Chapter XVIII: Act 1861: s. 31, any person may arrest any one whom he shall find (5)

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.

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_).

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.

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.

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.

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.

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.

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.)]

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.

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.

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.

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.

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).