to be entirely absentin part of the class Zoophyta (in the sponges and the lowestAcalephee), and to develop itself in a peculiar form in anotherpart (in the higher Acalephs).

In the Acoelomi it already begins to shape itself out asthe "intestinal muscular sheath," and has descended fromthese to the higher worms (the Coelomati), and from the latterto the four higher stem-groups of animals. There is nothingin the way of our recognising a universal homology in thiswithin these five groups of animals (the Bilateria).

The vascular system, which, as a whole, has developeditself in connection with the coelom, is, therefore, also to becompared within the five higher stem-groups of animals ; butthe question as to how far its separate parts, and especially theheart, are homologous, is very difficult to decide. Accord-ing to the sharp-sighted comparison of Gegenbaur, theheart of the Arthropoda and Mollusca is originally homolo-gous to a section of the dorsal main vascular stein of theVermes, while the heart of the Ascidia and Vertebrata ishomologous to a section of the ventral stem.

The intestinal glandular layer, which remains constant asthe epithelial outer covering of the intestinal canal and itsglandular appendages, is certainly, throughout the wholeanimal kingdom (only excepting the Protozoa), from the

THE GASTRAEA-THISORY, ETC. 2 3 5

sponges to the vertebrata, everywhere homologous, and every-where is derived directly from the entoderm of the gastrula.To be sure, Kowalevsky has lately arrived at the opinionthat the intestinal glandular layer of insects forms an excep-tion, and is much rather to be regarded as a special newformation sui generis •(' Embryologische Studien an "Wur-mern,' 1871, p. 58). This view seems to me untenable. Ifany organ can be homologous in all six phyla of the Metazoa,it is certainly the intestinal canal, with its outer coveringepithelium, the intestinal glandular layer. On the otherhand, the question of the homology of the openings of theintestine, the mouth, and anus, is at present still quite ob-scure, and so much only is certain that the opening of themouth is not always the same. The original oral opening•of the gastrula, the rudimentary mouth or the Prostoma,seems only to have descended to the Zoophyta, and, per-haps, to a part of the Vermes. It, nevertheless, seems toreappear in the Rusconian anus of the Vertebrata. On theother hand, the oral openings of the Vertebrata, the Arthro-poda, and the Echinodermata, are peculiar new formations,and certainly not homologous with the rudimentary mouth.

7. T H E PHYLOGENETIC SIGNIFICATION OF THE ONTO-GENETTC SUCCESSION OF THE SYSTEMS OF ORGANS.

The regularly graduated series in which the system of organsappear one after another in the different groups of animalsduring ontogenesis, furnishes us with a sure key, accordingto the biogenetic principle, to the historical series in whichthe animal systems of organs have developed themselves aftereach other and from each other, during the long and slowcourse of the organic history of the earth. This palaeonto-logical seniority of the systems of organs, as it is empiri-cally found a posteriori from the facts of ontogenesis, com-pletely anticipates "on the whole the demonstrations whichcould be formed on the subject d priori by physiologicalreflection and by philosophical consideration of forces at work(Causal-Momente).

In the first place, it follows from the comparison of thegastrula, and of the bilamellar cell-condition which representsit in the most dissimilar groups of animals, that two primary.systems of organs, the inner intestinal system, and the outertegumentary system, are simultaneously differentiated in.the first series, in the oldest Metazoa, the Gastraeada. Theoriginal and perfectly simple stomachic cavity or primaryintestine of the Gastraea is, indeed, the oldest organ, of the

VOL. XIV. NEW SER. Q

236 ERNST HAKCKEL.

body of the Metnzoa; but, simultaneously with its origin,has proceeded the separating of the two cell-layers of itswall, the inner nourishing epithelia (the gastral lamellae orentoderm), and the outer investing epithelia (the dermallamellae or exoderm).

In the second line of succession the elements of theskeleton-system (in the majority of the Metazoa?) formedthemselves, and this in the layer of the exoderm, as thesponges teach us. Although in the sponges the two pri-mordial germ-lamellae have (universally ?) remained constantin their original simplicity, and no third germ-lamella hasdeveloped itself from them, yet in the thickened exoderm ofmany of them we find present a very complicated and ex-tensively differentiated skeleton-system. Indeed, already theProtozoa have very generally formed skeleton-parts both forprotection and support. It is unnecessary to mention inaddition that the skeleton-system in the different groups ofanimals is of different epochs and of phylogenetic origin.

In the third line of succession the nervous and muscularsystems develop themselves simultaneously. The beautifulinvestigations of Kleinenberg on the ontogenesis of Hydra1

have informed us of the simultaneous origin of these twosystems of organs which here exist in the most intimatereciprocity. The highly interesting neuro-muscular systemof the Hydra is placed immediately before our eyes in statunascenti. The neuro-muscular cells developed from theexoderm of the Hydra show us the functions of both stillunited in a single individual of the first order. The twosystems of organs first appear independent and opposed toeach other, through a separation and a division of labourinto nerve-cells and muscle-cells. True muscles, in thestrictest sense of the term, therefore, occur first in thoseanimals in which true nerves also appear, and vice versa.As the Acalephas show us, only the dermal or parietal neuro-muscular system has originated at first from the outer germ-lamellse. The gastral or visceral neuro-muscular system(intestinal muscles and nerves) has probably originated in-dependently in a perfectly analogous manner from the intes-tinal glandular lamellae. Hitherto nothing has been saidagainst the view that the visceral nervous system has arisenindependently of the parietal; the former is just as much inconnection with the intestinal muscular layer as the latterwith the dermal muscular layer.

In the fourth line of succession the kidney or excretory1 An account of these investigations is given by Prof. Allman in the

January number of this Journal.—ED.

THE GASTRAISA-THEORY, ETC. 2 3 7

system has developed itself, the physiological signification ofwhich to the animal organism in general is greater than thatof the younger blood-vascular system and the coelom whichis connected with it. This opinion is confirmed by thePlathelminth.es, which still possess no coelom and blood-systern, but perhaps possess rudimentary kidneys (excretorycanals), and also by their universal occurrence throughoutthe whole animal series, and, lastly, especially by the earlyappearance of the " primary kidneys " in the embryo. Allwhich shows that we have here to do with a very old andimportant arrangement of organization, which already existedin the Accelomi before the formation of the blood-system andthe coelom, and has descended from thence to the highergroups of animals.

In the fifth line of succession the blood-vascular systemand the coelom developed themselves first after the kidneysystem. We have already shown that these two parts standin inseparable connection, and that the true body-cavity orthe coelom is to be considered as precisely the first commence-ment of the vascular system. After the commencement ofthe development of the intestinal fibrous layer, by its de-tachment from the adherent dermal fibrous layer, a cavity isfirst formed between these two muscular layers, which fillswith the chyle which has transuded through the intestinalwall. This was the coelom in its simplest form, and thishsemochylic-system or primordial primitive blood-system hassubsequently become differentiated into two different systemsof fluids, into the lymph-system and the true blood-system.1

In the sixth line of succession the genital system hasfirst developed itself morphologically as an independentsystem of organs (!) Certainly this has already been physio-logically present the longest of all, before any other systemof organs became differentiated. We certainly already meetwith single cells scattered in the endoderm of the intestinaltube in the sponges, some of which develop into germ-cells, and others into sperm-cells; and this was pro-bably already the case in the Gastraeada. Only in all

1 A very different view of the coelom and of the blood-system, as well asof the kidney-system, has been developed by B. R. Lankester in his oft-quotedarticle (' Annals and Magazine of Natural History,' May, 1873). He regardsthese two systems of organs as identical, and thinks that the "excretoryorgans or water-vessels " of the Accslomi form the first commencement of abody-cavity, arid that this coelom is therefore opened externally from thebeginning. On the contrary, my opinion is that the coelom is primarilyclosed, and originated subsequently to and independently of the olderprimary kidney-system. The connection of the two would then be secondary.The ontogenesis of the Bilateria seems to me to contradict E. 11. Lankester'sopinion.

238 ERNST HAECKEL.

the Zoophyta does tbe formation of both kinds of sexualcells from the epithelium remain confined to certain parts ofthe gastro-canal system; and even in many worms there arestill no independent persistent sexual organs present in amorphological sense. In many worms (Bryozoa, Annelida,&c.) individual coelom-cells, scattered cells of the " pleuro-peritoneal epithelia," develop themselves periodically intosexual cells. An independent differentiation of special sexualorgans seems, therefore, to occur later, perhaps at differenttimes in tbe different groups of animals. The decision of thisvery difficult question is, in general, connected with theproblem of the homology of tbe sexual organs, and with theprimary phyletic origin of the sexual cells, one of the mostdifficult problems of ontogenesis and phylogenesis. 1 wouldin addition here to the observations which I have made onthis subject in the * Biology of Calcareous Sponges ' (pp. 469,471), wish to hint as to the possibility of both primarygerm-lamellae sharing in the formation of sexual cells. For,although in most cases the origin of the sexual cells fromcells of the intestinal fibrous layer, or even of the primarygastral layer, is proved, yet in other cases they appear tooriginate just as certainly from the dermal muscular layer, oreven from the primary dermal layer (Hydra).

On account of the positiveness with which opposite viewsconcerning the origin of the sexual cells are maintained evenwithin the single group of Zoophyta, it may finally still besuggested whether a translocation of them has not occurredso early (already within the Laurentian period) that theirapparently original conditions may now, indeed, be theirsecond home. I have proved that in the calcareoussponges the egg-cells which originally arise in the endo-derm often pass very early into the exoderm by theiramoeboid movements, and there continue their growth.In many Calcispongise the egg-cells are much easier to befound in the exoderm (their secondary place of abode)than in the endoderm (their primary original position),so that I even believed at one time that they arose origi-nally in the former. We may now, perhaps, venture tosuppose that this early transport of the cells from oneprimary cell-layer to the other, by continued " shortened orcontracted inheritance," in the course of generations, wouldbe continually thrown further back in ontogenesis, till itfinally takes place already during the differentiation of similarfurrowed cells into the two forms of cells of the two primarygerm-lamellee. Then the cells which originally (phylo-genetically) belonged to the inner germ-lamellae nevertheless

THE GASTR.AEA-THEORY, ETC. 239

(ontogenetically) apparently occur first in the outer germ-lamellae, and vice versd. I suspect that this is often actuallythe case in the sexual cells, and that, generally, such anearly transport of the cells has played a significant partthrough the change of place and change from one germ-lamellEe to the other becoming constant by inheritance. Thistransport also possesses great significance for our above-statedview of the original difference of the two muscular layers,and may, for instance, explain much in the early axialconcrescence, in the blending of the germ-lamella in theaxial cord of the Vertebrata, as well as in their later diver-gence.

8. T H E SIGNIFICANCE OF THE GASTRAEA-THEOKY FORTHE THEORY OF TYPE.

If one judges the above-given confirmation of the Gastraea-theory as sufficient, and acknowledges the conclusions drawntherefrom as on the whole right, one will then have arrivedat the conviction that as a consequence the so-calledtype-theory—which to this very time is in general lookedon as the profoundest basis for a zoological system—hasbeen abolished, at least so far as its present significance goes,and an essentially different classification of the animal king-dom put in its place. As is known, this highly renownedand highly meritorious theory of types, which in the seconddecennium of our century two of the most important con-temporary zoologists attained to by different ways, culmi-nated in the idea that in the animal kingdom many funda-mentally different principal groups are to be discerned; foreach of which peculiar " types" there is a quite charac-teristically immanent and persistent " plan of structure."This plan of structure is determined through the peculiarposition and connection of the constitutive organs, and isentirely independent of the grade of perfection and develop-ment traversed by the various classes of animals of each typewithin its sphere. Both George Cuvier, who, by the path ofcomparative anatomy, and Carl Ernst Baer, who, alone andindependently of Cuvier, arrived at this idea by the path ofcomparative ontogenesis distinguished in the Avhole animalkingdom but four such types, which Baer, according to thedifferent manner of ontogenesis, characterised in the follow-ing manner :—(1) Radiata, with a radial development (evo-lutio radiata); (2) Mollusca, with a contorted development(evolutio contorta); (3) Articulata, with a symmetrical de-velopment (evolutio gemina) ; (4) Vertebrata, with a doublesymmetrical development {evolutio bigemina). Cuvier, as

240 ERNST HAECKEL.

also Baer, took each type for something absolutely persistent,and, in spite of all modifications, in the deepest sense un-alterable ; consequently here they allowed no connection ofany sort and no transition between the four different types.Baer, besides, insists that the type of the lowest forms ofeach of the four groups must be pronounced as well definedas in the highest, and that, consequently, the type of develop-ment is entirely independent of the grade of improvement.

In contrast with the earlier prevailing erroneous opinionthat the whole of the animal kingdom represented a single un-interrupted gradual scale of beings, and that a single con-tinuous succession of development proceeded from the lowestof the Infusoria through the different classes up to Man him-self, the light which the type-theory threw over the differentportions of zoology, but particularly over comparativeanatomy and over the history of development, procured forit a speedy entrance into the zoological system, and the fourtypes were soon pretty commonly looked upon as the basisof very exact scientific system of animals. One was, in-deed, soon compelled, through the advance in one's know-ledge of the lower animals, to pull to pieces that very un-natural type Radiata. First, Siebold in 1845 separated fromit the Protozoa, and at the same time he divided the Articu-lata into Arthropoda and Vermes. Leuckart, in 1848, wasthe first to distinguish as two distinct types the Ccelenterataand Echinodermata. So, from the original four types arosethe seven diverse main groups, which to this day are stillalso in vogue in most systems equally as the highest andmost general of the chief divisions of the animal kingdom.Hut the peculiar essence and the original signification of thetheory of types was not touched by the augmentation of thenumber of types. The aims of the later zoologists was muchmore directed to defining by the same standard the four newtypes (Protozoa, Coelenterata, Echinodermata, and Vermes),and combining each of these as an isolated form entity, withthe peculiar "plan of structure," in which was the basis ofarrangement for the three retained older types (Arthropoda,Mollusca, Vertebrata) of Baer and Cuvier. The idea, eversince then, growing stronger of the entirely independent cha-racter and the immanent''structure plan" of these seventypes of animals is to this day still generally prevalent; sothat, for example, Claus, even in the newest edition of his'Zoology' (1872, p. 41), points out the type-theory as themost important advance in science since Aristotle, and asthe very foundation of the natural symptom systems EvenHopkins names the types, moreover, " the Kepler's laws

THE GASTRAEA-THEORV, ETC. 241

of the animal kingdom," and sees in them, with Kefersteinand others, the " most brilliant confutation of the Darwinianheresy," and the strongest argument against the truth of thetheory of descent.

This last point our adversaries have themselves, withoutforeseeing it, pointed out as the Achilles' heel of the theoryof types. For it is quite certain that the theory of types, inthe original sense of its authors, does, without doubt, stRiidin a fundamental contradiction to the theory of descent.This contradiction lies not so much in this that the typesare considered as completely independent and separate highergroups of the animal kingdom, but rather in the teleogicalprincipal of their conception. The idea that the types formentirely independent groups of forms is of course inconsistentwith any monophyletic conception of the animal kingdom,which traces all animals as descendants from a single com-mon root-form; but it would allow itself to be broughtinto unison with the theory of descent in this that onerequires for each type an independent stem-form, conse-quently the entire animal kingdom requires a polyphyleticdescent—so many types, so many phyla. The conception ofthe immanent original " plan of structure of the types,"which forms the true teleogical ground principle of thetheory of types, is, on the contrary, perfectly inconsistentwith the theory of descent.

As soon, therefore, as the theory of descent reformed byDarwin attacked the Baer-Cuvier theory of types, it com-pelled the latter to defend itself by, first, freely giving up itsteleogical ground principle, and, secondly, at the same time,the connection of the types with one another had to bemodified. The first attempt towards this I made in 1866in my ' General History of Development' (" General Mor-phology," 2nd volume, chapters xvi, xix, xxiv, and xxv).First, 1 have there already pointed out that Baer's type ofdevelopment is nothing further than the consequence of in-heritance, and Baex's grade of improvement is nothing furtherthan the consequence of adaptability (1. c , p. 11); therewith,on the one side, the dualistic notion of types or the teleogicalplan of structure is brought back to the mechanical prin-ciple of inheritance (consequently to the physiological func-tion of increase), 1. c , p. 171; on the other hand, the dualisticidea of perfection or the teleogical aim of increase is conse-quently reduced to the mechanical principle of adaptability,that is, to the physiological function of nutrition (1. c , p.193). Secondly, I have, then, already shown that thedifferent higher types of the animal kingdom can be only

242 EIINST HAECKEL.

conceived in a genealogical sense as stems or phyla, but thatthe higher phyla of the animal kingdom (Vertebrata, Mol-lnsca, Arthropoda, Echinodermata) are to be considered asdiverging descendants from the lower stems of the Vermes,which have taken their origin from diverse branches of thisnumerous lower animal group; and that, lastly, theVermes and the Coelenterata must have started off from thosestill lower groups of organisms, the Protozoa or Protista (1. c ,pp. 413, 414). I have more definitively expressed thisopinion in the first edition of my ' Naturliche Schopfungs-geschichte ' (1868), and in the succeeding editions I havesought to state it more clearly. I failed in evolving it intoperfect clearness, because the Gastraea-theory, to which I firstof all was led by my ' Monograph of Calcareous Sponges,' Avasnot yet formed. It was only by means of the Gastraea-theoryand its consequences that the phylogenetic relationship ofthe types of animals to one another was completely cleared up.

It might be asserted that the Gastraea-theory is only areform or modification of the theory of types, because threeof the primitive four types (Vertebrata, Mollusca, and Arthro-poda) have been retained nearly within the original limits oftheir conception, but the content of this conception hasbecome completely different. Besides, moreover, betweenthe two theories there is this most essential difference^ thatin the " type-theory " the types appear as co-ordinate, self-existing groups of forms of equal morphological value, along-side each other, and yet independent one of another; whereasin the " Gastraea-theory" the phyla exist as partly co-ordi-nate, partly subordinate, groups of completely differentmorphological value • partly near, partly alongside each other,but all in a common connection.

In the excellent explanation which Gegenbaur has givenin the second edition of his ' Grundziige der vergleichendenAnatomie' (1872, p. 72) of the animal types, these variousreferences of types of different value to one another havebeen clearly explained, and, through the most sagaciousinvestigation of details, has been further strongly built onthe sure foundation of comparative anatomy. Gegenbaurshows that the seven types or phyla have their limits some-times tolerably distinctly fixed, and sometimes are by nomeans to be distinguished from One another; that one mustdistinguish between the lower and higher types, and that thedifferent higher types or phyla disclose in their commonpoint of departure the lower. Through this demonstrableconnection of the phyla it will appear tbat the whole of themembers of the animal kingdom can be placed in a near

THE GASTKAEA-THEORY, ETC. 243

alliance, whereby the ground is got ready for a monophyleticsystem. Through these cognisable connections the hardand fast conception of the stems, as derived from the earlierdoctrines of type, must become significantly more pliant, forwe find the relationship of the types to one another in themanner as we meet the subdivisions within the types, i. e., ingenealogical partition (1. c , p. 77).

With this conception the type-theory of Baer and Cuvieris at once destroyed, as well in the extent as in the contentof the idea of type. The type has consequently completelylost its earlier significance, and so far as it is a category ofthe system, possesses no other philosophical significance thanthe lowest category of class, order, genus, species, and soforth, it is now only relatively (through its height), notabsolutely, distinguished from the latter ; so even Gegenbaur,from the line of comparative anatomy, has attained to thesame position in respect to the type-theory as that to whichthe way of comparative ontogenesis has carried us. Thetype-theory has an extraordinary merit for zoology, and,as the highest principle of the classification of the animalkingdom, had effected on all sides an uncommonly fruitfuland stimulating work. Its efficaciousness is, however, to belooked on now as ended. The consistent application andcarrying out of the theory of descent which we have com-pared with it is no longer sufficient; in its place must nowcome the phylogenetic classification of the animal kingdom, theessential basis of which is formed by our Gastraea-theory.

APPENDIX.—SYNOPTIC PHYLOGENETIC TABLES.

For a hasty survey of the general results which appear todevelop themselves from the Gastraea-theory, the followingfour phylogenetic tables are appended. To avoid the manymisinterpretations which I have put on the similar tables andstem-structures in my ' General Morphology' and in my' Natural Creation,' as also in my ' Monograph of CalcareousSponges,' I may here expressly mention that these claimabsolutely no dogmatic currency, that they are merely essaysto give a clear insight, with the help of the Gastraea-theory,into the important relationships of the ontogenetic and thephylogenetic development of animals and their primary systemof organs. Should the attempt not be agreed with, let somebetter positive be put in its place, but let the objector not restcontented, as too often happens, with a mere negative rejec-tion. At all events, the herein proposed system of animalscoincide closer to the important facts of developmental historythan all other hitherto attempted experiments of classification.

I.—Ta

ble

of

the

Phy

loge

neti

cal D

evel

opm

ent

of

the

Syst

em o

f O

rgan

s of

Ver

tebr

ata

foun

ded

on t

he G

astr

aea-

The

ory

and

the

Ont

ogen

etic

Com

pari

son

of t

he V

erte

brat

a w

ith

the

Inve

rteb

rata

.

A.

EX

OD

EB

MA

.

Ext

erna

l pr

imit

ive

Ger

m-l

amel

la.

AH

IMA

I.

GE

BM

-LIM

EL

LA

.

Der

mal

lay

er.

LA

MIN

A D

EE

HA

LIS

.

Epi

blas

t.

B.

EN

TO

DE

EM

A.

Inne

r pr

imit

ive

Ger

m-l

amel

la.

VE

GE

TA

TIV

E

GE

BM

-LA

ME

LL

A.

Inte

stin

al l

ayer

.

LA

MIN

A G

AS

TE

AL

IS.

Hyp

obla

st.

Fir

st S

econ

dary

Ger

m-l

amel

la.

NE

UB

O-D

EB

MA

L L

AT

EB

(D

erm

alL

ayer

, B

oer)

, or

NEU

BOBL

AST

(Lam

ella

neu

rode

rmal

is).

I.Se

cond

Sec

onda

ry G

erm

-lam

ella

.F

IBB

O-D

EB

MA

L L

AT

EE (M

usc

ula

rL

ayer

, B

oer)

, or

INO

BLA

ST(L

amel

la i

node

rmal

is).

Thi

rd S

econ

dary

Ger

m-l

amel

la.

INT

ES

TIN

AL

FIB

EO

US

LA

YE

B(

Vas

-cu

lar

Lay

er,

Boe

r),

or I

LEM

O-

BLA

ST (

Lam

ella

ino

gast

rali

s).

Fou

rth

Seco

ndar

y G

erm

-lam

ella

.IN

TE

ST

INA

L G

LA

ND

UL

AB

LA

XE

B(M

ucou

s la

yer,

Boe

r), o

rMT

KO

-BL

AST

(Lam

ella

myk

ogas

tral

is).

HO

BN

Y

TU

BE

.T

ubus

cor

neus

.II

.N

EH

BA

I. T

UB

E.

Tub

us n

erve

us.

in.

SE

XU

AL

TU

BE

.T

ubus

uro

geni

tali

s.

IV.

CoB

iAC

Eon

a T

UB

E.

Tub

us c

oria

ceus

.v.

MU

SC

UL

AB

T

UB

E.

Tub

us c

arno

sus.

HJE

MA

L

TU

BE

.,

Tub

us s

angu

ineu

s.

VII

.M

ES

EN

TE

BIC

T

UB

E.

Tub

us m

esen

teri

cus.

VII

I.M

uco

us

TU

BE

.T

ubus

muc

osus

.

1. E

pide

rmis

(cu

ticl

e).

2. E

pide

rmal

app

enda

ges

(hai

r, n

ail?

, fe

athe

rs,

&c)

.3.

Epi

derm

al g

land

s (s

wea

t-gl

ands

,seb

aceo

usgl

ands

,&c)

.4.

Spi

nal

Mar

row

.5.

Bra

in.

6. S

ense

org

ans

(tli

e gr

eate

r p

art)

.7.

Pri

mit

ive

kidn

eys

(?)

(phy

loge

neti

call

y pr

imit

ive

epid

erm

al g

land

s).

8. U

roge

nita

l gl

ands

(?

) (p

hylo

gene

tica

lly

prim

itiv

eex

oder

m-c

ells

?).

9. C

oriu

.ni

(ski

n an

d sk

in m

uscl

es).

10.

Hol

low

mus

cles

.11

. E

ndos

kele

ton

(cho

rda,

ver

tebr

a, S

ec.)

.12

. E

xoco

elar

(?)

(pa

riet

al c

celo

m-e

pith

eliu

m).

' 13.

Hse

mol

ymph

(p

rim

itiv

e bl

ood—

prim

ordi

al

haem

alli

quid

).14

. E

ndoc

cula

r (?

) (v

isce

ral

ccel

om-e

pith

eliu

m).

15.

Pri

ncip

al v

ascu

lar

stem

s (l

ymph

- and

blo

od-s

tem

s,he

art)

.16

. V

ascu

lar

glan

ds (

lym

phat

ics,

spl

een,

&c)

.

17.

Mes

ente

ry.

18.

Inte

stin

al m

uscl

es (

and

cove

ring

s).

19.

Inte

stin

al e

pith

eliu

m.

20.

Inte

stin

al g

land

ular

epi

thel

ium

.

THE GASTRAEA-THEORY, ETC. 245

II.—Synoptical Table'of those Primitive Organs which, withprobability, are considered as homologous in the Vermes,Articulata, Molluscous and Vertebrated Animals.

ABTHBOPODA. MOLLTJSCA. VERTEBRATA.

I.—Products differentiated from the Neuro-dermal Layer.Epidermis.

Primitive brain(upper cesophageal (>

ganglia).

Organs of excre-tion

(" water vascular,segmental or-

gans")-

Hypodcrmis(chitine mem-

brane).

Brain[upper oesophageal

ganglia).

Shell-glands ofCrustacea.

Epidermis.

Brain(upper ccsophageal

ganglia).

Kidneys.

Epidermis.

Marrow-bone ormedullary tube(foremost part).

Primitive kidneyprocesses.

II.—Products differentiated from the Fibro-dermal Layer.

Coritim(and annular mus-

cular sheath).

Straight muscularsheath.

Corium (rudi-ment).

Hollow muscles(longitudinal).

Corium and j Corium and dermaldermal muscles I muscles

(First appearance!) (First appearance!)

Inner hollow mus-cles.

Lateral hollow

III.—Products differentiated from the Fibro-intestinal Layer.

Crelom.

Principal dorsalvessels.

Principal ventralvessels.

Intestinal wall(exclusive of epi-

thelium).

Body cavity.

Heart.

Intestinal wall(exclusive of epi-

thelium).

Body cavity.

Ventricles.

Intestinal wall(exclusive of epi-

thelium).

Pleuro-perifconealcavity.

Aorta (primordi-alis).

Heart (togetherwith bulbus arte-

riosus).

Fibro-intestinallayer and mesen-

tery.

IV.—Products differentiated from the Intestinal Glandular Layer.

Intestinal epithe-lium.

Intestinal hypo-dermis (for the

most part).

Intestinal epithe-lium (for the most

part).

Intestinal epithe-lium (exceptingmouth and anus).

246 ERNST HABCKEL.

III.—Sketch of a Phylogenetic Classification of the AnimalKingdom, founded on the Gastraea-Theory and theHomology of the Germ-lamella, the Primitive Intestine,and the Cadom.

3 SYNTAGMATA. 8 PHYLA. 16 PHYLOCLADI. 40 CLASSES.

First principal fgroup of theAnimal King-dom : PRO-TOZOA. PBI-MITIVE ANI-MALS, withoutGerra-lamellseor Intestine,or Ccelom orHsoraolymph.

I.Protozoa.

1. Ovularia. •< 2.I 3.

. Monera.Amcebina.Gregarinse.

J 4. Acinetoe.\ 5. Ciliata.

Second principal (group of theAnimal King-dom : ANM-MABIA. INTEB-MEDIATB ANI-MALS (blood-less intestinal -animals), withtwo primaryGerm-lamella)and Intestine,but withoutCcelom andHreraolymph.

II.Zoophytaltt

3. Spongise.

(Coelenterata). [ 4 - Acaleplias.

III.Acoelomi.

5. Acoelmio

8. CoraUa.9. Hydromedusse.

10. Ctenophoraj.J 11. Archelminthes.\J

(Verines I). \ 12. Plathelminthes.

Third principalgroup of theAnimal King,dom : HJKMA-TABIA.. BliOODA N I M A Ii S

(blood-carryingintestinal ani-mals), animalswith two pri-mary Germ-lamellae, withCoelom andHEemolymph ;all having atthe same timea muscular anda nervous BJS-tein.

IV.Ccelomati.

V.Mollusca.

VI.Echinodermata.

VILArthropods.

VIII.Vertebrata.

6. Coelomati<VermesII).

Brachiopoda. •{ 20.pi.

Otocardia. J. 22.[23.

Colobrachia. •< n~'

Lipobrachia. A at]

CaridBa. -{ 28.

[12. Tr;[29.

acheata. •< 30.I SI.

Acrania. <j 32.

Monorrhina. < ~/

T35.Anamnia. < 36.

[37.r38.

Amciota. < 39.1.40.

Neinatelminthes.Polyzoa.Tunica ta.Rhynchoccela.Gephyrea.Rotatoria.Annelida.Spirobranchia.Lamellibrauchia.Cochlides.Cephalopoda.Asterida.Ciinoida.Echinida.Holothurise.Cruistacea.Arachnida.Myriapoda.Insecta.Leptocardia.Cyclostoma.Pisces.Dipnensta.Halisauria.Amphibia.Keptilia.Avea.Mammalia.

THE GASTUAEA-THEOB.Y, ETC. 247

IV.—Monophyletic Stem-structure of the Animal Kingdom,founded on the Gastraea- Theory and the Homology of theGerm-lamellce.

•1

VEBTEBBATA.

ABTHBOPODA. I

ECItlNODEBMATA. MOLLTTSCA.

CCBLOMATI(Vermes with body-cavity.)

ZOOPHYTA(Ccelenterata.)

Acalephse.

Spongise.

Plathelminthes.

ACCELOMI(Vermes without body-cavity.)

Protascus.

Gastraea radialis(scdcns).

Prothelmis.

IGastraea bilaterali3

(repena).

GABTEABA.(Ontogenesis: Gastrula.)

si

K

S |w 4

-S S

tll

Planseada(Ontogenesis: Planula.)

PHOTOZOA.

Acinetic.Ciliata.

Synamo3b8e(Ontogenesis: Morula.)

Infusoria.

Gregarinaj

AMCEBJi.(Ontogenesis: Ovulum.) ? ? ?

I I I IMONEBA. Monera.

(Ontogenesis: Moneruia.)