SEGMENTATION AND PKYLOGBNY OF ARTHKOPODA. 4 6 9

Notes on the Segmentation and Phylogeny ofthe Arthropoda, with an Account of theMaxillae in Polyxenus lagurus.

George II. Carpenter, B.Sc.Loiid., M.R.I.A.,Professor of Zoology in the Royal College of Science, Dublin.

With Plate 28.

IN the introductory note appended to his article on "TheStructure and Classification of the Arthropoda" (19),Prof. B. Ray Lankester mentions " two authors who,while recently writing general essays" on the same subject,unfortunately overlooked his article, with its companionarticle on the Arachnida (20), as originally published in the'Encyclopaedia Britannica.' These authors are the late Pro-fessor A. S. Packard, whose paper (24) was read before theAmerican Philosophical Society in April, 1903, and the presentwriter, whose essay (3) was communicated to the Eoyal IrishAcademy in the succeeding month. It is a pleasure torespond to Prof. Lankester's courteous invitation to discussfurther in this JOURNAL the questions raised, in the lightof his latest invaluable contributions which, as he hopes,will now surely "not fail to come under the notice ofzoologists."

The Unity of the Arthropodan type.

The two papers published almost simultaneously by Packardand myself form a striking object-lesson on the wide diver-gence of opinion among students of Arthropod relationships.

470 GEORGE H. OAHPENTER.

While he advocates the dismemberment of the Arthropodainto five separate phyla, I support strongly the older viewthat they must be regarded as a " natural" monophyleticgroup. It must be admitted that most modern zoologistswho have written on the subject incline towards the poly-phyletic origin of the Arthropoda, though they may not goso far as Packard in separating from each other the variousclasses [Hutton, etc. (14)]. All the more cheering, therefore,has been Lankester's vindication of the essential unity of theArthropodan type, and in the ' Encyclopaedia' articles hisposition is even stronger than in the earlier paper (18) quotedin my recent essay.

It may be objected that, with such differences of opinionas to the theoretical interpretation of well-known facts,farther discussion must prove useless, and that fresh factsare needed before any secure conclusions can be reached.But, if the fundamental assumptions under ly ing allmodern phylogenet ic speculat ion be accepted, itseems to mo that we have plenty of facts from which to arriveat a conclusion, and that our conclusion can only beLankester's. Unless zoologists as a whole have " followedwandering fires," in believing that "community of descent isthe bond which is partially revealed to us by our classifica-tions," they must surely admit that the " remarkable anddistinctive features of structure which hold the Arthropodatogether render it impossible to conceive of them as having apolyphyletic origin."

Those zoologists who wish to divide the Arthropoda intoseveral distinct phyla constantly refer to the most obviousexternal features of the group—the hard, segmented exo-skeleton and jointed limbs—as conceivably due to " converg-ence " or " homoplasy." But such a superficial view over-looks the profoundly important internal characters whichdistinguish the Arthropoda: the reduced coelom; the heartwith paired openings, leading from a " pericardium" made upof greatly enlarged blood-spaces; the mesodermal excretorytubes; the apparent absence of true nephridia; the absence

SEGMENTATION AND PHYTOGENY OP ARTHROPODA. 471

of ciliated epithelium; the possession of striated musculartissue. And if it be objected that any of these are the neces-sary accompaniments of a hard exoskeleton and jointedlimbs, let it be remembered that all of them (except the twolast-mentioned)1 are displayed by that class of Arthropoda(the Malacopoda) whose members are not arthropodous. Imust confess that to me belief in the polyphylotic origin ofthe Vertebrata, the Echinoderma, or the Mollusca, might bemaintained with as great—or as little—reason as belief inthe multiple descent of the Arthropoda.

Numerical Correspondence in Segmentat ion.

Tn my own recent essay the endeavour was made to provean exact numerical correspondence in segmentation betweentypical Crustacea, Insecta, and Arachnida. The establish-ment of such a fact would conclusively demonstrate that theArthropoda are monophyletic, since the independent originof three classes of similarly formed animals with exactly thesame number of primitive segments is surely incredible. Intrying some years ago to trace homologies between theappendages "of the Crayfish, Cockroach and Scorpion, I wasastonished at the apparent identity in the number of segmentsin those three animals, and it was with much gratificationthat I afterwards found the same correspondence to havebeen suggested by Huxley (15) nearly fifty years ago, whenhe wrote : " I venture to think it a matter of no small momentif it can be proved that a Lobster, a Cockroach and a Scorpionare composed of the same primitive number of somites."

The facts and arguments that can be brought forward infavour of this contention have been set forth in my recentpaper with considerable detail. The appended table showingthe correspondence in segmentation between members of thevarious arthropod classes will serve for the present occasionas a sufficient summary of the evidence, and the referenceswill enable those interested to estimate the authority on

1 Hewitt lias, however, lately pointed out that the muscle-fibres which workthe jaw-levers of Peripaf.us are striated ('Manchester Memoirs,' vol. I, p. 4).

Somites.

i

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

472

SCOIil'tONIDA.

Eyes

Rostrum

Cheliceroe

[Vestigial seg-ment in Bpeira].

Lendl (22)Pedipalps

1st legs

2nd „

3rd „

4th „

Sternum (?). Pre-genital segment.

Brunei- (2)Genital oper-culuni, $ <j>]

Pectines

1st lung-books

2nd

3rd

4th

7th abdominalsegment

1st tail-segment

2nd

3rd

4th

Anal segment

Sting

TABM

ARACHNIDA,

MKKOSTOHATA.

Eyes

ChelioeriB

1st simple limbs

2nd

3rd

4th

Paddle-limbs

Chilaria

G-enital oper-culum, $ ?1st gill-plates

2nd

3rd

4th

5th

7th abdominalsegment

1st tail-segment

2nd

3rd

4th

Anal segment

Telson

SHOWING NUMERICAL CORRESPONDENCE

Pl'CNOGONIIIA.

Eyes

Cheliceroe

Palpi

Ovigerous legs

1st legs, ?

2nd „ ?

3rd „ $ ?

4th „ $ ?

(Abdomenreduced andcondensed)

-

-

—

-

—

—

-

Anal segment (?)

CRUSTACEA.

Eyes

Feelers (Triarth-rns) Beecher (1)

1st biramous heaclimbs

2nd

3rd

4th

1st trunk-limbs

2nd

3rd

4tU

5th

6th

7th

8th

9th

10th

11th

12th

13th

14th

15th'Number of trunksegments highly

variable)Anal segment

Telson

LKMOSTRACA.

Stalked eyes

Antennules

Antennae

Mandibles

Maxilluhe

Maxilloe

1st thoracic limbs

2nd

3rd „

4th „

5th

6th „ ?

7th

8th „ $

1st pleopods

2nd

3rd

4th

5th

6th

Limbless seg-ment

Anal segment

Furca

1 The signs <J $ indicate the

OF SEGMENTATION IN THE ARTHROPODA. 473

MllAOOSTUA.0CAsta-cus).

Stalked eyes

Antennules

Antenna)

Mandibles

MaxilluliB

Maxillse

1st maxillipeds

2nd

3rd

Chela;

1st walking legs

2nd „ ?

3rd

4th „ $

1st pleopods

2nd

3rd

4th

5th

[Segmentrepresented in

Grnathophausia],See Sars (26)

Uropods

/•Anal segment

1 Telson

Eyes

Feelers

Tritocerebralrudiments.Wheeler (30),

TJzcl (29)

Mandibles

Maxillulae.Hansen (6),Folsom (4)

1st niaxillae

2nd „(labium)

1st legs

2nd „

3rd „

1st abdominalsegment

2nd

Cereopods.Heymons (9)

Anal segment

Median cercopodin Thysanura

and Ephemerida

Feelers

Mandibles

Maxilluloe.Hanson (7;,

Carpenter (3)1st maxilla;

2nd „(labium).

Hausen (7)

1st legs

2nd „

3rd „ «??

4th „

5th „

6th „

7th „

8th „

9th „

10th „

11th „

12th „

Reduced limbs

Ceroopods

Anal segment

DIPLOPODA(Polyxenus).

Eyea

Peelers

Mandibles

Maxillute[Figs. 1—5, mxl, of

present paper)1st maxillae

2nd „(labium)

(Figs. 1—5, la, ofpreseat paper)

1st legs

2nd „

3rd „ 3 ?

4th „

5th „

6th „

7th „

8th „

9th „

10th „

ruth „Il2th „

13th „

Limbless seg-ment

Anal segment

CH1LOPODA(Iiithobiua).

Eyes.[Pre-antennalrudiment inSeolopendra.Heymons (11)J

Feelers

Tritocerebralrudiments.Heymons (11)

Mandibles

1st maxilla;

2nd „

Poison-feet

1st legs

2nd „

3rd „

4th „

5th „

6th „

7th „

8th „

9th „

10th „

11th „

12th „

13th „

14th „

15th „

Genital limbs,3 ?

Anal segment

positions of the genital openings.

474 GEORGE H. CARPENTER.

which rests the existence of each somite, not evidentlypresent in the various animals.

The conclusion to be drawn from this numerical corre-spondence in segmentation between typical members of theleading Arthropod classes is not merely that there must haveexisted common ancestors of all, with distinctively arthro-podous characters, but that these ancestors must havepossessed the definite number of segments still traceable intheir descendants. It follows from this that Arthropods withvery numerous segments—Apus, Julus, or G-eophilus, forexample—must be regarded as, in that respect at least,specialised forms. " Eich segmentation " has been so freelyassumed to be necessarily a primitive character, that myview will probably not find ready acceptance with manystudents. For the present I simply lay stress on the fact thatthe Symphyla (which combine so strikingly tlie characters ofInsects, Centipedes and Millipedes) and Polyxenus (whichbelongs to the primitive diplopodan order Pselaphognatha)both exhibit the typical and definite number of segmentscharacterising "a Lobster, a Cockroach, and a Scorpion."

Kinship between Insec ts and Crustaceans .

Perhaps the point which needs strongest enforcement inorder to recall zoologists to a reasonable view of the Arthro-poda as one phylum is the somewhat close relationship thatexists between Insects and Crustaceans. I have already (3,pp. 343-7) discussed this relationship at some length, and thearguments in favour of it are strongly pressed in Lankester'sarticle (19, p. 573). The existence in the Collembola andThysanura of maxillulas whose true nature as a pair of appen-dages between the mandibles and first maxillae has beenestablished by Hansen (6) and Folsom (4), shows the closestcorrespondence between the Insectan and the Crustaceanhead, with which the foremost trunk-segment undergoes moreor less complete fusion, its appendages forming the firstmaxillipeds in the Malacostraca and the second maxilla?

SEGMENTATION AND PHYLOGENY OF ARTHROPOD A. 475

(labium) in the Insects. Together with these correspondenceswe find (see Table, p. 472) an exact numerical agreement betweenthe trunk-segments of an Insect and those of a Malacostracan.Heymons has shown (9) that the cercopods of an insect belongto the embryonic eleventh abdominal segment, which, duringdevelopment, fuses with the tenth—the evident cercopod-bearing segment in the adult. That an exactly similar fusionof the hindmost trunk-segments has taken place in the Mala-costraca is shown by the presence of an additional somite inthe Leptostraca, and by the structure of the uropod-bearingsegment—evidently composed of two fused somites—in certainSchizopods of the genus Gnathophausia described and figuredby Sars (26)}

- I rejoice to find myself in such close agreement withLankester's main position, for if the kinship of Insects toCrustaceans be generally admitted, belief in the polyphyleticorigin of Arthropods must be rapidly given up. He, indeed,considers the relationship between the two classes closer thanI do, for he writes: " It seems probable that in the case ofthe Hexapoda, at any rate, the point of departure [from theCrustacean main stem] was subsequent to the attainment ofthe nomomeristic character presented by the higher grade ofCrustacea." My view is that the most primitive Crustaceawere nomomeristic, and that the ancestors of the Insects andtheir allies must be sought far down the Crustacean stem.This difference of opinion in details depends on the differencesbetween our respective estimates of the relationship of Insectsto the various classes of " Myriapods," and between our viewsas to the nature of the most primitive Crustaceans.

Relat ionships between Insecta , Chilopoda, andDiplopoda.

Turning, then, to the discussion of these details, I take firstthe question of the relationship between Insects and theother classes of tracheate Arthropods. It is satisfactory to

1 I am indebted to my friend Dr. W. T. Caiman for kindly bringing thisfact to my notice.

476 GEORGE H. CARPENTER.

find Lankester's support given to the necessary abolition ofthe " Class Myriapoda." It surely cannot be long beforezoologists generally will agree that the Chilopoda standnearer to the Insecta than to the Diplopoda. This relation-ship of Centipedes to Insects would, however, be distinctlyminimised by the acceptance of Lankester's suggestion (19,p. 570) that the pre-antennal, vestigial appendages in theembryo of Scolopendra, as described by Heymons (11),correspond to the functional feelers of Insects, and thatthe feelers of Centipedes must therefore be compared withthe trito-cerebral vestiges of Insects. Heymons refers thepre-antennal vestiges which he discovered to the hinderregion of the protocerebron, and they may be most probablyregarded as representing the eye-stalks of primitive Crusta-cean ancestors. Further, Heymons detected in Scolopendrathe presence of a trito-cerebral segment, thus showing theclosest possible agreement between the Chilopodan and theInsectan head. I regard the two pairs of maxilke in acentipede as corresponding with the maxillulee and firstniaxillas of the Aptera; the foremost trunk-segment, whoseappendages are the poison-feet, will then represent thesecond maxillary (labial) segment of Insects. This segmentis incompletely fused with the head in many insects—thecervical sclerites of the Cockroach apparently belong to it;and there can be little doubt that it has become associatedindependently with the head in various Crustacean orders(such as the Amphipoda and the Isopoda) as well as inInsects and Centipedes.

But while the Chilopoda must be regarded as near alliesof the Insecta, the Diplopoda are by Lankester removed faraway from these classes. This is one of the most importantof his interpretations with which I am unable to agree. Hecontrasts the mono-prosthomerous condition of the head inDiplopods with its triprosthomerous condition in Chilopodsand Insects, and states that in the first-mentioned class "onlyone somite following the first post-oral or mandibular seg-ment has its appendages modified as jaws " (19, p. 556).

SEGMENTATION AND PHTLOGENY OP ABTHEOPODA. 477

Now, the head of a Diplopod cannot be regarded as mono-prosthomerous. The ocular segment must be accounted for,and its presence makes the antennal segment deutocerebralas in Insects. As yet we have no embryological evidence ofthe tritocerebral segment of any millipede, but if the appen-dages of the diplopod head can be shown by comparativemorphology to correspond with those of the insectan head,we may expect with confidence that the existence of thetritocerebral segment in an embryo Millipede will one day bedemonstrated.

The Syrnphyla are regarded by Lankester (19, p. 567) asbelonging to the Diplopoda. The forward position of theirgenital aperture shows, indeed, that they have affinity withthat class, but it is impossible to accept Schmidt's contention(28) that they are specialised millipedes. The aspect of thehead and feelers of Scolopendrella has struck all observersas distinctively thysanuran. It is certain that there are twoevident pairs of maxilla; behind the mandibles, as shown byHansen (7) and Grassi (5); and it occurred to me that if thecharacteristic maxillulas of the Collembola and Thysanuracould be demonstrated in Scolopendrella, its affinity with thoselower insects could no longer be regarded as doubtful. Ifound the maxillulas, and figured them in my recent paper(3, fig. 3) ; and while that paper was in the press a briefaccount of them, with figures, was published by Hansen (7).

Scolopendrella, then, has three pairs of mouth-appendagesbehind the mandibles ; and when we find that in the numberof its trunk-segments it agrees exactly with a typical insect,and when we remember that in many Thysanura nearly allthe abdominal segments carry short appendages, we cannotdeny the near relationship of the Symphyla to the Aptera.And admitting such an affinity, without denying the affinityof the Symphyla to the Diplopoda, we bring all the tracheate 1

classes near to one another, for a rather close kinshipbetween the Insecta and the Ohilopoda is admitted on allhands.

1 I do not include the Arachnids among the " tracheate " classes.

478 GEOEGB H. CARPENTER.

The Maxillas of Polyxenus .

I am now glad to be able to show that Polyxenus—anundoubted Diplopod—closely resembles the lowest Insectaand the Symphyla in the structure of its mouth-organs.The gnatliochilarium of a millipede is generally believed, onembryological grounds, to consist of one pair only of maxilla?,although its structure strongly suggests the presence of twopairs, the anterior of which has come—as in Scolopendrella—to lie externally to the hinder. Latzel, in his description(21) - of Polyxenus, states that the nature of its gnathochi-larium is doubtful, a pair of reduced • appendages with con-spicuous palps being evident, and iii front of these a flatplate, apparently comparable to the hypopharynx of ordinarymillipedes.

The collection of Diplopoda in the Dublin Museum con-tains numerous specimens of Po lyxenus lagurus , collectedmany years ago by the late Robert Templeton j my friendDr. R. F. Scharff has kindly given me facilities for examiningthis material. By dissecting heads, partially cleared inpotash, under a compound erecting microscope, with fineneedles, one can determine accurately the arrangement ofthe parts. The palps, very imperfectly- segmented, rugose,and spiny (figs. 1, 4 mx. p.), and the rounded lobes, alsorugose and spiny (fig. 1, mx. lo.), each partially divided intoa larger posterior and a smaller anterior section (figs. 4, 5,mx. lo.), are borne upon basal sclerites (figs. 1, 4, 5, mx.)which are attached proximally to the ventral head-skeleton,and fused distally and centrally with the labium (see figs.1-4, la). Otherwise, however, they lie within (i. e. anteriorlyto) the labium. This latter organ consists of a nai-row,transverse basal membrane (fig. 1, la') attached to the ventralhead-skeleton, and bearing a pair of broadly trapezoidalsclei'ites (fig. 1, la) which meet in a central, longitudinalsuture. Tbis labium clearly corresponds with the internalstipites of the typical diplopodan gnathochilariunr, andrepresents, as I believe, a reduced second pair of maxillae.

SEGMENTATION AND PHYLOGENY OP AKTHllOFODA. 479

The basal sclerite of the palp-bearing maxilla in Polyxenusagrees with the external stipes of the Julid gnathochilarium.These are the first pair of maxilla?, and in Polyxenus theylie for the most part in front of, not exteriorly to, the secondniaxillaa (labium). Thus we see the specialised diplopodanstructure iu process of formation from two pairs of jaws.In the relative positions of maxillte and labium, Polyxenusis more primitive than Scolopendrella, in their partial fusionmore specialised.

When the hind head skeleton of Polyxenus is viewed from'within, we find that the flat plate seen and figured byLatzel, consists not of the tongue only but of that organ(figs. 1, 2, 4 li) together with a pair of maxillulas (figs.1-5, mxl). The outer edge of these jaws can be clearly seenfrom behind (fig. 1), and when the maxillula is viewed fromwithin (fig. 2) or isolated (fig. 3) its likeness to the corres-ponding appendage in Scolopendrella or a Springtail isunmistakable. It is a delicate, transparent, chitinous platewith outer and inner lobes, each with two or three promin-ences and the inner with several bristles at the tip. Onits posterior face, close to the tip, the inaxillula bears a longflagellate process (figs. 1, 3, 4<fl.) which assumes various posi-tions with reference to the other structures (cf. figs. 1, 3)in different individuals which I have studied. The inner lobeof the maxillula is in close contact with the tongue, and thelateral region of the tongue unites with the hinder face of themaxillula near its outer edge (figs. 1, 2 li).

When we remember that the maxillular segment has onlyonce been detected in an embryo insect, it is not surprisingthat it has hitherto been overlooked by the few zoologistswho have studied the development of millipedes. The generalagreement of embryologists that the diplopodan gnathochi-larium arises from one pair only of maxillary appendagesis, however, a more serious objection to the interpretation ofthe structure which I, following Hansen (6), advocate here.But Heymons (10), in his account of the germ-band of Julusand Glonieris, describes a post-maxillary segment which he

480 GEORGE H. CARPENTER.

states to be always limbless. I confidently anticipate futureproof that the central portions of the gnathochilarium reallybelong to this post-maxillary segment. In any case, thisembryonic segment supports the view that the head of aDiplopod agrees closely with that of an Insect—an agreementnow shown more clearly by the presence of maxillulas inPolyxenus. And considering that Polyxenus, which showsthis agreement so unmistakably, has exactly the same numberof trunk-segments as Scolopendrella or a Thysanuran, wecannot doubt the near relationship of Insects to Millipedes.

Isola ted Posit ion of the Malacopoda.

The Malacopoda (Peripatidae) must certainly be separatedwidely from the tracheates, as a grade of Arthropoda farlower than any other class. Belief in the polyphyletic originof Arthropods has been largely supported by a supposedclose relationship between the Malacopoda, the Chilopoda,and the Insecta. But a study of the characters of the" Protarthropoda" and " Euarthropoda" as given byLankester (19, pp. 564-6) must surely convince the readerthat Insects and Centipedes are much more nearly related toCrustaceans than to Peripatids.

Relat ionship between Crustacea and Arachnida.

My views on the general subject of the relationship betweenInsecta and Crustacea agree, then, in the main with Lankester's,while we differ on some points of detail. It is even so withregard to our interpretation of the kinship between Crustaceaand Arachnida. He suggests (19, p. 573) that the mostprimitive Crustaceans " were not very far removed from theaquatic ancestors (Trilobites) of the Arachnida, but difieredessentially from them by the higher specialisation of thehead," while in my essay (3, p. 349) occurs the statementthat " there is no difficulty in tracing back the Merostomata,the Xiphosura, and the Trilobita to a common ancestry; andthus the Arachnida as a class, like the Insecta, have been

SEGMENTATION AND PHYLOGKNY OF ARTHROPODA. 4 8 1

evolved from Crustaceans," I firmly believe that, in anyrational system of Arthropod classification, the Merostomataand Xiphosura must be regarded as Arachnids. Lankesterclaims that the Trilobita also are Arachnids, while in myopinion their possession of feelers and biramous limbs shouldlead us to consider them as Crustaceans. This is, however,rather a question of terminology than of principle. Lankesteradmits that the Trilobita had a common ancestry with theCrustacea, and I suggest " that the Arachnida arose from thebase of the Trilobitan branch rather than from the mainCrustacean stem."

The particular question on which Lankester and I are notin agreement is the nature of the segmentation of the proto-Arachnida. Regarding the Arachnida as descended fromthe Trilobita, and laying stress on the indefinite and oftenrich segmentation exhibited by many members of the latterorder, he suggests that the definite number of primitivesomites characterising typical Arachnids has been reachedby reduction from an originally anomomeristic condition.Believing, on the other hand, that the number of segments inan Arachnid agrees exactly with the number in a typicalCrustacean or Insect, I hold to the view that the ancestors ofall three classes possessed such a definite segmentation. Theindefinite segmentation of the Trilobita presents no difficultyto this view. I have shown in my recent paper (3, p. 333)that the average number of trunk segments in the genera ofTrilobites increases steadily as we trace their history fromCambrian to Carboniferous, and that among the most primitivemembers of the group known (Olenellus) were species withonly sixteen trunk-segments. It is reasonable, therefore, tosuppose that the richly-segmented Trilobites were developed,like the Centipedes and Millipedes, from ancestors withmoderate and definite segmentation.

This agreement in the number of body-segments among theCrustacea, Insecta and Arachnida, was long ago suggested,as mentioned above, by Huxley (15). Lankester, in referringto Huxley's view (19, p. 545), rejects it on the ground that

VOL. 49 , PAET 3.—NEW SERIES. 35

482 GEORGE H. CAHPENTER,

the older authors did not consider the nature (i. e. whetherprimitively pre-oral or post-oral) of the cephalic somites, andthat the head of an Arachnid is less complex than that of aCrustacean or an Insect. My own belief is that the olderauthors' concept of a primitive Arthropod head with fivelimb-bearing somites (six if the eyes be regarded as appendi-cular), all of which are now known to have been originallypost-oral, will yet be recognised as sound. Such a headclearly characterised the trilobites, and among them onesomite only—that bearing the simple feelers of Triarthrus—had, in addition to the ocular somite, become pre-oral. Thesefeelers must be compared with the antennules, not with theeye-stalks, of a typical Crustacean, and the foremost pair ofa trilobite's biramous head-limbs (on whose segment themouth apparently opened) with the Crustacean antennas.This latter pair of limbs—behind which the mouth has shiftedin Arachnids as in all recent Crustacea—are represented bythe Arachnid cheliceras. Embryological evidence for thiscomparison—suggested by many students of Arthropod mor-phology—might be found in the vestigial feelers describedby von Jaworowski (16) in the embryo of the spider Trochosa,but the appendicular nature of the structures seen by him isopen to doubt. More probably the trilobitan antennules arerepresented by the paired rudiments whence the Arachnidrostrum arises. The nerves of the rostrum take their origin,according to Korschelt and Heider (17, vol. 3, p. 13), from asmall unpaired section of the brain in front of the cheliceralganglia and behind the large protocerebral lobes whence theoptic nei'ves arise. Brauer's description and figures of thedeveloping scorpion's brain (2) further confirm the view thatthe cheliceral ganglia are tritocerebral.

The presence of three free leg-bearing somites in the Soli-fugida, the Palpigradi, and the Pycnogonida suggests thatthe arachnid cephalothorax has been formed by the union ofthree trunk-segments with the primitive Arthropodan head.If this suggestion be accepted it is found that the Arachnidaagree exactly with the typical Insecta and Crustacea in the

SEGMENTATION AND PHYLOGMY OK ABTHltOPODA. 483

number of their trunk-segments (see Table, p. 472), but thatthere is a missing head-segment in all Araclinida except thePycnogonida. The only embryological evidence for such a seg-ment is afforded by the evanescent somite with vestigial limbsdescribed by Lendl (22) in the embryo of the spider Epeira,between the somite of the cheliceras and that of the pedipalps.Unfortunately, Lendl gives no figures, but his description isprecise, and the late appearance of the head-segments in theembryonic development of Arachnids generally makes it easyto believe that a segment has disappeared from that region.The suggested homology between the appendages of this lostsegment and the palps of a Pycnogon brings the four pairs ofwalking-legs among the Pycnogonida into correspondencewith those of spiders. However, Hodgson's (12) recent dis-covery in the Antarctic seas of a Nymphonid, and his re-discovery of a Oollosendeid (13) with five pairs of legs maybethought to render this comparison of little value; for ifPentairymphon and Decalopoda be primitive forms, we mustconsider the Pycnogonida as probably descended from an-cestors with many pairs of similar legs, most of which havedisappeared successively from behind forwards. But the fifthpair of legs in these genera may possibly represent a com-paratively new development, and, in either case, if the viewnow advocated be accepted, we find that the head of aSolifugid, of a Pycnogon, of a Trilobite, and of a typicalCrustacean correspond exactly. Our conclusion is, there-fore, that the Aiachnida must be traced back to proto-Trilobitan ancestors which possessed a head with five and atrunk with fifteen limb-beai-ing segments.

As to the relationships between the orders of Arachnida,my views already published agree in all essential points withthose of Lankester and Pocock (20), except that I wouldassign to the Solif ugida and the Palpigradi a more primitiveposition than they do in the scheme of arrangement. Thefree thoracic segments of those animals forbid us to derivethem from a scorpionoid form in which the segments of thecephalothorax had already become aggregated. In their

484 GEORGE H. CARPENTER.

tracheal respiration, however, as in many other characters,the Solifngida are highly specialised. I must confess myselfunable to understand how some students of Arachuid mor-phology have concluded that tracheal tubes preceded lung-books in the evolution of the class.

The position assigned by Lankester to the Pycnogonida—within the Arachnid class, but at a lower grade than all theother orders from the Merostomata onwards—agrees closelywith my own views as shown in. the genealogical table whichaccompanies my recent paper (3, pi. vi). Pycnogons cer-tainly differ from typical Arachnids so markedly that theirseparation in a distinct sub-class, as proposed by Lankester,is abundantly justified. But their division into three orders,as proposed several years ago by Sars (27), and now, undernew ordinal names, by Pocock (20, pp. 224-5) is open toserious objection. Despite differences in points of detail, thegenera of Pycnogonida show such fundamental unity ofstructure combined with such varying relationships amongthemselves that it is best to comprise them all withm a singleorder. Certainly their separation into ordinal groups formedon the presence .or absence of the chelicerae or the palps mustlead to many unnatural associations. For example, therelationship of the Pallenidaa to the Nymphonidaj which isexpressed by the scheme of Sars and Pocock is undeniable.But within the Pallenidse must be included forms—Phoxi-chilidium and allies—with cheliceras variously developed andwith palps vestigial or absent, which lead on towardsPhoxichilus, a genus placed, however, by Sars and Pocock ina distinct " order" along with Pycnogonum. Beyond thetotal absence of cheliceras and palps, Phoxichilus and Pycno-gonum have little in common, and the loss of a particularpair of appendages might readily be sustained independentlyby two or more divergent genera of a degenerate group.

Ancestry of the Crustacea.

If, then, the common ancestors of Arachnids, Crustaceans,Insects, Centipedes and Millipedes, possessed a definite and

SEGMENTATION AND PHYLOGENY OF AETHEOFODA. 485

limited number of segments, and if these ancestors wereessentially Crustacean in nature, we are precluded from con-sidering A pus or any similar form as the representative ofsuch an ancestral stock. And this suggests the last, andperhaps the most important question on which my recently-published views are in disagreement with Lankester's: Whatwere the earliest Crustacea like, and what was their relation-ship to the Annelida ?

On the assumption that Apus and its allies represent themost primitive of living Crustaceans, the foliaceous appen-dages of those animals are derived directly from Annelidanparapodia^ and the ancestry of the Arthropoda is traceddirectly to richly-segmented Chsetopods. The typical bira-mous Crustacean limb is, on this view, to be understood as aspecialization of the Branchiopodan limb owing to the sup-pression of four of the endites, and the elongation of theremaining two, as explained by Lankester (19, pp. 551-9).But there is much reason for regarding the Branchiopoda assomewhat specialized animals. The extreme reduction ofboth pairs of feelers, the absence of a palp on eithermandible or first maxilla (maxillula) and its vestigial con-dition on the second maxilla, the modification of certainendites of the first trunk limb as tactile organs—all these areundoubtedly specializations, and it is not unreasonable tosuppose that the foliaceous condition of the other trunk limbmay be regarded as a specialization. Such foliaceous deve-lopment is most nearly matched, among other Crustacea, inthe maxillae and anterior maxillipeds of many genera belong-ing to different orders. It is a modification thataccompanies excessive crowding of the appendages,and in Apus we may readily conceive that the secondarydevelopment of very numerous pairs of limbs has led to theircrowding together and to the correlated development of theleaf-like endites and exites.

For limbs like those of Apus are quite exceptional amongthe Crustacea; while the typical biramous limb, with or with-out an exite (epipodite) occurs in members of every crustacean

486 GEORGE H. CARPENTER.

order and is present in every Nauplius larva. Surely, there-fore, the presumption is that the foliaceous limb has beenelaborated from the biramous and not that the latter hasbeen simplified from the former. Now, the Crustacean ordersin which the biramous (naupliar) condition of the limbs ismost strikingly preserved are the Copepoda, the Trilobita andthe Leptostraca. The trunk-limbs of the last are often men-tioned as phyllopodous in character, but the foliaceous struc-tures on the thoracic leg of Nebalia are the exopoditeand epi-podite of a typical crustacean appendage, while in Paranebalia—see Sars (25)—the exopodite is slender and fringed, andthe epipodite quite small. Similarly, the close comparisonoften made between the Branchiopoda and the Trilobita ismisleading, for trilobitan limbs—according to Beecher (1)—are biramous, and their foliaceous condition in the tail-regionis due to a flattening of the segments of a typical endopodite(20, p. 217, fig. 35), not to the presence of numerous enditeslike those of Apus. My ideal ancestral crustacean is nottherefore a Branchiopodan, but a form combining the mostprimitive characters of Triarthrus, Nebalia and Calanus. Inthis view I am largely in agreement with Sars (25), whosuggests that the Nebaliidas sprang from Copepod-likeancestors, and that the Branchiopoda are a " considerablymodified" offshoot (less primitive than the Leptostraca) fromthe same stem. I cannot follow Sars, however, in denying nearrelationship between the Leptostraca and the Malacostraca.He would derive the latter group independently fromnaupliiform ancestors, but the generally accepted view thatthe Leptostraca are to some extent ancestral to the Mala-costraca has much evidence in its favour. I have alreadysuggested (p. 475, supra) how the two hindmost abdominalsegments have undergone fusion in the Malacostraca as in theInsecta.

If we accept the nauplius larva as representing the ancestralstock of the Crustacea, and therefore of all Arthropods, andif we follow many special students of the Crustacea in con-sidering the Copepoda as the most primitive living order of

SEGMENTATION AND PHYLOGENY OP ARTHROPODA. 4 8 7

the class, we may be tempted to regard the small number oftrunk-segments in a typical Copepod as indicating a stagebetween the naupliar condition and the segmentation of thecommon ancestors of Araclmida, Crustacea, and Insecta withtheir fifteen limb-bearing trunk-segments. But when weconsider the shortening that has undoubtedly taken place inmany Crustaceans—e. g. the Cladocera, the Ostracoda, theCirripedia—we must admit the great probability that theCopepoda have also undergone reduction in their segmenta-tion, and that the Leptostraca represent in this respect theprimitive Arthropodous condition.

Arthropoda and Annelida.

The ancestral standing of the nauplius suggested nearlyforty years ago by Miiller (23) has not been abandoned byall modern students of the Crustacea, although the tendencyof most present-day zoologists is to derive Arthropods directlyfrom elongate Annelid worms. No student can deny somerelationship between Arthropods and Annelids, but we mustask whether the Annelida were the direct ancestors of theArthropoda or whether both phyla must be traced back to animmensely remote common ancestry ?

The opinion that the Apodidae are the most primitive ofCrustacea is held by those zoologists who believe in thehomology between the parts of a polychaetan parapodinmand those of a branchiopodan limb, and in the direct deriva-tion of Arthropods from Annelids. This position, firstsuggested, I believe, by Hatschek (8), is virtually acceptedby Lankester, who considers that the most primitive Arthro-poda " arose by modification of parapodiate annulate wormsnot very unlike some of the existing Chsetopods" (19, p. 527).I have always regarded with suspicion this modern hypo-thesis, because its acceptance has led to the present wide-spread disbelief in the monophyletic origin of the Arthropoda.And it seems that many recent morphological advances havetended to sharpen the distinction between all Annelids and all

488 GEORGK H. CARPENTER.

Arthi'opods. The hasmoccelic body cavity and reduced coelomof Arthropods offer the most marked contrast to the closedvascular system and extensive coelom of Annelids. Truenephridia, so characteristic of the Annelids, are apparentlyunknown among the Arthropoda. The only characterscommon to the two groups are the metameric segmentation,the presence of paired hollow limbs, and paired coelomoductsarranged segmentally, and the general structure of thenervous system. This last-named character—on which especialstress has been laid as indicative of affinity—is a necessaryaccompaniment of metameric segmentation in animals whosebrain—whether archicerebrum or syncerebrum—is connected,by a nerve-ring surrounding the gullet, with paired lateral orventral cords, so soon as the nerve-cells become aggregatedsegmentally into ganglia. And that such aggregation hasarisen independently among Annelids and among Arthropodsis strongly suggested by the condition of the nervous systemin the Malacopoda (Onychophora), which, though essentiallyArthropoda, possess laterally-situated nerve-cords withoutdistinct ganglia. Thus the most primitive of living Arthro-pods exhibit a condition of the nervous system more primitivethan is to be found among those Annelids from which theArthropoda are believed by most zoologists to have beenevolved. The structure of the eye is, indeed, the only un-equivocal annelidan character exhibited by a Peripatid.

The more probable conclusion therefore seems to be notthat Arthropods and Polycheete Annelids stand to each otherin the relation of descendants to ancestors, but that the twogroups represent specialised collateral branches from a com-mon stock. My own view is that these common ancestorswere microscopic animals, unsegmented, or with compara-tively few segments between a broad head-lobe and a narrowtail-somite. The occurrence of the nauplius larva in somemembers of all the great Crustacean groups justifies thephylogenetic importance attached to that form by Miiller,whose views will probably, in the near future, again dominatezoological opinion on the subject. Comparison of a Nauplius,

SEGMENTATION AND PHTLOGENJ OF ARTHROPODA. 489

a Kotifer like Pedalion, and aTrocophore helps us to conceivethe hypothetical ancestral stock of Annelids and Arthropods.But the lines of evolution must, on that assumption, convergein a past so immensely remote that we are hardly justified inincluding both Annelids and Arthropods within the limits ofa single phylum. Good reason might indeed be shown forderiving the Mollusca from the same common ancestry. Weconclude, then, that the divergences between Annelids andArthropods are profound, while their correspondences arecomparatively superficial. The Arthropoda must stand, there-fore, in our zoological systems, not only as a natural monophy-letic group, but also sharply marked off from other phyla asa great primary division of the Animal Kingdom.

REFERENCES.

1. BEECHER, C. E.—" Trilobita," in Zittel's ' Text-book of Palaeontology '(English edition), London, 1900.

2. BRATJER, A.—" Beitrage zur Kenntnis der Enlwicklungsgeschichte desSkorpions," ' Zeit. wiss. Zool.,' vol. lix, 1895.

3. CARPENTER, G. H.—" On the ^Relationships between the Classes of theArthropoda," 'Proc. It. Irish Acad.,' vol. xxiv, Sect. B, 1903.

4. FOLSOM, J. W.—"The Development of the Mouth-parts of Anuridamaritima," ibid., vol. xxxvi, 1900.

5. GRASSI, B.—"Morfologia delle Scolopendrelle," 'Mem. Reale Accad.Scienze di Torino,' vol. xxxvii, 1886.

6. HANSEN, H. J.—"A Contribution to the Morphology of the Limbs andMoutli-parts of Crustaceans and Insects," 'Ann. Mag. Nat. Hist.1 (6),vol. xii, 1894 (trans), from ' Zool. Anz.,' 1893).

7. HANSEN, H. J.—"The Genera and Species of the Order Symphyla,"' Quart. Journ. Micr. Sci.,' vol. xlvii 1903.

8. HATSCHEK, B.—" Studien zur Entwickluugsgeschickte der Anneliden,"•Art. Zool. Inst. Wien,' vol. i, 1878.

9. HEYMOKS, 11.—" Zur Morphologie der Abdominalanhange bei denInsekten," ' Morph. Jahrb.,' vol. xxiv, 1896.

10. HEYMONS, 11.—"Miltheilungen fiber die Segmentierung und denKorperbau der Myriopoden," ' Sitzsb. k. Preuss. Akad. Wiss.,' 1897.

11. HEYMONS, R.—"Die Entwicklungsgeschichte der Scolopender,"'Zoolo-gica,' vol. xiii, 1901.

490 • GEORGE H. CARPENTER.

12. HODGSON, T. V.—" On a New Pjcnogonid from the South Polar Regions,"' Ann. Mag. Nat. Hist.' (ser. 7), vol. xiv, 1904.

18. HODGSON, T. V.—Scotia Collections. "On Decalopoda australis,Eights, an old Pycnogonid re-discovered," 'Proc. R. Phys. Soc. Edin.,'vol. xvi, 1905.

14. HUTTON, P. W., and OTHERS.—" Are the Arthropoda a Natural Group?"' Nat. Sci.,' vol. x, 1897.

15. HUXLEY, T. H.—" On the Agamic Reproduction and Morphology ofAphis," 'Trans. Linn. Soc.,' vol.xxii, 1859.

18. JAWOROWSXI, A. V.—" Ueber die Extremilaten bei den Embrjonen derArachniden und Insekten," ' Zool. Anz.,' vol. xiv, 1891-92.

17. KOHSCHELT, E., and HEIDER, K.—' Text-Book of the Embryology ofInvertebrates' (English edition by Bernard and Woodward), vols. iiand iii, London, 1899.

18. LANKESTER, E. R.—"Are the Arthropoda a Natural Group?" 'Nat.Sci.,' vol. x, 1897.

19. LANKESTEB, E. It.—" The Structure and Classification of the Arthro-poda," ' Quart. Journ. Micr. Sci.,' vol. xlvii, 1904.

20. LANKESTER, E. R.—"The Structure and Classification of the Arach-nida," ibid., vol. xlviii, 1904.

21. LATZEL, R.—'Die Myriopoden der Oesterreich-Ungaiischeh Monarchie,'Band 2, Wien, 1884.

22. LEKDL, A.—"Ueber die Morphologische Bedeutung der Gliedmaassenbei den Spinnen," 'Math. Naturwiss Berichte aus Ungarn.,' vol. iv,1886.

23. MULLEE, F.—'Facts and Figures in Aid of Darwin' (English transl. byDallas), London, 1869.

24. PACKAUD, A. S.—" Hints on the Classification of the Arthropoda : theGroup a Polyphyletic One," ' Proc. Amer. Phil. Soc.,' vol. xlii, 1903.

25. SARS, G. O.—' Report on the Phyllocarida collected by H.M.S.Challenger,' London, 1887.

26. SAKS, G. 0.—' Report on the Schizopoda collected by H.M.S.Challenger,' London, 1885.

27. SAKS, G. 0.—'The Norwegian North Atlantic Expedition: Pycno-gonida, Christiania,' 1891.

28. SCHMIDT, P.—" Beitrage zur Kenntnis der Niederen Myriapoden,"' Zeit. wiss. Zool.,' vol. lix, 1895.

29. UZEL, H.—"Beitrage zur Entwicklungsgeschichte von Campodeas t a p h y 1 i n u s," ' Zool. Anz.,' vol. xs, 1897.

30. WHEELEB, W. M.—" A Contribution to Insect Embryology," ' Journ,Morph.,' vol. viii, 1893.

SEGMENTATION AND PHYLOGENY OF ABTHEOPODA. 491

E X P L A N A T I O N OF P L A T E 28,

I l lustrat ing Professor Carpenter 's paper on " Notes on theSegmentation and Phylogeny of the Arfchropoda, with anAccount of the Maxillae in P o l y x e n u s l a g u r u s . "

REFERENCE LETTERS.

It. Tongue or hypopharynx. la. Labium. la'. Basal membrane of labium.mx. Maxilla, basal sclerite. mx. lob. Maxilla, lobe. mx. p. Maxilla, palp.mxl. Maxillula. jl. Flagellate process, m. add. Adductor muscle, m. aid.Abductor muscle, md. Tip of mandible. Ibr- Edge of labrum.

FIG. ].—External (posterior) view of the maxillBe of Polyxenus lagurus.The left maxilla has been removed to show the maxillula and tongue.

FIG. 2.—Internal (anterior) view of left maxillula and tongue of Poly-xenus lagurus. The central region of the tongue only is shown.

FIG. 3.—Left maxillula of Polyxenus isolated ; external (posterior) view.FIG. 4.—-llight maxilla, maxillula and apex of tongue; ventral view.FIG. 5.—Semi-diagrammatic lateral view, showing relations of maxillula,

maxilla, and labium ; the palp is displaced and truncated.

All figures are magnified 285 diameters.

Quart. Journ. Micr. Sci., Vol. 4g, N.S., PI. 28.

mxl.nx. lo.

mxl.

mx.p.

G. II. C, del. F. Buttcraiorth, iAdlani & San, ill