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Originally appearing in Volume V28, Page 1031 of the 1911 Encyclopedia Britannica.
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PTEROPODA. ,, IV. ACEPHALA. Sub-Branch 2. Molluscoidea, Class I. TUNICATA. Class II. BRYOZOA. Branch IV. Zoophytes. Sub-Branch 1. Radiaria. Class I. ECHINODERMS. Class III. CORALLARIA Or , II. ACALEPHS. POLYPI. Sub-Branch 2. Sarcodaria. Class I. INFUSORIA. Class II. SPONGIARIA. In England T. H. Huxley adopted in his lectures Huxley's (1869) a classification which was in many respects classifisimilar to both of the foregoing, but embodied im- taboo. provements of his own. It is as follows: Sub-Kingdom I. Protozoa. Classes: RHIZOPODA, GREGARINIDA, RADIOLARIA, SPONGIDA. Sub-Kingdom II. Infusoria. Sub-Kingdom III. Coelenterata. Classes: HYDROZOA, ACTINOZOA. Sub-Kingdom IV. Annuloida. Classes: SCOLECIDA, ECHINODERMATA. Sub-Kingdom V. Annulosa. Classes: CRUSTACEA, ARACHNIDA,MYRIAPODA,INSECTA,CHAETOGNATHA, ANNELIDA. Sub-Kingdom VI. Molluscoida. Classes: POLYZOA, BRACHIOPODA, TUNICATA. Sub-Kingdom VII. Molluscs. Classes: LAMELLIBRANCHIATA,BRANCHIOGASTROPODA,PULMOGASTROPODA,PTEROPODA,CEPHALOPODA. Sub-Kingdom VIII. Vertebrata. ClaSSeS: PISCES, AMPHIBIA, REPTILIA, AYES, MAMMALIA. We now arrive at the period when the doctrine of organic evolution was established by Darwin, and when naturalists,. being convinced by him as they had not been by the transmutationists of fifty years' earlier date, were compelled to take an entirely new view of the significance of all attempts at framing a " natural " classification. Many zoologists—prominent among them in Great Britain being Huxley—had been repelled by the airy fancies and assumptions of the " philosophical " morphologists. Ciassiii-The efforts of the best minds in zoology had been cations directed for thirty years or more to ascertaining based on with increased accuracy and minuteness the struc- structure, microscopic and gross, of all possible forms of lure. animals, and not only of the adult structure but of the steps of development of that structure in the growth of each kind of organism from the egg to maturity. Putting aside fantastic theories, these observers endeavoured to give in their classifications a strictly objective representation of the facts of animal structure and of the structural relationships of animals to one another capable of demonstration. The groups within groups adopted for this purpose were necessarily wanting in symmetry: the whole system presented a strangely irregular character. From time to time efforts were made by those who believed that the Creator must have followed a symmetrical system in his production of animals to force one or other artificial, neatly balanced scheme of classification upon the zoological world. The last of these was that of Louis Agassiz (1807-1873), who, whilst surveying all previous Agassiz. classifications, propounded a scheme of his own (Essay on Classification, 1859), in which, as well as in the criticisms he applies to other systems, the leading notion is that sub-kingdoms, classes, orders and families have a real existence, and that it is possible to ascertain and distinguish characters which are of class value, others which are only of ordinal value, and so on, so that the classes of one sub-kingdom should on paper, and in nature actually do, correspond in relative value to those of another sub-kingdom, and the orders of any one class similarly should be so taken as to be of equal Sub-Branch 2. Vermes. Class I. ANNELIDS. Class IV. CESTOIDEA. „ II. HELMINTHS. ,, V. ROTATORIA. „ III. TURBELLARIA. value with those of another class, and have been actually so created. The whole position was changed by the acquiescence, which became universal, in the doctrine of Darwin. That doctrine took some few years to produce its effect, but it Influence became evident at once to those who accepted Dar-of Dar- winian winism that the natural classification of animals, doctrine after which collectors and anatomists, morphologists, on taxo- philosophers and embryologists had been so long now. striving, was nothing more nor less than a genea- logical tree, with breaks and gaps of various extent in its record. The facts of the relationships of animals to one another, which had been treated as the outcome of an inscrutable law by most zoologists and glibly explained by the transcendental morphologists, were amongst the most powerful arguments in support of Darwin's, theory, since they, together with all other vital phenomena, received a sufficient explanation through it. It is to be noted that, whilst the zoological system took the form of a genealogical tree, with main stem and numerous diverging branches, the actual form of that tree, its limitation to a certain number of branches corresponding to a limited number of divergences in structure, came to be regarded as the necessary consequence of the operation of the physico-chemical laws of the universe, and it was recognized that the ultimate explanation of that limitation is to be found only in the constitution of matter itself. The first naturalist to put into practical form the conse- quences of the new theory, in so far as it affected zoological Haeckel. classification, was Ernst Haeckel of Jena (b. 1834), who in 1866, seven years after the publication of Darwin's Origin of Species, published his suggestive Generelle Morphologic. Haeckel introduced into classification a number of terms intended to indicate the branchings of a genealogical tree. The whole "system " or scheme of classification was termed a genealogical tree (Stammbaum); the main branches were termed " phyla," their branchings " sub-phyla "; the great branches of the sub-phyla were termed " cladi," and the " cladi " divided into " classes," these into sub-classes, these into legions, legions into orders, orders into sub-orders, sub- orders into tribes, tribes into families, families into genera, genera into species. Additional branchings could be indicated by similar terms where necessary. There was no attempt in Haeckel's use of these terms to make them exactly or more than approximately equal in significance; such attempts were clearly futile and unimportant where the purpose was the exhibition of lines of descent, and where no natural equality of groups was to be expected ex hypothesi. Haeckel's classifica- tion of 1866 was only a first attempt. In the edition of the Natiirliche Schopfungsgeschichte published in 1868 he made a great advance in his genealogical classification, since he now introduced the results of the extraordinary activity in the study of embryology which followed on the publication of the Origin Schopfungsgeschichte. Haeckel himself, with his pupil Miklucho-Maclay, had in the meantime made studies on the growth from the egg of Sponges—studies which resulted in the complete separation of the unicellular or equicellular Protozoa from the Sponges, hitherto confounded with them. It is this introduction of the consideration of cell-structure and cell-development which, subsequently to the establishment of Darwinism, has most profoundly modified the views of systematists, and led in conjunction with the genealogical doctrine to the greatest activity in research—an activity which culminated in the work (1873–1882) of F. M. Balfour, and produced the profoundest modifications in classification. HaeckePs 1868 arrangement. Zoophyta. Vermes. Mollusca. Echinoderma. Arthropoda. Vertebrata. In representing pictorially the groups of the animal kingdom as the branches of a tree, it becomes obvious that a distinction may be drawn, not merely between the individual main branches, but further a.s to the level at which they are given off from the main stem, so that one branch or set of branches may be marked off as be-longing to an earlier or lower level than another set of branches; and the same plan may be adopted with regard to the caries, classes and smaller branches. The term " grade " was introduced by Ray Lankester (" Notes on Embryology and Classification," in Quart. Journ. Micr. Sci. 1877), to indicate this giving off of branches at a higher or lower, i.e. a later or earlier, level of a main stem.' The mechanism for the statement of the genealogical relationships of the groups of the animal kingdom was thus completed. Renewed study of every group was the result of the acceptance of the genealogical idea and of the recognition of the importance ' Sir Edwin Ray Lankester (b. 1847) was the eldest son of Edwin Lankester (1814–1874), a physician and naturalist (F.R.S. 1845), who became well known as a scientific writer and lecturer, editor of the Quarterly Journal of Microscopical Science from 1853 to 1871, and from 1862, in succession to Thomas Wakley, coroner for Central Middlesex. Educated at St Pall's and both at Downing College, Cambridge, and Christ Church, Oxford, E. Ray Lankester obtained the Radcliffe Travelling Fellowship at Oxford in 187o, and became a fellow and lecturer at Exeter College in 1872. From 1874 to 1890 he was professor of zoology and comparative anatomy at University College, London; and from 1891 to 1898 Linacre professor of comparative anatomy at Oxford. From 1898 to 1907 he was director of the Natural History Department of the British Museum. He was made K.C.B. in 1907. [Ed. E. B.]. of Species. The pre-Darwinian systematists since the time of Von Baer had attached very great importance to embryological facts, holding that the stages in an animal's development were often more significant of its true affinities than its adult structure. Von Baer had gained unanimous support for his dictum, " Die Entwickelungsgeschichte ist der wahre Lichttrager fur Untersuchungen uber organische Korper." Thus J. Miiller's studies on the larval forms of Echinoderms and the discoveries of Vaughan Thompson were appreciated. But it was only after Darwin that the cell-theory of Schwann was extended to the embryology of the animal kingdom generally, and that the knowledge of the development of an animal became a knowledge of the way in which the millions of cells of which its body is composed take their origin by fission from a smaller number of cells, and these at last from the single egg-cell. Kolliker (Development of Cephalopods, 1844), Remak (Development of the Frog, 1850), and others had laid the foundations of this knowledge in isolated examples; but it was Kovalevsky, by his accounts of the development of Ascidians and of Amphioxus (1866), who really made zoologists see that a strict and complete cellular embryology of animals was as necessary and feasible a factor in the comprehension of their relationships as at the beginning of the century the coarse anatomy had been shown to be by Cuvier. Kovalevsky's work appeared between the dates of the Generelle Morphologic and the Protozoa. Haeckel's second pedigree is as follows: Phyla. Clades. Classes. Archezoa.
End of Article: PTEROPODA
PTERON (Gr. arepov, a wing)

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