Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/711

member only of which, Peneus, has a Nauplius stage been detected. The Zoëæ and the Insect possess alike three pairs of limbs for locomotion, and three for ingestion of food. The abdomen in both is without appendages, and the mandibles are without palps. Admitting the resemblances, there is a prior question to be settled in the case of Insects, Myriapods, and Arachnids. Are the temporary embryonal investments of these animals, the cellular and the structureless membranes, to be compared with the blastodermic moultings of Crustaceans, with that membrane whose presence in the Amphipods is accepted by some observers, as the last trace of the Nauplius stage? In the Acarids Claparède found the inner layer to invest the embryo after the outer gave way, and Metschnikoff recognises the deutovum in Platygaster also. The identification with the Nauplius is strongly denied by the last-named observer; but there is still room for further investigation, since embryologists of high reputation differ so entirely on the matter of fact, irrespective of phylogenetic theories. Should the identification be accepted, the Arthropods would, as a group, argree in having a Nauplius stage, different in detail in each class; the second, or Zoëa stage, would differ still more in each, and the homologies of the parts would thus become obscured in details, the identity of the general plan being clearly recognisable. All the Arthropods agree in having the terminal portion of the intestinal canal derived from the outer, the middle portion from the inner embryonic layer. Lastly, the Arthropods, in common with the Molluscs and Annelids, have their body-cavity, or perivisceral space, formed by the splitting of the mesoblast, or derivative layer formed between the outer and inner layer (epiblast and hypoblast, and ectoderm and endoderm). In Insects, in the higher Arachnids, and Crustaceans, yelk segmentation is partial; in Myriapods, and the lower Arachnids, and Crustaceans, it is total; but as in Insects, for example, the unsegmented yelk undergoes at a later period a division into polygonal masses, the difference, though of value in classification, is not of primary importance. The relation of the branchiate Arthropods, the Crustaceans, to the other three tracheate classes, has been discussed chiefly from a phylogenetic point of view. The priority of aquatic to terrestrial forms is assumed, and the derivation of the latter from the former is traced in various ways. The somites or metamera of the Arthropod retain, more than the nervous system which is derived from the epiblast, and still more than the alimentary canal and its appendages, the annulose characteristic that each represents a unit; each may, and many do, give rise to appendages originally similar, but afterwards modified for special functions. Hence the same limbs are tactile in Crustaceans, prehensile in Arachnids, ambulatory in some Crustaceans, accessories of mastication in others, locomotive in some, respiratory in others. In Insects the abdomen of the adult is destitute of appendages, but many larvæ are provided with tracheal gills, that is, external processes in which air-canals ramify, and in which a large quantity of blood is received within the cavity of the thin-walled dilatable process. These processes are destitute of external apertures, the tracheal system is in them closed. Such structures are found but rarely to co-exist with the open condition of this tracheal system. But Pteornarcys, one of the Orthopterous order, is remarkable for this conjunction, the branchial processes of the adult overhanging the stigmata of the tracheæ. Considerable variety exists in the position of the gills. The larvæ of Perla have three thoracic pairs of gills, and are terminal of the abdomen. In other cases they are confined to the abdomen. Now, it is to be noted that these gill projections are not at first tracheal; in Chloea the tracheæ appear after another moult, and they are then vibratile. Further, they are developed from the upper surface of the body. Now, in the Annelids the limbs are typically double pairs projected from the sides of the body; the parapodia consist of two branches, notopodium and neuropodium, and the gills when present are modifications of the notopodium. In the Crustaceans this bipartite condition is indicated by the exopodial and endopodial divisions of the limbs. In the adult insect this duplicity has disappeared, unless we recognise in the position of the gill tracheæ the equivalent of the branchiferous notopodium. Gegenbaur and Lubbock regard the wings as tracheal gills transferred from locomotive organs in water to locomotive functions in air. Gegenbaur thinks that the dropping off of the gills determines the opening of the tracheal system by the stigmata or pores. Further, he assigns to the closed tracheal system a function similar to that of the swim-bladder of fishes, structures primarily useful in flotation, subordinately respiratory in function.

Natural as may seem the assemblage included under Arthropoda, there is no group in which adaptive modifications have introduced so much diversity of anatomical and physiological relations. Metamorphoses, the changes of form which changes of external conditions have promoted, are met with of very various amount. The progress of the embryo from the first appearance of the blastoderm up to sexual maturity of the adult may be direct, without metamorphosis, or may be retarded by changes of form and habit, rendering the young animal capable of sustaining life under very various conditions. In any one of these stages, even in the adult, multiplication may be provided for by a process of budding, the bud from which the new form emerges being in essence undistinguishable from the ovum for whose further development impregnation is necessary. These metamorphoses are probably of late origin in the history of the group, their perpetuation being due to change in their surroundings. Their relations may be “falsified by the struggle for existence,” the details of the developmental history of the family (phylogenesis) may be crowded into a short space in the development of the individual (Ontogenesis). The description of these variations belongs to the particular treatment of the Crustaceans, Myriapods, Arachnids, and Insects.

(j. y.)

ARTHUR, or Artus, a hero of the Welsh Tales, the Chronicles of Geoffrey of Monmouth, and the Romances of the Round Table. His exploits, even the most fabulous, passed with historians, before the days of historical criti cism; subsequently a reaction led to the figure of Arthur being regarded aa nothing but a Celtic myth. The truth, so far as it is possible to arrive at it, lies between these two extremes. There was a real Arthur, one of the last Celtic chiefs in Great Britain ; but there is no single trait of his real character and exploits which legends, working according to laws to be presently discussed, have not re modelled and transfigured or disfigured; while the scarcity of documents makes it impossible to reconstruct a coherent historical picture. Thus the work of comparison between the historical and the legendary personages, such as has been performed for Charlemagne by MM. Gaston Paris and Leon Gautier, is impossible in the case of Arthur. We can only study the legend and analyse its elements.

There is an error, not so popular as it once was, which

supposes that myths and legends arc arbitrary creations, and does not recognise them as having an origin in regular causes, and therefore a rational history, before the period when they are crystallised into their final legendary form, or are merged in the current of a literature in that later and artificial stage when it disinters and refashions old materials. Before Arthur took his final French form in the Romances of the Round Table, he was a Celtic hero in the Breton, and more specifically still in the yet earlier Welsh, legends.

And behind these is the original Arthur, of whom we must