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 Grade 1 (of the Arthropoda).—.

With a single pair of parapodial jaws carried by the somite which is perforated by the mouth; this is not the first somite, but the second. The first somite has become a prosthomere, and carries a pair of extensile antennae.

＝ (Peripatus, &c.).

Grade 2 (of the Arthropoda).—.

The third somite as well as the second develops a pair of parapodial jaws; the first somite is a prosthomere carrying jointed antennae.

.

Grade 3 (of the Arthropoda).—.

A gnathobase is developed (in the primitive stock) on every pair of post-oral appendages; two prosthomeres present, the second somite as well as the first having passed in front of the mouth, but only the second has appendages.

Grade 4 (of the Arthropoda).—.

The original stock, like that of the last grade, has a gnathobase on every post-oral appendage, but three prosthomeres are now present, in consequence of the movement of the oral aperture from the third to the fourth somite. The later eyes are polymeniscous, with specialized vitrellae and retinulae of a definite type peculiar to this grade.

According to older views the increase of the number of somites in front of the mouth would have been regarded as a case of intercalation by new somite-budding of new prae-oral somites in the series. We are prohibited by a general consideration of metamerism in the Arthropoda from adopting the hypothesis of intercalation of somites. However strange it may seem, we have to suppose that one by one in the course of long historical evolution somites have passed forwards and the mouth has passed backwards. In fact, we have to suppose that the actual somite which in grades 1 and 2 bore the mandibles lost those mandibles, developed their rami as tactile organs, and came to occupy a position in front of the mouth, whilst its previous jaw-bearing function was taken up by the next somite in order, into which the oral aperture had passed. A similar history must have been slowly brought about when this second mandibulate somite in its turn became agnathous and passed in front of the mouth. The mandibular parapodia may be supposed during the successive stages of this history to have had, from the first, well-developed rami (one or two) of a palp-like form, so that the change required when the mouth passed away from them would merely consist in the suppression of the gnathobase. The solid palpless mandible such as we now see in some Arthropoda is, necessarily, a late specialization. Moreover, it appears probable that the first somite never had its parapodia modified as jaws, but became a prosthomere with tactile appendages before parapodial jaws were developed at all, or rather pari passu with their development on the second somite. It is worth while bearing in mind a second possibility as to the history of the prosthomeres, viz. that the buccal gnathobasic parapodia (the mandibles) were in each of the three grades of prosthomerism only developed after the recession of the mouth and the addition of one, of two, or of three post-oral somites to the prae-oral region had taken place. In fact, we may imagine that the characteristic adaptation of one or more pairs of post-oral parapodia to the purposes of the mouth as jaws did not occur until after ancestral forms with one, with two, and with three prosthomeres had come into existence. On the whole the facts seem to be against this supposition, though we need not suppose that the gnathobase was very large or the rami undeveloped in the buccal parapodia which were destined to lose their mandibular features and pass in front of the mouth.

.—1. Bateson, Materials for the Study of Variation (Macmillan, 1894), p. 85; 2. Lankester, “Primitive Cell-layers of the Embryo.” ''Annals and Mag. Nat. Hist. (1873), p. 336; 3. Korschelt and Heider, Entwickelungsgeschichte'' (Jena, 1892), cap. xv. p. 389; 4. Folsom, “Development of the Mouth Parts of Anurida,” ''Bulletin Mus. Comp. Zool. Harvard College'', vol. xxxvi. No. 5 (1900), pp. 142-146; 5. Lankester, “Observations and Reflections on the Appendages and Nervous System of Apus Cancriformis,” ''Quart. Journ. Micr. Sci.'' vol. xxi. (1881); 6. Hofer, “Ein Krebs mit einer Extremität statt eines Stielauges,” ''Verhandl. d. deutschen zool. Gesellsch. (1894); 7. Watase, “On the Morphology of the Compound Eyes of Arthropods,” Studies from the Biol. Lab. of the Johns Hopkins University'', vol. iv. pp. 287-334; 8. Benham describes backward shifting of the oral aperture in certain Chaetopods, ''Proc. Zoolog. Soc. London'' (1900), No. lxiv. p. 976. N.B.—References to the early literature concerning the group Arthropoda will be found in Carus, Geschichte der Zoologie. The more important literature up to 1892 is given in the admirable treatise on Embryology by Professors Korschelt and Heider. Detailed references will be found under the articles on the separate groups of Arthropoda.

 ARTHUR (Fr. Artus), the central hero of the cycle of romance known as the Matière de Bretagne (see ). Whether there was an historic Arthur has been much debated; undoubtedly for many centuries after the appearance of Geoffrey of Monmouth’s Historia Britonum (circ. 1136), the statements therein recorded of a mighty monarch, who ruled over Britain in the 5th-6th centuries, and carried his conquests far afield, even to the gates of Rome, obtained general, though not universal, credence. Even in the 12th century there were some who detected, and derided, the fictitious character of Geoffrey’s “History.” As was naturally to be expected, the pendulum swung to the other extreme, and in a more critical age the existence of Arthur was roundly denied. The truth probably lies midway between the two. The words of Wace, the Norman poet who translated the Historia into verse, are here admirably to the point. Speaking of the tales told of Arthur, he says:— The opinion now generally accepted by scholars is that the evidence of Nennius, whose Historia Britonum preceded that of Geoffrey by some 400 years, is in the main to be relied on. He tells us that Arthur was Dux bellorum, and led the armies of the British kings against the Saxon invaders, whom he defeated in twelve great battles. Tunc Arthur pugnabat cum regibus Britonum, sed ipse dux erat bellorum.

The traditional site of these battles covers a very wide area, and it is supposed that Arthur held a post analogous to that of the general who, under the Roman occupation, was known as Comes Britanniae, and held a roving commission to defend the island wherever attacked, in contradistinction to the Dux Britanniarum, who had charge of the forces in the north, and the Comes Littoris Saxonici, whose task it was to defend the south-east line. The Welsh texts never call Arthur gwledig (prince), but amheradawr (Latin imperator) or emperor, a title which would be bestowed on the highest official in the island. The truth thus appears to be that, while there was never a King Arthur, there was a noted chieftain and general of that name. If we say that he carried on a successful war against the Saxons, was probably betrayed by his wife and a near kinsman, and fell in battle, we have stated all which can be claimed as an historical nucleus for his legend. It is now generally admitted that the representation of Arthur as world conqueror, Welt-Kaiser, is due to the influence of the Charlemagne cycle. In the 12th century the Matière de France was waning, the Matière de Bretagne waxing in popularity, and public opinion demanded that the central figure of the younger cycle (for whatever the date of the subject matter, as a literary cycle the Arthurian is the younger) should not be inferior in dignity and importance to that of the earlier. When we add to this the fact that the writers of the 12th century represented the personages and events of the 6th in the garb, and under the conditions, of their own time, we can understand the reason of the manifold difficulties which beset the study of the cycle.

But into the figure of Arthur as we know him, other elements have entered; he is not merely an historic personality, but at the same time a survival of pre-historic myth, a hero of romance, and a fairy king; and all these threads are woven together in one fascinating but bewildering web. It is only possible here to summarize the leading features which may be claimed as characteristic of each phase.

Mythic.—Certain elements of the story point to Arthur as a culture hero; as such his name has been identified with the Mercurius Artaius of the Gauls. In this rôle he slays monsters, the boar Twrch Trwyth, the giant of Mont St Michel and the Demon Cat of Losanne (André de Coutances tells us that Arthur was really vanquished and carried off by the Cat, but that one durst not tell that tale before Britons!). He never, it should be