The literature of the Comparative Anatomy of the Arteries up to 1902

will be found in R. Wiedersheim's _Vergleichende Anatomie der

Wirbeltiere_ (Jena, 1902). The morphology of the Iliac Arteries is

described by G. Levi, _Archivio Italiano di Anat. ed Embriol._, vol.

i. (1902). (F. G. P.)

ARTERN, a town of Germany, in Prussian Saxony, on the Unstrut, at the

influx of the Helme, at the junction of railways to Erfurt, Naumburg and

Sangerhausen, 8 m. S. of the last named. Pop. 5000. It has an

Evangelical church, an agricultural college and some manufactures of

machinery, sugar and boots. Its brine springs, known as early as the

15th century, are still frequented.

ARTESIAN WELLS, the name properly applied to water-springs rising above

the surface of the ground by natural hydrostatic pressure, on boring a

small hole down through a series of strata to a water-carrying bed

enclosed between two impervious layers; the name is, however, sometimes

loosely applied to any deep well, even when the water is obtained by

pumping. In Europe this mode of well-boring was first practised in the

French province of Artois, whence the name of Artesian is derived. At

Aire, in that province, there is a well from which the water has

continued steadily to flow to a height of 11 feet above the ground for

more than a century; and there is, within the old Carthusian convent at

Lillers, another which dates from the 12th century, and which still

flows. But unmistakable traces of much more ancient bored springs appear

in Lombardy, in Asia Minor, in Persia, in China, in Egypt, in Algeria,

and even in the great desert of Sahara. (See WELL.)

ARTEVELDE, JACOB VAN (c. 1290-1345), Flemish statesman, was born at

Ghent about 1290. He sprang from one of the wealthy commercial families

of this great industrial city, his father's name being probably William

van Artevelde. His brother John, a rich cloth merchant, took a leading

part in public affairs during the first decades of the 14th century.

Jacob, who according to tradition was a brewer by trade, spent three

years in amassing quietly a large fortune. He was twice married, the

second time to Catherine de Coster, whose family was of considerable

influence in Ghent. Not till 1337, when the outbreak of hostilities

between France and England threatened to injure seriously the industrial

welfare of his native town, did Jacob van Artevelde make his first

appearance as a political leader. As the Flemish cities depended upon

England for the supply of the wool for their staple industry of weaving,

he boldly came forward, as a tribune of the people, and at a great

meeting at the monastery of Biloke unfolded his scheme of an alliance of

the Flemish towns, with those of Brabant, Holland and Hainaut, to

maintain an armed neutrality in the dynastic struggle between Edward

III. and Philip VI. of France. His efforts were successful. Bruges,

Ypres and other towns formed a league with Ghent, in which town

Artevelde, with the title of captain-general, henceforth until his death

exercised almost dictatorial authority. His first step was to conclude a

commercial treaty with England. The efforts of the count of Flanders to

overthrow the power of Artevelde by force of arms completely failed, and

he was compelled at Bruges to sign a treaty (June 21, 1338) sanctioning

the federation of the three towns, Ghent, Bruges and Ypres, henceforth

known as the "Three members of Flanders." This was the first of a series

of treaties, made during the year 1339-1340, which gradually brought

into the federation all the towns and provinces of the Netherlands. The

policy of neutrality, however, proved impracticable, and the Flemish

towns, under the leadership of Artevelde, openly took the side of the

English king, with whom a close alliance was concluded. Artevelde now

reached the height of his power, concluding alliances with kings, and

publicly associating with them on equal terms. Under his able

administration trade flourished, and Ghent rose rapidly in wealth and

importance. His well-nigh despotic rule awoke at last among his

compatriots jealousy and resentment. The proposal of Artevelde to disown

the sovereignty of Louis, count of Flanders, and to recognize in its

place that of Edward, prince of Wales (the Black Prince), gave rise to

violent dissatisfaction. A popular insurrection broke out in Ghent, and

Artevelde fell into the hands of the crowd and was murdered on the 24th

of July 1345.

The great services that he rendered to Ghent and to his country have in

later times been recognized. A statue was erected in his native town on

the Marche du Vendredi, and was unveiled by Leopold I., king of the

Belgians, on the 13th of September 1863.

See J. Hutten, _James and Philip van Artevelde_ (London, 1882); W.J.

Ashley, _James and Philip van Artevelde_ (London, 1883); P. Nameche,

_Les van Artevelde et leur epoque_ (Louvain, 1887); L. Vanderkindere,

_Le Siecle des Arteveldes_ (Brussels, 1879).

ARTEVELDE, PHILIP VAN (c. 1340-1382), youngest son of the above, and

godson of Queen Philippa of England, who held him in her arms at his

baptism, lived in retirement until 1381. The Ghenters had in that year

risen in revolt against the oppression of the count of Flanders, and

Philip, now forty years of age, and without any military or political

experience, was offered the supreme command. His name awakened general

enthusiasm. At first his efforts were attended by considerable success.

He defeated Louis de Male, count of Flanders, before Bruges, entered

that city in triumph, and was soon master of all Flanders. But France

took up the cause of the Flemish count, and a splendid French army was

led across the frontier by the young king Charles VI. in person.

Artevelde advanced to meet the enemy at the head of a burgher army of

some 50,000 Flemings. The armies met at Roosebeke near Courtrai, with

the result that the Flemings were routed with terrible loss, Philip

himself being among the slain. This happened on the 27th of November

1382.

The brief but stirring career of this popular leader is admirably

treated in Sir Henry Taylor's drama, _Philip van Artevelde_.

ART GALLERIES. An art gallery (by which, as distinguished from more

general MUSEUMS OF ART, q.v., is here meant one specially for pictures)

epitomizes so many phases of human thought and imagination that it

connotes much more than a mere collection of paintings. In its technical

and aesthetic aspect the gallery shows the treatment of colour, form and

composition. In its historical aspect we find the true portraits of

great men of the past; we can observe their habits of life, their

manners, their dress, the architecture of their times, and the religious

worship of the period in which they lived. Regarded collectively, the

art of a country epitomizes the whole development of the people that

produced it. Most important of all is the emotional aspect of painting,

which must enter less or more into every picture worthy of notice. To

take examples from the British National Gallery: pathos in its most

intense degree will be found in Francia's "Pieta"; dignity in Velasquez'

portrait of Admiral Pareja; homeliness in Van Eyck's portrait of Jan

Arnolfini and his wife; the interpretation of the varying moods of

nature in the work of Turner or Hobbema; nothing can be more devotional

than the canvases of Bellini or his Umbrian contemporaries. So also the

ruling sentiments of mankind--mysticism, drama and imagination--are the

keynotes of other great conceptions of the artist. All this may be at

the command of those who visit the art gallery; but without patience,

care and study the higher meaning will be lost to the spectator. The

picture which "tells its own story" is often the least didactic, for it

has no inner or deeper lesson to reveal; it gives no stimulus or

training to the eye, quick as that organ may be--_segnius irritant

animos_--to translate sight into thought. In brief, the painter asks

that his [Greek: aethos] may be shared as much as possible by the man

who looks at the painting--the art above all others in which it is most

needful to share the master's spirit if his work is to be fully

appreciated. So, too, the art gallery, recalling the gentler

associations of the past amidst surroundings of harmonious beauty and

its attendant sense of comfort, is essentially a place of rest for the

mind and eye. In the more famous galleries where the wealth of paintings

allows a grouping of pictures according to their respective schools, one

may choose the country, the epoch, the style or even the emotion best

suited to one's taste. According to this theory, though imperfectly

realized owing to the paucity of examples, the philosophic influence of

art galleries is becoming more widely extended; and in its further

development will be found an ever-growing source of interest,

instruction and scholarship to the community. The most suitable method

of describing art galleries is to classify them by their types and

contents rather than by the various countries to which they belong. Thus

the great representative galleries of the world which possess works of

every school are grouped together, followed by state galleries which are

not remarkable for more than one school of national art. Municipal

galleries are divided into those which have general collections, and

those which are notable for special collections. Churches which have

good paintings, together with those which are now secularized, are

treated separately; while the collections in the Vatican and private

houses are described together. The remaining galleries, such as the

Salon or the Royal Academy, are periodical or commercial in character,

and are important in the development of modern art.

[Illustration: FIG. 1.--Plan of the National Gallery, London.

North Vestibule, Early Italian Schools:

I. Tuscan School (15th and 16th centuries).

II. Sienese School, &c.

III. Tuscan School.

IV. Lombard School.

V. Ferrarese and Bolognese Schools.

VI. Umbrian School, &c.

VII. Venetian and Brescian Schools.

VIII. Paduan and Early Venetian Schools.

IX. Later Venetian School.

X. Flemish School.

XI. Early Dutch and Flemish Schools.

XII. Dutch and Flemish Schools.

XIII. Flemish School.

XIV. Spanish School.

XV. German Schools.

XVI. French School.

XVII. French School.

XVIII. British School.

XIX. Old British School.

XX. British School.

XXI. British School.

XXII. Turner Collection.

Octagonal Hall: Miscellaneous.

East Vestibule: British School.

West Vestibule: Italian School.]

State galleries of international schools.

The collections most worthy of attention are the state galleries

representative of international schools. Among these the British

National Gallery holds a high place. The collection was founded in 1824

by the acquisition of the Angerstein pictures. Its accessions are mainly

governed by the parliamentary grant of L5000 to L10,000 a year, a sum

which has occasionally been enlarged to permit special purchases. Thus,

in 1871, the Peel collection of seventy-seven pictures was bought for

L75,000, and in 1885 the Ansidei Madonna (Raphael) and Van Dyck's

portrait of Charles I. were bought, the one for L70,000 and the other

for L17,500. In 1890 the government gave L25,000 to meet a gift of

L30,000 made by three gentlemen to acquire three portraits by Moroni,

Velazquez and Holbein. The most important private gifts were the Vernon

gift in 1847, the Turner bequest in 1856 and the Wynne-Ellis legacy in

1876. Since 1905 the Art Collections Fund, a society of private

subscribers, has also been responsible for important additions to the

gallery, notably the Venus of Velazquez (1907). The gallery contains

very few poor works and all schools are well represented, with the sole

exception of the French school. This, however, can be amply studied at

Hertford House (Wallace Collection), which, besides Dutch, Spanish and

British pictures of the highest value, contains twenty examples of

Greuze, fifteen by Pater, nineteen by Boucher, eleven by Watteau and

fifteen by Meissonier. The national gallery of pictures at Berlin

(Kaiser Friedrich Museum), like the British National Gallery, is

remarkable for its variety of schools and painters, and for the select

type of pictures shown. During the last twenty-five years of the 19th

century, the development of this collection was even more striking than

that of the English gallery. Italian and Dutch examples are specially

numerous, though every school but the British (here as elsewhere) is

really well seen. The purchase grant is considerable, and is well

applied. Two other German capitals have collections of international

importance--Dresden and Munich. The former is famous for the Sistine

Madonna by Raphael, a work of such supreme excellence that there is a

tendency to overlook other Italian pictures of celebrity by Titian,

Giorgione and Correggio. Munich (Old Pinakothek) has examples of all the

best masters, the South German school being particularly noticeable. The

arrangement is good, and the methods of exhibition make this one of the

most pleasant galleries on the continent. Vienna has the Imperial

Gallery, a collection which in point of number cannot be considered

large, as there are not more than 1700 pictures. This, however, is in

itself a safeguard, like the wise provision in a statute of 1856 for

enabling the English authorities to dispose of pictures "unfit for the

collection, or not required." It avoids the undue multiplication of

canvases, and the overcrowding so noticeable in many Italian galleries

where first-rate pictures hang too high to be examined. Thus the

Viennese gallery, besides the intrinsic value of its pictures (Albert

Durer's chief work is there), is admirably adapted for study. The best

gallery in Russia (St Petersburg, Hermitage) was made entirely by royal

efforts, having been founded by Peter the Great, and much enlarged by

the empress Catherine. It contains the collections of Crozat, Bruhl and

Walpole. There are about 1800 works, the schools of Flanders and Italy

being of signal merit; and there are at least thirty-five genuine

examples by Rembrandt. The French collection (Louvre Palace, Paris) is

one of the most important of all. In 1880 it was undoubtedly the first

gallery in Europe, but its supremacy has since been menaced by other

establishments where acquisitions are made more frequently and with

greater care, and where the system of classification is such that the

value of the pictures is enhanced rather than diminished by their

display. In 1900 it was partly rearranged with great effect. The feature

of the Louvre is the Salon Carre, a room in which the supposed finest

canvases in the collection are kept together, pictures of world-wide

fame, representing all schools. It is now generally accepted that this

system of selection not only lowers the standard of individual schools

elsewhere by withdrawing their best pictures, but does not add to the

aesthetic or educational value of the masterpieces themselves. In

Florence the Tribuna room of the Uffizi gallery is a similar case in

point. Probably the two most widely known pictures in the Louvre are

Watteau's second "Embarquement pour Cythere," and the "Monna Lisa," a

portrait by Leonardo da Vinci, but each school has many unique examples.

The original drawings should be noted, being of equal importance to the

collection preserved at the British Museum. The last collection to be

mentioned under this heading is that known as the Royal Galleries in

Florence, housed in the Pitti and Uffizi palaces. In some ways this

collection does not represent general painting sufficiently to justify

its inclusion with the galleries of Berlin, Paris and London. On the

other hand, the great number of Italian pictures of vital importance to

the history of international art makes this one of the finest existing

collections. The two great palaces, dating from the 15th and 16th

centuries, are joined together and contain the Medici pictures. They

form the largest gallery in the world, and though many of the rooms are

small and badly lighted, and although many paintings have suffered from

thoughtless restoration, they have a charm and attraction which

certainly make them the most popular galleries in Europe. The Pitti has

ten Raphaels and excellent examples of Andrea del Sarto, Giorgione and

Perugino. The Uffizi is more representative of non-Italian schools, but

is best known for its works by Botticelli, Leonardo da Vinci,

Michelangelo and Sodoma, the schools of Tuscany and Umbria forming the

bulk of both collections. Admission to the galleries is by payment, and

the small income derived from this source is devoted to maintaining and

enlarging the collections.

[Illustration: FIG. 2.--Plan of the first and second floors of the

Imperial Gallery, Vienna.]

As to the ground plans of the National Gallery, London (fig. 1), and of

the Imperial Gallery at Vienna (fig. 2), it will be observed that while

the former has the advantage of uniform top-light, the galleries at

Vienna possess the most ample facilities for minute classification,

small rooms or "cabinets" opening from each large room. Special rooms

are also provided for drawings and water-colours, while special ranges

of rooms are used by copyists and those responsible for the repair and

preservation of the pictures.

State galleries of national schools.

Though not so comprehensive as the great collections just described, the

state galleries showing national schools of painting and little else are

of striking interest. In England the National Gallery of British Art

(known as the Tate Gallery) contains British pictures. The corresponding

collection of modern French art is at Paris (Luxembourg Palace), Berlin,

Rome, Dresden, Vienna and Madrid having analogous galleries. The

Victoria and Albert Museum has also numerous British pictures,

especially in water-colour, and the National Portrait Gallery, founded

in 1856, and since 1896 housed in its permanent home, is instructive in

this connexion, though many of its pictures are the work of foreign

artists. The national collections at Dublin and Edinburgh may be

mentioned here, though most schools are represented. Brussels and

Antwerp are remarkable for fine examples of Flemish art--Matsys, Memlinc

and Van Eyck of the primitive schools, Rubens and Van Dyck of the later

period. The collections at Amsterdam (Ryks Museum) and the Hague

(Mauritshuis) are a revelation to those who have only studied Rembrandt,

Franz Hals, Van der Helst, and other Dutch portrait painters outside

Holland; and in the former gallery especially, the pictures are arranged

in a manner showing them to the best advantage. The Museo del Prado is

even more noteworthy, for the fifty examples of Velasquez (outrivalling

the Italian pictures, important as they are) make a visit to Madrid

imperative to those who wish to realize the achievements of Spanish art.

Christiania, Stockholm and Copenhagen have large collections of

Scandinavian art, and the cities of Budapest and Basel have galleries of

some importance. In Italy the state maintains twelve collections, mainly

devoted to pictorial art. Of these the best are situated at Bologna,

Lucca, Parma, Venice, Modena, Turin and Milan. In each case the local

school of painting is fully represented. In Rome the Corsini and

Borghese Galleries, the latter being the most catholic in the city,

contain superb examples, some of them accepted masterpieces of Italian

art; there are also good foreign pictures, but their number is limited.

The Accademia at Florence should also be noted as the most important

state gallery of early Italian art. The central Italian Renaissance can

be more adequately studied here than in the Pitti. The "Primavera" of

Botticelli, and the "Last Judgment" by Fra Angelico are perhaps the

best-known works. The large statue of David by Michelangelo is also in

this gallery, which, on the whole, is one of the most remarkable in

Italy. Speaking broadly, these national galleries scattered throughout

the country are not well arranged or classified; and though some are

kept in fine old buildings, beautiful in themselves, the lighting is

often indifferent, and it is with difficulty that the pictures can be

seen. In nearly every case admission fees are charged every day,

festivals and Sundays excepted; few pictures are bought, acquisitions

being chiefly made by removing pictures from churches.

Municipal galleries of special schools.

Many towns own collections of well-merited repute. In Italy such

galleries are common, and among them may be noted Siena, with Sodoma and

his school; Venice with Tintoretto (Doge's Palace); Genoa, with the

great palaces Balbi and Rosso; Vicenza (Montagna and school), Ferrara

(Dosso and school), Bergamo and Milan (north Italian schools). Other

civic collections of Italian art are maintained at Verona, Pisa, Rome,

Perugia and Padua. In Holland, Haarlem, Leiden, Rotterdam and the Hague

have galleries supplemental to those of the state, and are remarkable in

showing the brilliance of artists like Grebber, de Bray and Ravesteyn,

who are usually ignored. Birmingham and Manchester have good examples of

modern British art. Moscow (Tretiakoff collection) has modern Russian

pictures, and contemporary German and French work will be found in all

the galleries of these two countries included in the municipal group.

Collections of French work are found at Amiens, Rouen, Nancy, Tours, Le

Mans and Angers, but large as these civic collections are, sometimes

containing six and eight hundred canvases, few of their pictures are

really good, many being the enormous patriotic canvases marked "Don de

l'Etat," which do not confer distinction on the galleries. Cologne has

the central collection of the early Rhenish school; Nuremberg is

remarkable for early German work (Wohlgemut, &c.). Stuttgart, Cassel

(Dutch) and Hamburg (with a considerable number of British pictures) are

also noteworthy, together with Brunswick, Hanover, Augsburg, Darmstadt

and Dusseldorf, where German and Dutch art preponderate. Seville is

famous for twenty-five examples of Murillo, and there are old Spanish

paintings at Valencia, Cordova and Cadiz.

Municipal galleries of general schools.

In Great Britain the best of the municipal galleries of general schools

are at Liverpool (early Flemish and British), and at Glasgow (Scottish

painters, Rembrandt, Van der Goes and Venetian schools). In France there

are very large galleries at Tours, Montpellier, Lyons (Perugino,

Rubens), Dijon and Grenoble (Italian), Valenciennes (Watteau and

school), while Rennes, Lille and Marseilles have first-rate collections.

Nantes, Orleans, Besancon, Cherbourg and Caen have also many paintings,

French for the most part, but with occasional foreign pictures of real

importance, presented by the state during the Napoleonic conquests, and

not returned on the declaration of peace as were the works of art

amassed in Paris. Some of the American collections have in recent years

made a great advance in their acquisition of good pictures. At Boston

(Museum of Fine Arts) all schools are represented, so too at the

Metropolitan Museum of Art in New York, which is strong in Italian and

Dutch works. Modern French and Flemish art is a feature of the Academy

at Philadelphia, at the Lenox Library (New York), and at Chicago, where

there are good examples of Millet, Constable and Rembrandt. The Corcoran

bequest at Washington is of minor importance. The best civic collection

in Germany of this class is the Stadel Institute at Frankfort (Van Eyck,

Christus, early Flemish and Italian).

Churches.

As the great bulk of religious painting was executed for church

decoration, there are still numberless churches which may be considered

picture galleries. Thus at Antwerp cathedral the Rubens paintings are

remarkable; at Ghent, Van Eyck; at Bruges (hospital of St John),

Memlinc; at Pisa, the Campo Santo (early Tuscan schools); at Sant'

Apollinare, Ravenna, primitive Italo-Byzantine mosaics; at Siena,

Pinturichio. Examples could be multiplied indefinitely--in Italy alone

there are 80,000 churches and chapels, in all of which pictorial art has

been employed. In Italy, besides the church "galleries" still used for

religious services, there are some which have been secularized and are

now used as museums, e.g. Certosa at Pavia, and San Vitale at Ravenna

(mosaics); at Florence, the Scalzo (Andrea del Sarto); San Marco (Fra

Angelico); the Riccardi and Pazzi chapels (Gozzoli and Perugino); at

Milan, in the Santa Maria delle Grazie, the "Last Supper," by Leonardo,

and at Padua, the famous Arena chapel (Giotto).

Private and semi-private galleries.

The Vatican galleries, though best known for their statuary, have fine

examples of painting, chiefly of the Italian school; the most famous

easel picture is Raphael's "Transfiguration," but the Stanze, apartments

entirely decorated by painting, are even more famous. In England three

royal palaces are open to the public--Hampton Court (Mantegna), Windsor

(Van Dyck, Zuccarelli), and Kensington (portraits). At Buckingham Palace

the Dutch pictures are admirable, and Queen Victoria lent the celebrated

Raphael cartoons to the Victoria and Albert Museum. Semi-private

collections belong to Dulwich College (Velasquez and Watteau), Oxford

University (Italian drawings), the Soane Museum (Hogarth and English

school), and the Royal Academy (Leonardo). Among private collections the

most important are the Harrach, and Prince Liechtenstein (Vienna), J.

Pierpont Morgan (including miniatures), Mrs J. Gardner of Boston

(Italian), Prince Corsini (Florence). In Great Britain there are immense

riches in private houses, though many collections have been dispersed.

The most noteworthy (1909) belong to the dukes of Devonshire and

Westminster, Lord Ellesmere, Captain Holford (including the masterpiece

of Cuyp), Ludwig Mond, Lord Lansdowne, Miss Rothschild. The finest

private collection is at Panshanger, formerly the seat of Lord Cowper,

the gallery of Van Dyck's work being quite the best in the world.

Periodical and commercial.

Many galleries are devoted to periodical exhibitions in London; the

Royal Academy is the leading agency of this character, having held

exhibitions since 1769. Its loan exhibitions of Old Masters are most

important. Similar enterprises are the New Gallery, opened in 1888, the

Grafton Gallery, and others. There are also old-established societies of

etchers, water-colourists, &c. A feature common to these exhibitions is

that the public always pays for admission, though they differ from the

commercial exhibitions, becoming more common every year, in which the

work of a single school or painter is shown for profit. But the annual

exhibitions at the Guildhall, under the auspices of the corporation, are

free. The great periodical exhibition of French art is known as the

Salon, and for some years it has had a rival in the Champ de Mars

exhibition. These two societies are now respectively housed in the Grand

Palais and Petit Palais, in the Champs Elysees, which were erected in

connexion with the Paris Exhibition of 1900, but with the ultimate

object of being devoted to the service of the two Salons. Berlin, Rome,

Vienna and other Continental towns have regular exhibitions of original

work.

The best history of art galleries is found in their official and other

catalogues, see article MUSEUMS. See also L. Viardot, _Les Musees

d'Italie, &c._ (3 vols., Paris, 1842, 1843, 1844); Annual Reports,

official, of National Portrait Gallery, National Galleries of England,

Ireland and Scotland; Civil Service Estimates, class iv. official. See

also the series edited by Lafenestre and E. Richtenberger: _Le Louvre,

La Belgique, Le Hollande, Florence, Belgique_; A. Lavice, _Revue des

musees de France,... d'Allemagne,... d'Angleterre, ... d'Espagnc,...

d'Italie,... de Belgique, de Hollande et de Russie_ (Paris,

1862-1872); E. Michel, _Les Musees d'Allemagne_ (Paris, 1886); Kate

Thompson, _Public Picture Galleries of Europe_ (1880); C.L. Eastlake,

_Notes on Foreign Picture Galleries_; Lord Ronald Gower, _Pocket Guide

to Art Galleries (public and private) of Belgium and Holland_ (1875);

and many works, albums, and so forth, issued mainly for the sake of

the illustrations. (B.)

ARTHRITIS (from Gr. [Greek: arthron], a joint), inflammation of the

joints, in various forms of what are generally called gout and

rheumatism (qq.v.).

ARTHROPODA, a name, denoting the possession by certain animals of

jointed limbs, now applied to one of the three sub-phyla into which one

of the great phyla (or primary branches) of coelomocoelous animals--the

Appendiculata-is divided; the other two being respectively the

Chaetopoda and the Rotifera. The word "Arthropoda" was first used in

classification by Siebold and Stannius (_Lehrbuch der vergleich.

Anatomie_, Berlin, 1845) as that of a primary division of animals, the

others recognized in that treatise being Protozoa, Zoophyta, Vermes,

Mollusca and Vertebrata. The names Condylopoda and Gnathopoda have been

subsequently proposed for the same group. The word refers to the

jointing of the chitinized exo-skeleton of the limbs or lateral

appendages of the animals included, which are, roughly speaking, the

Crustacea, Arachnida, Hexapoda (so-called "true insects"), Centipedes

and Millipedes. This primary group was set up to indicate the residuum

of Cuvier's Articulata when his class Annelides (the modern Chaetopoda)

was removed from that _embranchement_. At the same time C.T.E. von

Siebold and H. Stannius renovated the group Vermes of Linnaeus, and

placed in it the Chaetopods and the parasitic worms of Cuvier, besides

the Rotifers and Turbellarian worms.[1]

The result of the knowledge gained in the last quarter of the 19th

century has been to discredit altogether the group Vermes (see WORM),

thus set up and so largely accepted by German writers even at the

present day. We have, in fact, returned very nearly to Cuvier's

conception of a great division or branch, which he called Articulata,

including the Arthropoda and the Chaetopoda (Annelides of Lamarck, a

name adopted by Cuvier), and differing from it only by the inclusion of

the Rotifera. The name Articulata, introduced by Cuvier, has not been

retained by subsequent writers. The same, or nearly the same, assemblage

of animals has been called Entomozoaria by de Blainville (1822),

Arthrozoa by Burmeister (1843), Entomozoa or Annellata by H.

Milne-Edwards (1855), and Annulosa by Alexander M'Leay (1819), who was

followed by Huxley (1856). The character pointed to by all these terms

is that of a ring-like segmentation of the body. This, however, is not

the character to which we now ascribe the chief weight as evidence of

the genetic affinity and monophyletic (uni-ancestral) origin of the

Chaetopods, Rotifers and Arthropods. It is the existence in each ring of

the body of a pair of hollow _lateral appendages_ or _parapodia_, moved

by intrinsic muscles and penetrated by blood-spaces, which is the

leading fact indicating the affinities of these great sub-phyla, and

uniting them as blood-relations. The parapodia (fig. 8) of the marine

branchiate worms are the same things genetically as the "legs" of

Crustacea and Insects (figs. 10 and 11). Hence the term Appendiculata

was introduced by Lankester (preface to the English edition of

Gegenbaur's _Comparative Anatomy_, 1878) to indicate the group. The

relationships of the Arthropoda thus stated are shown in the subjoined

table:--

/ Sub-phylum 1. Rotifera.

Phylum--APPENDICULATA < " 2. Chaetopoda.

\ " 3. Arthropoda.

The ROTIFERA are characterized by the retention of what appears in

Molluscs and Chaetopods as an embryonic organ, the velum or ciliated

prae-oral girdle, as a locomotor and food-seizing apparatus, and by the

reduction of the muscular parapodia to a rudimentary or non-existent

condition in all present surviving forms except _Pedalion_. In many

important respects they are degenerate--reduced both in size and

elaboration of structure.

The CHAETOPODA are characterized by the possession of horny epidermic

chaetae embedded in the integument and moved by muscles. Probably the

chaetae preceded the development of parapodia, and by their

concentration and that of the muscular bundles connected with them at

the sides of each segment, led directly to the evolution of the

parapodia. The parapodia of Chaetopoda are never coated with dense

chitin, and are, therefore, never converted into jaws; the primitive

"head-lobe" or prostomium persists, and frequently carries eyes and

sensory tentacles. Further, in all members of the sub-phylum Chaetopoda

the relative position of the prostomium, mouth and peristomium or first

ring of the body, retains its primitive character. We do not find in

Chaetopoda that parapodia, belonging to primitively post-oral rings or

body-segments (called "somites," as proposed by H. Milne-Edwards), pass

in front of the mouth by adaptational shifting of the oral aperture.

(See, however, 8.)

The ARTHROPODA might be better called the "Gnathopoda," since their

distinctive character is, that one or more pairs of appendages behind

the mouth are densely chitinized and turned (fellow to fellow on

opposite sides) towards one another so as to act as jaws. This is

facilitated by an important general change in the position of the

parapodia; their basal attachments are all more ventral in position than

in the Chaetopoda, and tend to approach from the two sides towards the

mid-ventral line. Very usually (but not in the Onychophora =

_Peripatus_) all the parapodia are plated with chitin secreted by the

epidermis, and divided into a series of joints--giving the

"arthropodous" or hinged character.

There are other remarkable and distinctive features of structure which

hold the Arthropoda together, and render it impossible to conceive of

them as having a polyphyletic origin, that is to say, as having

originated separately by two or three distinct lines of descent from

lower animals; and, on the contrary, establish the view that they have

been developed from a single line of primitive Gnathopods which arose by

modification of parapodiate annulate worms not very unlike some of the

existing Chaetopods. These additional features are the following--(1)

All existing Arthropoda have an ostiate heart and have undergone

"phleboedesis," that is to say, the peripheral portions of the

blood-vascular system are not fine tubes as they are in the Chaetopoda

and as they were in the hypothetical ancestors of Arthropoda, but are

swollen so as to obliterate to a large extent the coelom, whilst the

separate veins entering the dorsal vessel or heart have coalesced,

leaving valvate ostia (see fig. 1) by which the blood passes from a

pericardial blood-sinus formed by the fused veins into the dorsal vessel

or heart (see Lankester's _Zoology_, part ii., introductory chapter,

1900). The only exception to this is in the case of minute degenerate

forms where the heart has disappeared altogether. The rigidity of the

integument caused by the deposition of dense chitin upon it is

intimately connected with the physiological activity and form of all the

internal organs, and is undoubtedly correlated with the total

disappearance of the circular muscular layer of the body-wall present in

Chaetopods. (2) In all existing Arthropoda the region in front of the

mouth is no longer formed by the primitive prostomium or head-lobe, but

one or more segments, originally post-oral, with their appendages have

passed in front of the mouth (prosthomeres). At the same time the

prostomium and its appendages cease to be recognizable as distinct

elements of the head. The brain no longer consists solely of the

nerve-ganglion-mass proper to the prostomial lobe, as in Chaetopoda, but

is a composite (syncerebrum) produced by the fusion of this and the

nerve-ganglion-masses proper to the prosthomeres or segments which pass

forwards, whilst their parapodia (= appendages) become converted into

eye-stalks, and antennae, or more rarely grasping organs. (3) As in

Chaetopoda, coelomic funnels (coelomoducts) _may_ occur right and left

as pairs in each ring-like segment or somite of the body, and some of

these are in all cases retained as gonoducts and often as renal

excretory organs (green glands, coxal glands of Arachnida, _not_ crural

glands, which are epidermal in origin); but true nephridia, genetically

identical with the nephridia of earthworms, do not occur (on the subject

of coelom, coelomoducts and nephridia, see the introductory chapter of

part ii. of Lankester's _Treatise on Zoology_).

[Illustration: After Lankester, _Q. J. Mic. Sci._ vol. xxxiv., 1893.

FIG. 1.--Diagram to show the gradual formation of the Arthropod

pericardial blood-sinus and "ostiate" heart by the swelling up

(phleboedesis) of the veins entering the dorsal vessel or heart of a

Chaetopod-like ancestor. The figure on the left represents the condition

in a Chaetopod, that on the right the condition in an Arthropod, the

other two are hypothetical intermediate forms.]

_Tabular Statement of the Grades, Classes and Sub-classes of the

Arthropoda._--It will be convenient now to give in the clearest form a

statement of the larger subdivisions of the Arthropoda which it seems

necessary to recognize at the present day. The justification of the

arrangement adopted will form the substance of the rest of the present

article. The orders included in the various classes are not discussed

here, but are treated of under the following titles:--PERIPATUS

(Onychophora), CENTIPEDE and MILLIPEDE (Myriapoda), HEXAPODA (Insecta),

ARACHNIDA and CRUSTACEA.

SUB-PHYLUM ARTHROPODA (of the Phylum Appendiculata).

Grade A. HYPARTHROPODA (hypothetical forms connecting ancestors of

Chaetopoda with those of Arthropoda).

Grade B. PROTARTHROPODA.

Class ONYCHOPHORA.

Ex.--_Peripatus._

Grade C. EUARTHROPODA.

Class 1. DIPLOPODA.

Ex.--_Julus._

Class 2. ARACHNIDA.

Grade a. Anomomeristica.

Ex.--_Phacops._

Grade b. Nomomeristica.

(a) Pantopoda.

Ex.--_Pycnogonum._

(b) Euarachnida.

Ex.--_Limulus, Scorpio, Mygale, Acarus._

Class 3. CRUSTACEA.

Grade a. Entomostraca.

Ex.--_Apus, Branchipus, Cyclops, Balanus._

Grade b. Malacostraca.

Ex.--_Nebalia, Astacus, Oniscus, Gammarus._

Class 4. CHILOPODA.

Ex.--_Scolopendra._

Class 5. HEXAPODA (syn. Insecta Pterygota).

Ex.--_Locusta, Phryganea, Papilio, Apis, Mnsca, Cimex, Lucanus,

Machilis._

_Incertae sedis_--Tardigrada, Pentastomidae (degenerate forms).

_The Segmentation of the Body of Arthropoda._--The body of the

Arthropoda is more or less clearly divided into a series of rings,

segments, or somites which can be shown to be repetitions one of

another, possessing identical parts and organs which may be larger or

smaller, modified in shape or altogether suppressed in one somite as

compared with another. A similar constitution of the body is more

clearly seen in the Chaetopod worms. In the Vertebrata also a

repetition of units of structure (myotomes, vertebrae, &c.)--which is

essentially of the same nature as the repetition in Arthropods and

Chaetopods, but in many respects subject to peculiar developments--is

observed. The name "metamerism" has been given to this structural

phenomenon because the "meres," or repeated units, follow one another

in line. Each such "mere" is often called a "metamere." A satisfactory

consideration of the structure of the Arthropods demands a knowledge

of what may be called the laws of metamerism, and reference should be

made to the article under that head.

[Illustration: From Goodrich, _Q. J. Micr. Sci._ vol. xi p. 247.

FIG. 2.--Diagram of the head and adjacent region of an Ohgochaet

Chaetopod.

Pr, The prostomium.

m, The mouth.

A, The prostomial ganglion-mass or archi-cerebrum.

I, II, III, coelom of the first, second and third somites.]

_The Theory of the Arthropod Head._--The Arthropod head is a tagma or

group of somites which differ in number and in their relative position

in regard to the mouth, in different classes. In a simple Chaetopod

(fig. 2) the head consists of the first somite only; that somite is

perforated by the mouth, and is provided with a prostomium or

prae-oral lobe. The prostomium is essentially a part or outgrowth of

the first somite, and cannot be regarded as itself a somite. It gives

rise to a nerve-ganglion mass, the prostomial ganglion. In the marine

Chaetopods (the Polychaeta) (fig. 3), we find the same essential

structure, but the prostomium may give rise to two or more tactile

tentacles, and to the vesicular eyes. The somites have well-marked

parapodia, and the second and third, as well as the first, may give

rise to tentacles which are directed forward, and thus contribute to

form "the head." But the mouth remains as an inpushing of the wall of

the first somite.

[Illustration: FIG. 3.--Diagram of the head and adjacent region of a

Polychaet Chaetopod Letters as in fig. 1, with the addition of T,

prostomial tentacle; _Pa_, parapodium. (From Goodrich.)]

The Arthropoda are all distinguished from the Chaetopoda by the fact

that the head consists of one or more somites which lie _in front of

the mouth_ (now called prosthomeres), as well as of one or more

somites behind it (opisthomeres). The first of the post-oral somites

invariably has its parapodia modified so as to form a pair of

hemignaths (mandibles). About 1870 the question arose for discussion

whether the somites in front of the mouth are to be considered as

derived from the prostomium of a Chaetopod-like ancestor.

Milne-Edwards and Huxley had satisfied themselves with discussing and

establishing, according to the data at their command, the number of

somites in the Arthropod head, but had not considered the question of

the _nature_ of the prae-oral somites. Lankester (2) was the first to

suggest that (as is actually the fact in the Nauplius larva of the

Crustacea) the prae-oral somites or prosthomeres and their appendages

were ancestrally post-oral, but have become prae-oral "by adaptational

shifting of the oral aperture." This has proved to be a sound

hypothesis and is now accepted as the basis upon which the Arthropod

head must be interpreted (see Korschelt and Heider (3)). Further, the

morphologists of the 'fifties appear, with few exceptions, to have

accepted a preliminary scheme with regard to the Arthropod head and

Arthropod segmentation generally, which was misleading and caused them

to adopt forced conclusions and interpretations. It was conceived by

Huxley, among others, that the same number of cephalic somites would

be found to be characteristic of all the diverse classes of

Arthropoda, and that the somites, not only of the head but of the

various regions of the body, could be closely compared in their

numerical sequence in classes so distinct as the Hexapods, Crustaceans

and Arachnids.

The view which it now appears necessary to take is, on the contrary,

this--viz that all the Arthropoda are to be traced to a common

ancestor resembling a Chaetopod worm, but differing from it in having

lost its chaetae and in having a prosthomere in front of the mouth

(instead of prostomium only) and a pair of hemignaths (mandibles) on

the parapodia of the buccal somite. From this ancestor Arthropods with

heads of varying degrees of complexity have been developed

characteristic of the different classes, whilst the parapodia and

somites of the body have become variously modified and grouped in

these different classes. The resemblances which the members of one

class often present to the members of another class in regard to the

form of the limb-branches (rami) of the parapodia and the formation of

tagmata (regions) are not hastily to be ascribed to common

inheritance, but we must consider whether they are not due to

homoplasy--that is, to the moulding of natural selection acting in the

different classes upon fairly similar elements under like exigencies.

[Illustration: FIG. 4.--Diagram of the head and adjacent region of

_Peripatus_. Monoprosthomerous.

m, Mouth.

I, Coelom of the first somite which carries the antennae and is in

front of the mouth.

II, Coelom of the second somite which carries the mandibles (hence

deuterognathous).

III and IV, Coelom of the third and fourth somites.

FP, Rudimentary frontal processes perhaps representing the

prostomial tentacles of Polychaeta.

Ant, Antenna or tactile tentacle.

Md, Mandible.

Op, Oral-papilla.

P, Protocerebrum or foremost cerebral mass belonging to the first

somite.

D, Deuterocerebrum, consisting of ganglion cells belonging to the

second or mandibular somite. (After Goodrich.)]

The structure of the head in Arthropods presents _three_ profoundly

separated grades of structure dependent upon the number of

prosthomeres which have been assimilated by the prae-oral region. The

classes presenting these distinct plans of head-structure cannot be

closely associated in any scheme of classification professing to be

natural. Penpatus, the type-genus of the class Onychophora, stands at

the base of the series with only a single prosthomere (fig. 4). In

Peripatus the prostomium of the Chaetopod-like ancestor is atrophied,

but it is possible that two processes on the front of the head (FP)

represent in the embryo the dwindled prostomial tentacles. The single

prosthomere carries the retractile tentacles as its "parapodia." The

second somite is the buccal somite (II, fig. 4); its parapodia have

horny jaws on their ends, like the claws on the following legs (fig.

9), and act as hemignaths (mandibles). The study of sections of the

embryo establishes these facts beyond doubt. It also shows us that the

neuromeres, no less than the embryonic coelomic cavities, point to the

existence of one, and only one, prosthomerp in Peripatus, of which the

"protocerebrum," P, is the neuromere, whilst the deuterocerebrum, D,

is the neuromere of the second or buccal somite. A brief indication of

these facts is given by saying that the Onychophora are

"deuterognathous"--that is to say, that the buccal somite carrying the

mandibular hemignaths is the second of the whole series.

What has become of the nerve-ganglion of the prostomial lobe of the

Chaetopod in Peripatus is not clearly ascertained, nor is its fate

indicated by the study of the embryonic head of other Arthropods so

far. Probably it is fused with the protocerebrum, and may also be

concerned in the history of the very peculiar paired eyes of

Peripatus, which are like those of Chaetopods in structure--viz

vesicles with an intravesicular lens, whereas the eyes of all other

Arthropods have essentially another structure, being "cups" of the

epidermis, in which a knob-like or rod-like thickening of the cuticle

is fitted as refractive medium.

In Diplopoda (_Julus_, &c.) the results of embryological study point

to a composition of the front part of the head exactly similar to that

which we find in Onychophora. They are deuterognathous.

[Illustration: FIG. 5.--Diagram of the head and adjacent region of an

Arachnid. Diprosthomerous in the adult condition, though

embryologically the appendages of somite II and the somite itself are,

as here drawn, not actually in front of the mouth.

E, Lateral eye.

Ch, Chelicera.

m, Mouth.

P, Protocerebrum,

D, Deuterocerebrum.

I, II, III, IV. Coelom of the first, second, third and fourth

somites. (After Goodrich.)]

The Arachnida present the first stage of progress. Here embryology

shows that there are two prosthomeres (fig. 5), and that the

gnathobases of the chelae which act as the first pair of hemignaths

are carried by the third somite. The Arachnida are therefore

tritognathous. The two prosthomeres are indicated by their coelomic

cavities in the embryo (I and II, fig. 5), and by two neuromeres, the

protocerebrum and the deuterocerebrum. The appendages of the first

prosthomere are not present as tentacles, as in Peripatus and

Diplopods, but are possibly represented by the eyes or possibly

altogether aborted. The appendages of the second prosthomere are the

well-known chelicerae of the Arachnids, rarely, if ever, antenniform,

but modified as "retroverts" or clasp-knife tangs in spiders.

The Crustacea (fig. 6) and the Hexapoda (fig. 7) agree in having three

somites in front of the mouth, and it is probable, though not

ascertained, that the Chilopoda (Scolopendra, &c.) are in the same

case. The three prosthomeres or prae-oral somites of Crustacea due to

the sinking back of the mouth one somite farther than in Arachnida are

not clearly indicated by coelomic cavities in the embryo, but their

existence is clearly established by the development and position of

the appendages and by the neuromeres.

The eyes in some Crustacea are mounted on articulated stalks, and from

the fact that they can after injury be replaced by antenna-like

appendages it is inferred that they represent the parapodia of the

most anterior prosthomere. The second prosthomere carries the first

pair of antennae and the third the second pair of antennae. Sometimes

the pair of appendages has not a merely tactile jointed ramus, but is

converted into a claw or clasper. Three neuromeres--a proto-,

deutero-, and trito-cerebrum--corresponding to those three

prosthomeres are sharply marked in the embryo. The fourth somite is

that in which the mouth now opens, and which accordingly has its

appendages converted into hemignathous mandibles. The Crustacea are

tetartognathous.

[Illustration: FIG. 6.--Diagram of the head of a Crustacean.

Tri-prosthomerous.

FP, Frontal processes (observed in Cirrhiped nauplius-larvae)

probably representing the prostomial tentacles of Chaetopods.

e, Eye.

Ant^1, First pair of antennae.

Ant^2, Second pair of antennae.

md, Mandible.

mx^1, mx^2, First and second pairs of maxillae.

m, Mouth.

I, II, and III, The three prosthomeres.

IV, V, VI, The three somites following the mouth.

P, Protocerebrum.

D, Deuterocerebrum.

T, Tritocerebrum.

(After Goodrich.)]

[Illustration: FIG. 7.--Diagram of the head of a Hexapod insect.

e, Eye.

ant, Antenna.

md, Mandible.

mx^1, First maxilla.

mx^2, Second maxilla.

m, Mouth.

I, Region of the first or eye-bearing prosthomere.

II, Coelom of the second antenna-bearing prosthomere.

III, Coelom of the third prosthomere devoid of appendages.

IV, V, and VI, Coelom of the fourth, fifth and sixth somites.

P, Protocerebrum belonging to the first prosthomere.

D, Deuterocerebrum belonging to the second prosthomere.

T, Tritocerebrum belonging to the third prosthomere.

(After Goodrich.)]

The history of the development of the head has been carefully worked

out in the Hexapod insects. As in Crustacea and Arachnida, a first

prosthomere is indicated by the paired eyes and the protocerebrum; the

second prosthomere has a well-marked coelomic cavity, carries the

antennae, and has the deuterocerebrum for its neuromere. The third

prosthomere is represented by a well-marked pair of coelomic cavities

and the tritocerebrum (III, fig. 7), but has no appendages. They

appear to have aborted. The existence of this third prosthomere

corresponding to the third prosthomere of the Crustacea is a strong

argument for the derivation of the Hexapoda, and with them the

Chilopoda, from some offshoot of the Crustacean stem or class. The

buccal somite, with its mandibles, is in Hexapoda, as in Crustacea,

the fourth: they are tetartognathous.

The adhesion of a greater or less number of somites to the buccal

somite posteriorly (opisthomeres) is a matter of importance, but of

minor importance, in the theory and history of the Arthropod head. In

Peripatus no such adhesion or fusion occurs. In Diplopoda two

opisthomeres--that is to say, one in addition to the buccal

somite--are united by a fusion of their terga with the terga of the

prosthomeres. Their appendages are respectively the mandibles and the

gnathochilarium.

In Arachnida the highest forms exhibit a fusion of the tergites of

five post-oral somites to form one continuous carapace united with the

terga of the two prosthomeres. The five pairs of appendages of the

post-oral somites of the head or prosoma thus constituted all

primitively carry gnathobasic projections on their coxal joints, which

act as hemignaths: in the more specialized forms the mandibular

gnathobases cease to develop.

In Crustacea the fourth or mandibular somite never has less than the

two following somites associated with it by the adaptation of their

appendages as jaws, and the ankylosis of their terga with that of the

prosthomeres. But in higher Crustacea the cephalic "tagma" is

extended, and more somites are added to the fusion, and their

appendages adapted as jaws of a kind.

The Hexapoda are not known to us in their earlier or more primitive

manifestations; we only know them as possessed of a definite number of

somites arranged in definite numbers in three great tagmata. The head

shows two jaw-bearing somites besides the mandibular somite (V, VI, in

fig. 7)--thus six in all (as in some Crustacea), including

prosthomeres, all ankylosed by their terga to form a cephalic shield.

There is, however, good embryological evidence in some Hexapods of the

existence of a seventh somite, the supra-lingual, occurring between

the somite of the mandibles and the somite of the first maxillae (4).

This segment is indicated embryologically by its paired coelomic

cavities. It is practically an excalated somite, having no existence

in the adult. It is probably not a mere coincidence that the Hexapod,

with its two rudimentary somites devoid of appendages, is thus found

to possess twenty-one somites, including that which carries the anus,

and that this is also the number present in the Malacostracous

Crustacea.

[Illustration: FIG. 8.--Diagram of the somite-appendage or parapodium

of a Polychaet Chaetopod. The chaetae are omitted.

Ax, The axis.

nr.c, Neuropodial cirrhus.

nr.l^1, nr.l^2, Neuropodial lobes or endites.

nt.c. Notopodial cirrhus.

nt.l^1, nt.l^2, Notopodial lobes or exites.

The parapodium is represented with its neural or ventral surface

uppermost. (Original).]

_The Segmented Lateral Appendages or Limbs of Arthropoda._--It has

taken some time to obtain any general acceptance of the view that the

parapodia of the Chaetopoda and the limbs of Arthropoda are

genetically identical structures; yet if we compare the parapodium of

Tomopteris or of Phyllodoce with one of the foliaceous limbs of

Branchipus or Apus, the correspondences of the two are striking. An

erroneous view of the fundamental morphology of the Crustacean limb,

and consequently of that of other Arthropoda, came into favour owing

to the acceptance of the highly modified limbs of Astacus as typical.

Protopodite, endopodite, exopodite, and epipodite were considered to

be the morphological units of the crustacean limb. Lankester (5) has

shown (and his views have been accepted by Professors Korschelt and

Heider in their treatise on _Embryology_) that the limb of the lowest

Crustacea, such as Apus, consists of a corm or axis which may be

jointed, and gives rise to outgrowths, either leaf-like or filiform,

on its inner and outer margins (endites and exites). Such a corm (see

figs. 10 and 11), with its outgrowths, may be compared to the simple

parapodia of Chaetopoda with cirrhi and branchial lobe (fig. 8). It is

by the specialization of two "endites" that the endopodite and

exopodite of higher Crustacea are formed, whilst a flabelliform exite

is the homogen or genetic equivalent of the epipodite (see Lankester,

"Observations and Reflections on Apus Cancriformis," _Q. J. Micr.

Sci._). The reduction of the outgrowth-bearing "corm" of the

parapodium of either a Chaetopod or an Arthropod to a simple

cylindrical stump, devoid of outgrowths, is brought about when

mechanical conditions favour such a shape. We see it in certain

Chaetopods (e.g. Hesione) and in the Arthropod Peripatus (fig. 9). The

conversion of the Arthropod's limb into a jaw, as a rule, is effected

by the development of an endite near its base into a hard, chitinized,

and often toothed gnathobase (see figs. 10 and 11, _en^1_). It is not

true that all the biting processes of the Arthropod limb are thus

produced--for instance, the jaws of Peripatus are formed by the axis

or corm itself, whilst the poison-jaws of Chilopods, as also their

maxillae, appear to be formed rather by the apex or terminal region of

the ramus of the limb; but the opposing jaws (= hemignaths) of

Crustacea, Arachnida and Hexapoda are gnathobases, and not the axis or

corm. The endopodite (corresponding to the fifth endite of the limb of

Apus, see fig. 10) becomes in Crustacea the "walking leg" of the

mid-region of the body; it becomes the palp or jointed process of

anterior segments. A second ramus, the "exopodite," often is also

retained in the form of a palp or feeler. In Apus, as the figure

shows, there are four of these "antenna-like" palps or filaments on

the first thoracic limb. A common modification of the chief ramus of

the Arthropod parapodium is the chela or nipper formed by the

elongation of the penultimate joint of the ramus, so that the last

joint works on it--as, for instance, in the lobster's claw. Such

chelate rami or limb-branches are independently developed in Crustacea

and in Arachnida, and are carried by somites of the body which do not

correspond in position in the two groups. The range of modification of

which the rami or limb-branches of the limbs of Arthropoda are capable

is very large, and in allied orders or even families or genera we

often find what is certainly the palp of the same appendage (as

determined by numerical position of the segments)--in one case

antenniform, in another chelate, in another pediform, and in another

reduced to a mere stump or absent altogether. Very probably the power

which the appendage of a given segment has of assuming the perfected

form and proportions previously attained by the appendage of another

segment must be classed as an instance of "homoeosis," not only where

such a change is obviously due to abnormal development or injury, but

also where it constitutes a difference permanently established between

allied orders or smaller groups, or between the two sexes.

[Illustration: FIG. 9.--Three somite-appendages or parapodia of

_Peripatus._

A, A walking leg; p^1 to p^4, the characteristic "pads"; f, the

foot; cl^1, cl^2, the two claws.

B, An oral papilla, one of the second pair of post-oral appendages.

C, One of the first post-oral pair of appendages or mandibles; cl^1,

cl^2, the greatly enlarged claws. (Compare A.)

The appendages are represented with the neural or ventral surface

uppermost.]

The most extreme disguise assumed by the Arthropod parapodium or

appendage is that of becoming a mere stalk supporting an eye--a fact

which did not obtain general credence until the experiments of Herbst

in 1895, who found, on cutting off the eye-stalk of Palaemon, that a

jointed antenna-like appendage was regenerated in its place. Since the

eye-stalks of Podophthalmate Crustacea represent appendages, we are

forced to the conclusion that the sessile eyes of other Crustacea, and

of other Arthropoda generally, indicate the position of appendages

which have atrophied.[2]

From what has been said, it is apparent that we cannot, in attempting

to discover the affinities and divergences of the various forms of

Arthropoda, attach a very high phylogenetic value to the coincidence

or divergence in form of the appendages belonging to the somites

compared with one another.

[Illustration: After Lankester, _Q. J. Mic. Sci._ vol. xxi., 1881.

FIG. 10.--The second thoracic (fifth post-oral) appendage of the left

side of _Apus cancriformis_, placed with its ventral or neural surface

uppermost to compare with figs. 8 and 9.

1, 2, The two segments of the axis.

en^1, The gnathobase.

en^2 to en^6, The five following "endites."

fl, The flabellum or anterior exite.

br, The bract or posterior exite.]

The principal forms assumed by the Arthropod parapodium and its rami

may be thus enumerated:--

(1) Axial corm well developed, unsegmented or with two to four

segments; lateral endites and exites (rami) numerous and of various

lengths (certain limbs of lower Crustacea).

(2) Corm, with short unsegmented rami, forming a flattened foliaceous

appendage, adapted to swimming and respiration (trunk-limbs of

Phyllopods).

(3) Corm alone developed; with no endites or exites, but provided with

terminal chitinous claws (ordinary leg of Peripatus), with terminal

jaw teeth (jaw of Peripatus), or with blunt extremity (oral papilla of

same) (see fig. 9).

(4) Three of the rami of the primitive limb (endites 5 and 6, and

exite 1) specially developed as endopodite, exopodite, and

epipodite--the first two often as firm and strongly chitinized,

segmented, leg-like structures; the original axis or corm reduced to a

basal piece, with or without a distinct gnathobase (endite 1)--typical

tri-ramose limb of higher Crustacea.

(5) One ramus (the endopodite) alone developed--the original axis or

corm serving as its basal joint with or without gnathobase. This is

the usual uni-ramose limb found in the various classes of Arthropoda.

It varies as to the presence or absence of the jaw-process and as to

the stoutness of the segments of the ramus, their number (frequently

six, plus the basal corm), and the modification of the free end. This

may be filiform or brush-like or lamellate when it is an antenna or

palp; a simple spike (walking leg of Crustacea, of other aquatic

forms, and of Chilopods and Diplopods); the terminal joint flattened

(swimming leg of Crustacea and Gigantostraca); the terminal joint

provided with two or with three recurved claws (walking leg of many

terrestrial forms--e.g. Hexapoda and Arachnida); the penultimate joint

with a process equal in length to the last joint, so as to form a

nipping organ (chelae of Crustaceans and Arachnids); the last joint

reflected and movable on the penultimate, as the blade of a

clasp-knife on its handle (the retrovert, toothed so as to act as a

biting jaw in the Hexapod _Mantis_, the Crustacean _Squilla_ and

others); with the last joint produced into a needle-like stabbing

process in spiders.

[Illustration: After Lankester, _Q. J. Mic. Sci._ vol. xxi., 1881.

FIG. 11.--The first thoracic (fourth post-oral) appendage of _Apus

cancriformis_ (right side).

Ax^1 to Ax^4, the four segments of the axis with muscular bands.

En^1, Gnathobase.

En^2 to En^5, The elongated jointed endites (rami).

En^6, The rudimentary sixth endite (exopodite of higher Crustacea).

Fl, The flabellum which becomes the epipodite of higher forms.

Br, The bract devoid of muscles and respiratory in function.]

(6) Two rami developed (usually, but perhaps not always, the

equivalents of the endopodite and exopodite) supported on the somewhat

elongated corm (basal segment). This is the typical "bi-ramose limb"

often found in Crustacea. The rami may be flattened for swimming, when

it is "a bi-ramose swimmeret," or both or only one may be filiform and

finely annulate; this is the form often presented by the antennae of

Crustacea, and rarely by prae-oral appendages in other Arthropods.

(7) The endopoditic ramus is greatly enlarged and flattened, without

or with only one jointing, the corm (basal segment) is evanescent;

often the plate-like endopodites of a pair of such appendages unite in

the middle line with one another or by the intermediary of a sternal

up-growth and form a single broad plate. These are the plate-like

swimmerets and opercula of Gigantostraca and Limulus among Arachnids

and of Isopod Crustaceans. They may have rudimentary exopodites, and

may or may not have branchial filaments or lamellae developed on their

posterior faces. The simplest form to which they may be reduced is

seen in the genital operculum of the scorpion.

(8) The gnathobase becomes greatly enlarged and not separated by a

joint from the corm; it acts as a hemignath or half jaw working

against its fellow of the opposite side. The endopodite may be

retained as a small segmented palp at the side of the gnathobase or

disappear (mandible of Crustacea, Chilopoda and Hexapoda).

(9) The corm becomes the seat of a development of a special visual

organ, the Arthropod eye (as opposed to the Chaetopod eye). Its

jointing (segmentation) may be retained, but its rami disappear

(Podophthalmous Crustacea). Usually it becomes atrophied, leaving the

eye as a sessile organ upon the prae-oral region of the body (the

eye-stalk and sessile lateral eyes of Arthropoda generally, exclusive

of Peripatus).

(10) The forms assumed by special modification of the elements of the

parapodium in the maxillae, labium, &c., of Hexapods, Chilopods,

Diplopods, and of various Crustacea, deserve special enumeration, but

cannot be dealt with without ample space and illustration.

It may be pointed out that the most radical difference presented in

this list is that between appendages consisting of the corm alone

without rami (Onychophora) and those with more or less developed rami

(the rest of the Arthropoda). In the latter class we should

distinguish three phases: (a) those with numerous and comparatively

undeveloped rami; (b) those with three, or two highly developed rami,

or with only one--the corm being reduced to the dimensions of a mere

basal segment; (c) those reduced to a secondary simplicity

(degeneration) by overwhelming development of one segment (e.g. the

isolated gnathobase often seen as "mandible" and the genital

operculum).

There is no reason to suppose that any of the forms of limb observed

in Arthropoda may not have been independently developed in two or more

separate diverging lines of descent.

_Branchiae._--In connexion with the discussion of the limbs of

Arthropods, a few words should be devoted to the gill-processes. It

seems probable that there are branchial plumes or filaments in some

Arthropoda (some Crustacea) which can be identified with the distinct

branchial organs of Chaetopoda, which lie dorsal of the parapodia and

are not part of the parapodium. On the other hand, we cannot refuse to

admit that any of the processes of an Arthropod parapodium may become

modified as branchial organs, and that, as a rule, branchial

out-growths are easily developed, _de novo_, in all the higher groups

of animals. Therefore, it seems to be, with our present knowledge, a

hopeless task to analyse the branchial organs of Arthropoda and to

identify them genetically in groups.

A brief notice must suffice of the structure and history of the

_Eyes_, the _Tracheae_ and the so-called _Malpighian tubes_ of

Arthropoda, though special importance attaches to each in regard to

the determination of the affinities of the various animals included in

this great sub-phylum.

_The Eyes._--The Arthropod eye appears to be an organ of special

character developed in the common ancestor of the Euarthropoda, and

distinct from the Chaetopod eye, which is found only in the

Onychophora where the true Arthropod eye is absent. The essential

difference between these two kinds of eye appears to be that the

Chaetopod eye (in its higher developments) is a vesicle enclosing the

lens, whereas the Arthropod eye is a pit or series of pits into which

the heavy chitinous cuticle dips and enlarges knobwise as a lens. Two

distinct forms of the Arthropod eye are observed--the monomeniscous

(simple) and the polymeniscous (compound). The nerve-end-cells, which

lie below the lens, are part of the general epidermis. They show in

the monomeniscous eye (see article ARACHNIDA, fig. 26) a tendency to

group themselves into "retinulae," consisting of five to twelve cells

united by vertical deposits of chitin (rhabdoms). In the case of the

polymeniscous eye (fig. 23, article ARACHNIDA) a single retinula or

group of nerve-end-cells is grouped beneath each associated lens. A

further complication occurs in each of these two classes of eye. The

monomeniscous eye is rarely provided with a single layer of cells

beneath its lens; when it is so, it is called monostichous (simple

lateral eye of Scorpion, fig. 22, article ARACHNIDA). More usually, by

an infolding of the layer of cells in development, we get three layers

under the lens; the front layer is the corneagen layer, and is

separated by a membrane from the other two which, more or less, fuse

and contain the nerve-end-cells (retinal layer). These eyes are called

diplostichous, and occur in Arachnida and Hexapoda (fig. 24, article

ARACHNIDA).

On the other hand, the polymeniscous eye undergoes special elaboration

on its lines. The retinulae become elongated as deep and very narrow

pits (fig. 12 and explanation), and develop additional cells near the

mouth of the narrow pit. Those nearest to the lens are the corneagen

cells of this more elaborated eye, and those between the original

retinula cells and the corneagen cells become firm and transparent.

They are the crystalline cells or vitrella (see Watase, 7). Each such

complex of cells underlying the lenticle of a compound eye is called

an "ommatidium"; the entire mass of cells underlying a monomeniscous

eye is an "ommataeum." The ommataeum, as already stated, tends to

segregate into retinulae which correspond potentially each to an

ommatidium of the compound eye. The ommatidium is from the first

segregate and consists of few cells. The compound eye of the king-crab

(Limulus) is the only recognized instance of ommatidia in their

simplest state. Each can be readily compared with the single-layered

lateral eye of the scorpion. In Crustacea and Hexapoda of all grades

we find compound eyes with the more complicated ommatidia described

above. We do not find them in any Arachnida.

It is difficult in the absence of more detailed knowledge as to the

eyes of Chilopoda and Diplopoda to give full value to these facts in

tracing the affinities of the various classes of Arthropods. But they

seem to point to a community of origin of Hexapods and Crustacea in

regard to the complicated ommatidia of the compound eye, and to a

certain isolation of the Arachnida, which are, however, traceable, so

far as the eyes are concerned, to a distant common origin with

Crustacea and Hexapoda through the very simple compound eyes

(monostichous, polymeniscous) of Limulus.

[Illustration: FIG. 12.--Diagram to show the derivation of the unit or

"ommatidium" of the compound eye of Crustacea and Hexapoda, C, from a

simple monomeniscous monostichous eye resembling the lateral eye of a

scorpion, A, or the unit of the compound lateral eye of Limulus (see

article ARACHNIDA, figs. 22 and 23). B represents an intermediate

hypothetical form in which the cells beneath the lens are beginning to

be superimposed as corneagen, vitrella and retinula, instead of

standing side by side in horizontal series. The black represents the

cuticular product of the epidermal cells of the ocular area, taking

the form either of lens, _cl_, of crystalline body, _cry_, or of

rhabdom, _rhab_; _hy_, hypodermis or epidermal cells; _corn^1_,

laterally-placed cells in the simpler stage, A, which like the

nerve-end cells, _vit^1_ and _ret^1_, are corneagens or

lens-producing; _corn_, specialized corneagen or lens-producing cells;

_vit^1_, potential vitrella cells with _cry^1_, potential crystalline

body now indistinguishable from retinula cells and rhabdomeres; _vit_,

vitrella cell with _cry_, its contained cuticular product, the

crystalline cone or body; _ret^1_, _rhab^1_, retinula cells and

rhabdom of scorpion undifferentiated from adjacent cells, _vit^1;

ret_, retinula cell; _rhab_, rhabdom; _nf_, optic nerve-fibres.

(Modified from Watase.)]

_The Tracheae._--In regard to tracheae the very natural tendency of

zoologists has been until lately to consider them as having once

developed and once only, and therefore to hold that a group

"Tracheata" should be recognized, including all tracheate Arthropods.

We are driven by the conclusions arrived at as to the derivation of

the Arachnida from branchiate ancestors, independently of the other

tracheate Arthropods, to formulate the conclusion that tracheae have

been independently developed in the Arachnidan class. We are also, by

the isolation of Peripatus and the impossibility of tracing to it all

other tracheate Arthropoda, or of regarding it as a degenerate offset

from some one of the tracheate classes, forced to the conclusion that

the tracheae of the Onychophora have been independently acquired.

Having accepted these two conclusions, we formulate the generalization

that tracheae can be independently acquired by various branches of

Arthropod descent in adaptation to a terrestrial as opposed to an

aquatic mode of life. A great point of interest therefore exists in

the knowledge of the structure and embryology of tracheae in the

different groups. It must be confessed that we have not such full

knowledge on this head as could be wished for. Tracheae are

essentially tubes like blood-vessels--apparently formed from the same

tissue elements as blood-vessels--which contain air in place of blood,

and usually communicate by definite orifices, the tracheal stigmata,

with the atmosphere. They are lined internally by a cuticular deposit

of chitin. In Peripatus and the Diplopods they consist of bunches of

fine tubes which do not branch but diverge from one another; the

chitinous lining is smooth. In the Hexapods and Chilopods, and the

Arachnids (usually), they form tree-like branching structures, and

their finest branches are finer than any blood-capillary, actually in

some cases penetrating a single cell and supplying it with gaseous

oxygen. In these forms the chitinous lining of the tubes is thickened

by a close-set spiral ridge similar to the spiral thickening of the

cellulose wall of the spiral vessels of plants. It is a noteworthy

fact that other tubes in these same terrestrial Arthropoda--namely,

the ducts of glands--are similarly strengthened by a chitinous

cuticle, and that a spiral or annular thickening of the cuticle is

developed in them also. Chitin is _not_ exclusively an ectodermal

product, but occurs also in cartilaginous skeletal plates of

mesoblastic origin (connective tissue). The immediate cavities or pits

into which the tracheal stigmata open appear to be in many cases

ectodermic in sinkings, but there seems to be no reason (based on

embryological observation) for regarding the tracheae as an ingrowth

of the ectoderm. They appear, in fact, to be an air-holding

modification of the vasifactive connective tissue. Tracheae are

abundant just in proportion as blood-vessels become suppressed. They

are reciprocally exclusive. It seems not improbable that they are two

modifications of the same tissue-elements. In Peripatus the stigmatic

pits at which the tracheae communicate with the atmosphere are

scattered and not definite in their position. In other cases the

stigmata are definitely paired and placed in a few segments or in

several. It seems that we have to suppose that the vasifactive tissue

of Arthropoda can readily take the form of air-holding instead of

blood-holding tubes, and that this somewhat startling change in its

character has taken place independently in several instances--viz. in

the Onychophora, in more than one group of Arachnida, in Diplopoda,

and again in the Hexapoda and Chilopoda.

_The Malpighian Tubes._--This name is applied to the numerous fine

caecal tubes of noticeable length developed from the proctodaeal

invert of ectodermal origin in Hexapods. These tubes are shown to

excrete nitrogenous waste products similar to uric acid. Tubes of

renal excretory function in a like position occur in most terrestrial

Arthropoda--viz. in Chilopoda, Diplopoda and Arachnida. They are also

found in some of the semi-terrestrial and purely aquatic Amphipod

Crustaceans. But the conclusion that all such tubes are identical in

essential character seems to be without foundation. The Malpighian

tubes of Hexapods are outgrowths of the proctodaeum, but those of

Scorpion and the Amphipod Crustacea are part of the metenteron or

endodermal gut, though originating near its junction with the

proctodaeum. Hence the presence or absence of such tubes cannot be

used as an argument as to affinity without some discrimination. The

Scorpion's so-called Malpighian tubes are _not_ the same organs as

those so named in the other Tracheata. Such renal caecal tubes seem to

be readily evolved from either metenteron or proctodaeum when the

conditions of the out-wash of nitrogenous waste-products are changed

by the transference from aquatic to terrestrial life. The absence of

such renal caeca in Limulus and their presence in the terrestrial

Arachnida is precisely on a parallel with their absence in aquatic

Crustacea and their presence in the feebly branchiate Amphipoda.

_Group Characters._--We shall now pass the groups of the Arthropoda in

review, attempting to characterize them in such a way as will indicate

their probable affinities and genetic history.

SUB-PHYLUM ARTHROPODA.--The characters of the sub-phylum and those of

the associated sub-phyla Chaetopoda and Rotifera have been given

above, as well as the general characters of the phylum Appendiculata

which comprises these great sub-phyla.

Grade A.--Hyparthropoda.

Hypothetical forms.

Grade B.--Protarthropoda.

(a) The integument is covered by a delicate soft cuticle (not firm or

plated) which allows the body and its appendages great range of

extension and contraction.

(b) The paired claws on the ends of the parapodia and the fang-like

modifications of these on the first post-oral appendages (mandibles)

are the only hard chitinous portions of the integument.

(c) The head is deuterognathous--that is to say, there is only one

prosthomere, and accordingly the first and only pair of hemignaths is

developed by adaptation of the appendages of the second somite.

(d) The appendages of the third somite (second post-oral) are clawless

oral papillae.

(e) The rest of the somites carry equi-formal simple appendages,

consisting of a corm or axis tipped with two chitinous claws and

devoid of rami.

(f) The segmentation of the body is anomomeristic, there being no

fixed number of somites characterizing all the forms included.

(g) The pair of eyes situated on the prosthomere are not of the

Euarthropod type, but resemble those of Chaetopods (hence

Nereid-ophthalmous).

(h) The muscles of the body-wall and gut do not consist of

transversely-striped muscular fibre, but of the unstriped tissue

observed also in Chaetopoda.

(i) A pair of coelomoducts is developed in every somite including the

prosthomere, in which alone it atrophies in later development.

(j) The ventral nerve-cords are widely separated--in fact, lateral in

position.

(k) There are no masses of nerve-cells forming a ganglion (neuromere)

in each somite. (In this respect the Protarthropoda are at a lower

stage than most of the existing Chaetopoda.)

(l) The genital ducts are formed by the enlargement of the

coelomoducts of the penultimate somite.

Class (Unica).--ONYCHOPHORA.

With the characters of the grade: add the presence within the body of

fine unbranched tracheal tubes, devoid of spiral thickening, opening

to the exterior by numerous irregularly scattered tracheal pits.

Genera--Eoperipatus, Peripatopsis, Opisthopatus, &c. (See PERIPATUS.)

Grade C (of the Arthropoda).--Euarthropoda.

(a) Integument heavily plated with firm chitinous cuticle, allowing no

expansion and retraction of regions of the body nor change of

dimensions, except, in some cases, a dorso-ventral bellows movement.

The separation of the heavier plates of chitin by grooves of delicate

cuticle results in the hinging or jointing of the body and its

appendages, and the consequent flexing and extending of the jointed

pieces.

(b) Claws and fangs are developed on the branches or rami of the

parapodia, not on the end of the axis or corm.

(c) The head is either deuterognathous, tritognathous, or

tetartognathous.

(d) Rarely only one, and usually at least two, of the somites

following the mandibular somite carry appendages modified as jaws

(with exceptions of a secondary origin).

(e) The rest of the somites may all carry appendages, or only a

limited number may carry appendages. In all cases the appendages

primarily develop rami or branches which form the limbs, the primitive

axis or corm being reduced and of insignificant size. In the most

primitive stock all the post-oral appendages had gnathobasic

outgrowths.

(f) The segmentation of the body is anomomeristic in the more archaic

members of each class, nomomeristic in the higher members.

(g) The two eyes of Chaetopod structure have disappeared, and are

replaced by the Euarthropod eyes.

(h) The muscles in all parts of the body consist of striped muscular

fibre, never of unstriped muscular tissue.

(i) The coelomoducts are suppressed in most somites, and retained only

as the single pair of genital ducts (very rarely more numerous) and in

some also as the excretory glands (one or two pairs).

(j) The ventral nerve-cords approach one another in the mid-ventral

line behind the mouth.

(k) The nerve-cells of the ventral nerve cords are segregated as

paired ganglia in each somite, often united by meristic dislocation

into composite ganglia.

(l) The genital ducts may be the coelomoducts of the penultimate or

ante-penultimate or adjacent somite, or of a somite placed near the

middle of the series, or of a somite far forward in the series.

Class 1 (of the Euarthropoda).--DIPLOPODA.

The head has but one prosthomere (monoprosthomerous), and is

accordingly deuterognathous. This carries short-jointed antennae (in

one case bi-ramose) and eyes, the structure and development of which

require further elucidation. Only one somite following the first

post-oral or mandibular segment has its appendages modified as jaws.

The somites of the body, except in Pauropus, either fuse after early

development and form double somites with two pairs of appendages

(Julus, &c.), or present legless and leg-bearing somites alternating.

Somites, anomomeristic, from 12 to 150 in the post-cephalic series.

The genital ducts open in the fourth, or between the fourth and fifth

post-oral somite.

Terrestrial forms with small-jointed legs formed by adaptation of a

single ramus of the appendage. Tracheae are present.

_Note._--The Diplopoda include the Juliformia, the Symphyla

(Scolopendrella), and Pauropoda (Pauropus). They were until recently

classified with the Chilopoda (Centipedes), with which they have no

close affinity, but only a superficial resemblance. (Compare the

definition of the class Chilopoda.)

The movement of the legs in Diplopoda is like that of those of

Peripatus, of the Phyllopod Crustacea, and of the parapodia of

Chaetopoda, symmetrical and identical on the two sides of the body.

The legs of Chilopoda move in alternating groups on the two sides of

the body. This implies a very much higher development of nerves and

muscles in the latter. (See MILLIPEDE.)

Class 2 (of the Euarthropoda).--ARACHNIDA.

Head tritognathous and diprosthomerous--that is to say, with two

prosthomeres, the first bearing typical eyes, the second a pair of

appendages reduced to a single ramus, which is in more primitive forms

antenniform, in higher forms chelate or retrovert. The ancestral stock

was pantognathobasic--i.e. had a gnathobase or jaw process on every

parapodium. As many as six pairs of appendages following the mouth may

have an enlarged gnathobase actually functional as a jaw or hemignath,

but a ramus is well developed on each of these appendages either as a

simple walking leg, a palp or a chela. In the more primitive forms the

appendage of every post-oral somite has a gnathobase and two rami; in

higher specialized forms the gnathobases may be atrophied in every

appendage, even in the first post-oral.

The more primitive forms are anomomeristic; the higher forms

nomomeristic, showing typically three groups or tagmata of six somites

each.

The genital apertures are placed on the first somite of the second

tagma or mesosoma. Their position is unknown in the more primitive

forms. The more primitive forms have branchial respiratory processes

developed on a ramus of each of the post-oral appendages. In higher

specialized forms these branchial processes become first of all

limited to five segments of the mesosoma, then sunk beneath the

surface as pulmonary organs, and finally atrophied, their place being

taken by a well-developed tracheal system.

A character of great diagnostic value in the more primitive Arachnida

is the tendency of the chitinous investment of the tergal surface of

the telson to unite during growth with that of the free somites in

front of it, so as to form a pygidial shield or posterior carapace,

often comprising as many as fifteen somites (Trilobites, Limulus).

A pair of central monomeniscous diplostichous eyes is often present on

the head. Lateral eyes also are often present which are monostichous

with aggregated lenses (_Limulus_) or with isolated lenses (Scorpio),

or are diplostichous with simple lens (_Pedipalpi_, Araneae, &c.).

Class 3 (of the Euarthropoda).--CRUSTACEA.

Head tetartognathous and triprosthomerous--that is to say, with three

prosthomeres; the first bearing typical eyes, the second a pair of

antenniform appendages (often bi-ramose), the third a pair of

appendages usually antenniform, sometimes claw-like. The ancestral

stock was (as in the Arachnida) pantognathobasic, that is to say, had

a gnathobase or jaw-process on the base of every post-oral appendage.

Besides the first post-oral or mandibular pair, at least two

succeeding pairs of appendages are modified as jaws. These have small

and insignificant rami, or none at all, a feature in which the

Arachnida differ from them. The appendages of four or more additional

following somites may be turned upwards towards the mouth and assist

in the taking of food.

The more primitive forms (Entomostraca) are anomomeristic, presenting

great variety as to number of somites, form of appendages, and

tagmatic grouping; the higher forms (Malacostraca) are nomomeristic,

showing in front of the telson twenty somites, of which the six hinder

carry swimmerets and the five next in front ambulatory limbs. The

genital apertures are neither far forward nor far backward in the

series of somites, e.g. on the fourteenth post-oral in Apus, on the

ninth post-oral in female Astacus and in Cyclops.

With rare exceptions, branchial plates are developed either by

modification of a ramus of the limbs or as processes on a ramus, or

upon the sides of the body. No tracheate Crustacea are known, but some

terrestrial Isopoda develop pulmonary in-sinkings of the integument. A

characteristic, comparable in value to that presented by the pygidial

shield of Arachnida, is the frequent development of a pair of long

appendages by the penultimate somite, which with the telson form a

trifid, or, when that is small, a bifid termination to the body.

The lateral eyes of Crustacea are polymeniscous, with highly

specialized retinulae like those of Hexapoda, and unlike the simpler

compound lateral eyes of lower Arachnida. Monomeniscous eyes are

rarely present, and when present, single, minute, and central in

position.

_Note._--The Crustacea exhibit a longer and more complete series of

forms than any other class of Arthropoda, and may be regarded as

preserving the most completely represented line of descent.

Class 4.--CHILOPODA.

Head triprosthomerous[3] and tetartognathous. The two somites

following the mandibular or first post-oral or buccal somite carry

appendages modified as maxillae. The fourth post-oral somite has its

appendages converted into very large and powerful hemignaths, which

are provided with poison-glands. The remaining somites carry

single-clawed walking legs, a single pair to each somite. The body is

anomomeristic, showing in different genera from 17 (inclusive of the

anal and genital) to 175 somites behind that which bears the poison

jaws. No tagmata are developed. The genital ducts open on the

penultimate somite.

Tracheae are developed which are dendriform and with spiral thickening

of their lining. Their trunks open at paired stigmata placed laterally

in each somite of the trunk or in alternate somites. Usually the

tracheae open by paired stigmata placed upon the sides of a greater or

less number of the somites, but never quite regularly on alternating

somites. At most they are present on all the pedigerous somites

excepting the first and the last. In _Scutigera_ there are seven

unpaired dorsal stigmata, each leading into a sac whence a number of

air-holding tubes project into the pencardial blood-sinus.

Renal caecal tubes (Malpighian tubes) open into the proctodaeum. (See

CENTIPEDE.)

Class 5.--HEXAPODA.

Head shown by its early development to be triprosthomerous and

consequently tetartognathous. The first prosthomere has its appendages

represented by the compound eyes and a protocerebrum, the second has

the antennae for its appendages and a deutocerebral neuromere, the

third has suffered suppression of its appendages (which corresponded

to the second pair of antennae of Crustacea), but has a tritocerebrum

and coelomic chamber. The mandibular somite bears a pair of

gnathobasic hemignaths without rami or palps, and is followed by two

jaw-bearing somites (maxillary and labial). This enumeration would

give six somites in all to the head--three prosthomeres and three

opisthomeres. Recent investigations (Folsom, 4) show the existence in

the embryo of a prae-maxillary or supra-lingual somite which is

suppressed during development. This gives seven somites to the

Hexapod's head, the tergites of which are fused to form a cephalic

carapace or box. The number is significant, since it agrees with that

found in Edriophthalmous Crustacea, and assigns the labium of the

Hexapod to the same somite numerically as that which carries the

labium-like maxillipedes of those Crustacea.

The somites following the head are strictly nomomeristic and

nomotagmic. The first three form the thorax, the appendages of which

are the walking legs, tipped with paired claws or ungues (compare the

homoplastic claws of Scorpio and Peripatus). Eleven somites follow

these, forming the abdominal "tagma," giving thus twenty-one somites

in all (as in the higher Crustacea). The somites of the abdomen all

may carry rudimentary appendages in the embryo, and some of the hinder

somites may retain their appendages in a modified form in adult life.

Terminal telescoping of the abdominal somites and excalation may occur

in the adult, reducing the obvious abdominal somites to as few as

eight. The genital apertures are median and placed far back in the

series of somites, viz. the female on the seventh abdominal

(seventeenth of the whole series) and the male on the ninth or

ante-penultimate abdominal (nineteenth of the whole series). The

appendages of the eighth and tenth abdominal somites are modified as

gonapophyses. The eleventh abdominal segment is the telson, usually

small and soft; it carries the anus.

The Hexapoda are not only all confined to a very definite disposition

of the somites, appendages and apertures, as thus indicated, but in

other characters also they present the specialization of a

narrowly-limited highly-developed order of such a class as the

Crustacea rather than a range from lower more generalized to higher

more specialized forms such as that group and also the Arachnida

present. It seems to be a legitimate conclusion that the most

primitive Hexapoda were provided with wings, and that the term

Pterygota might be used as a synonym of Hexapoda. Many Hexapoda have

lost either one pair or both pairs of wings; cases are common of

wingless genera allied to ordinary Pterygote genera. Sdme Hexapods

which are very primitive in other respects happen to be also Apterous,

but this cannot be held to prove that the possession of wings is not a

primitive character of Hexapods (compare the case of the Struthious

Birds). The wings of Hexapoda are lateral expansions of the terga of

the second and third thoracic somites. They appear to be serial

equivalents (homogenous meromes) of the tracheal gills, which develop

in a like position on the abdominal segments of some aquatic Hexapods.

The Hexapoda are all provided with a highly developed tracheal system,

which presents considerable variation in regard to its stigmata or

orifices of communication with the exterior. In some a serial

arrangement of stigmata comparable to that observed in Chilopoda is

found. In other cases (some larvae) stigmata are absent; in other

cases again a single stigma is developed, as in the smaller Arachnida

and Chilopoda, in the median dorsal line or other unexpected position.

When the facile tendency of Arthropoda to develop tracheal air-tubes

is admitted, it becomes probable that the tracheae of Hexapods do not

all belong to one original system, but may be accounted for by new

developments within the group. Whether the primitive tracheal system

of Hexapoda was a closed one or open by serial stigmata in every

somite remains at present doubtful, but the intimate relation of the

system to the wings and tracheal gills cannot be overlooked.

The lateral eyes of Hexapoda, like those of Crustacea, belong to the

most specialized type of "compound eye," found only in these two

classes. Simple monomeniscous eyes are also present in many Hexapods.

Renal excretory caeca (Malpighian tubes) are developed from the

proctodaeum (not from mesenteron as in scorpion and Amphipoda).