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ANAXONIA

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Originally appearing in Volume V18, Page 869 of the 1911 Encyclopedia Britannica.
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ANAXONIA : Forms destitute of axes, and consequently wholly irregular in See also:

form, e.g. Amoebae and many See also:sponges. B. AXONIA : Forms with definite axes. I. HoMAxoNIA, all axes equal. (a) See also:Spheres, where an indefinite number of equal axes can be See also:drawn through the See also:middle point, e.g. Sphaerozoum. (b) Polyhedra, with a definite number of like axes. Of these a considerable number occur in nature, for example, many radiolarians (fig. 3), See also:pollen-grains, &c., and they are again classifiable by the number and regularity of their faces. II.

PROTAXONIA, where all the parts are arranged See also:

round a See also:main See also:axis, and of these we distinguish I. Monaxonia, with not more than one definite axis. Here are distinguished (a) those with similar poles, See also:spheroid (Coccodiscus) and See also:cylinder (Pyrosoma) and (b) those with dissimilar poles, See also:cone (Conulina). 2. Stauraxonia, where, besides the main axes, a definite number of secondary FIGE t h.mo shaeRaa ¢diolarian axes are placed at right angles, and the p endos ,hae an stereometric ground-form becomes a rP See also:pyramid. Here, again, may be dis- See also:polyhedron with equitinguished (a) those with poles similar, angular faces. Type Stauraxonia homopola, where the stereo- of Homaxonia. metric form is the See also:double pyramid (fig. 4), and (b) those with poles dissimilar, Stauraxonia heteropola, where the stereometric form is the single pyramid, and where we distinguish a basal, usually oral, See also:pole from an apical, aboral or anal pole. The bases of these may be either See also:regular or irregular polygons, and thus a new See also:classification into Homostaura and Heterostaura naturally arises. The simpler See also:group, the I-Iomostaura, may have either an even or an See also:odd number of sides, and thus among the Homostaura we have even-sided and odd-sided, single and double pyramids. In those Homostaura with an even number of sides, such as medusae, the radial and inter-radial axes have similar poles; but in the See also:series with an odd number of sides, like most echinoderms, each of the transverse axes is See also:half radial and half semi-radial (fig. 5).

Of the group of regular double pyramids the twelve-sided pollen-See also:

grain of Passiflora (fig. 4) may be taken as an example, having the ground-form of the hexagonal See also:system, the hexagonal See also:dodecahedron. Of the equal even-sided single pyramids 0Heteropola homostaura), Alcyonium, eryonia, See also:Aurelia may be taken as examples of the eight-sided, six-sided, and four-sided, pyramids while those with an odd number of sides may be illustrated by Ophiura or Primula with five sides, and the See also:flower of See also:lily or See also:rush with three sides. In the highest and most complicated group, the Heterostaura, the basal See also:polygon is no longer regular but amphithect (ltu LBnsrso =double-edged). Such a polygon has an even number of sides, • and can be divided into symmetrical halves by each of two planes intersecting at right angles in the middle point, and thus dividing the whole figure into four congruent polygons. The longer of these axes may be termed lateral, the shorter the See also:equatorial or dorsoventral; and these two axes, along with the main axes, always define the three dimensions of space. Ctenophores (fig. 6) furnish examples of eight-sided amphithect pyramids, some Madrepore See also:Corals of six-sided, Crucifers, some Medusae, and Cestodes of four-sided amphithect pyramids. In these forms the poles of the dorso- ventral and lateral axes are similar, and, as in the preceding Monaxonia and Stauraxonia, the centre of the See also:body is defined by a See also:line; and they are therefore termed Centraxonia, while the See also:Pro- taxonia which are defined by their central point are called Centrostigma. There are, however, other forms, and these the most complicated, in which the poles of at Ground-form an eight-sided form a five-sided half amphi- double amphithect pyramid. thect pyramid. least the dorso-ventral axis are unlike, and in which the body is thus defined not with reference to a line but to a median See also:plane, and these have accordingly received the name of Centropipeda. Their ground-form is a polygon with an even number of sides, which can only be divided into two symmetrical halves by the one median plane.

It can be obtained by halving an amphithect pyramid of double the number of sides, and is consequently termed a half amphithect pyramid (fig. 7). The whole amphithect pyramid may be most conveniently obtained by the reduplication of the ground-form as if in a See also:

mirror. Of half amphithect pyramids'there are again two forms, termed by See also:Haeckel Amphipleura and Zygopleura, the former including the " bilaterally symmetrical " or irregularly radiate forms of previous authors, such as Spatdngus See also:Viola, Orchis, while the Zygopleura include forms bilaterally symmetrical in the strictest sense, in which not more than two radial planes, and these at right angles to each other, are See also:present. The sterometric ground-form is a half rhombic pyramid. Haeckel again divides these, according to the number of antimeres, into Tetra pleura and Dipleura. Promorphology has thus shown that the reigning See also:dogma of the fundamental difference of organic and See also:mineral forms is false, and that a See also:crystallography of organic forms is possible—the form of the See also:cell or the cell-aggregate differing from the crystal merely by its more or less viscous See also:state of See also:aggregation, its inherited peculiarities, and its greater adaptability to the environment. The classification into bilateral and radiate forms which usually does See also:duty for more precise promorphological conceptions must be abandoned as hopelessly confusing essentially different forms, or at least must be rigidly restricted—the See also:term radial to regular and double pyramids, the term bilateral to the Centropipeda if not indeed to dipleural forms. Similarly the topographical and relative terms, anterior and posterior, upper and under, See also:horizontal and See also:vertical, must be superseded by the terms above applied to the axes and their poles, oral and aboral, dorsal and ventral, right and See also:left. Nature of Morphological Changes.—The main forms of organic structure being analysed and classified and their See also:stage of individuality being ascertained, the question next arises, by what morphological changes have they arisen, and into what categories can these modes of differentiation be grouped? They at first sight seem innumerable, yet in reality are few. See also:Goethe somewhat vaguely generalized them for the flower as ascending and descending See also:meta-morphosis, expansion and contraction of See also:organs, &c.; but the first See also:attempt at careful enumeration seems to be that of Auguste de St Hilaire, who recognized defects of development, adherences, excesses of See also:production or " dedoublements," See also:metamorphosis and displacement of organs.

Subsequent authors have variously treated the subject; thus See also:

Asa See also:Gray enumerates as modifications of theflower—coalescence, adnation, irregularity, See also:abortion, non-See also:alternation or anteposition, multiplication,. enation, unusual development of the axis, and other morphological modifications connected with fertilization. These are obviously too numerous, as may be best shown by a single comparison with the view of an See also:animal morphologist. Thus See also:Huxley, in discussing the arrangement of the See also:Vertebrata, recognizes only three processes of modification, not only in the ancestral See also:evolution of the Eguidae, but in the individual development of animals generally; these are " (I) excess of development of some parts in relation to others, (2) partial or See also:complete suppression of certain parts, (3) coalescence of parts originally distinct." The particular form of excess of development which results in the repetition of parts, and the morphological changes due to partial or complete See also:fusion, of such repeated parts receive See also:special treatment in the See also:article See also:METAMERISM. Nature of Morphological Correspondence—Categories of Homology. —To indicate all the steps by which the See also:idea of morphological re-semblance has been distinguished from that of physiological would be to examine the whole See also:history of See also:morphology; it must suffice to discuss the terminology of the subject which has, as ever, served not only as an See also:index but as an See also:engine of progress. For these two distinct forms of resemblance the terms homology and See also:analogy gradually became specialized, and were finally established and clearly defined by See also:Owen in 1843—" the former as the same See also:organ in different animals under every variety of form and See also:function (e.g. fore-limbs of See also:Draco volans and wings of See also:Bird) ; the second as a See also:part or organ in one animal which has the same function as another part or organ in a different animal (e.g. See also:parachute of Draco and wings of Bird)." He further distinguishes three kinds of homology: (1) special, being " that above defined, namely, the See also:correspondence of a part or organ determined by its relative position and connexions with a part or organ in a different animal, the determination of which homology indicates that such animals are constituted on a See also:common type," e.g. basilar See also:process of human occipital with basi-occipital of See also:fish; (2) See also:general, that " higher relation in which a part or series of parts stands to the fundamental or general type, involving a knowledge of the type on which the group in question is constituted," e.g. the same human See also:bone and centrum of the last See also:cranial vertebra; (3) serial homology, " representative or repetitive relation in the segments of the same See also:skeleton " (demonstrated when general and special homologies have been determined) ; thus usually the basi-occipital and basi-sphenoid are " homotypes." These terms were henceforth accepted by naturalists; but the criterion of analogy and homology became for L. See also:Agassiz and other embryologists developmental as well as See also:comparative, reference to the ideal archetype becoming less and less frequent. Passing over the discussions of L. Agassiz and See also:Bronn, of which the latter is criticized and partly incorporated by Haeckel, we find the last-named (1) placing serial under general homology; (2) erecting categories of homology partially corresponding to those of individuality—(a) homotypy (of antimeres), hence distinct from that of Owen, (b) homodynamy (of metameres), (c) homonomy (of parts arranged on transverse axes) ; (3) defining special homology in terms of identity of embryonic origin. In 1870 this latter point was more fully insisted upon by See also:Ray Lankester, who, decomposing it into two others, proposed to supersede the term homology by homogeny, being the correspondence of common descent, and homoplasy, denoting any superinduced correspondence of position and structure in .parts embryonically distinct. Thus, the fore-See also:limb of a mammal is homogenous with that of a bird, but the right and left ventricles of the See also:heart in both are only homoplastic, these having arisen independently since the divergence of both See also:groups from a uni-ventriculate ancestor in relation to similarity of physiological needs. St G.

See also:

Mivart next proposed to retain homology as a generic term, with homogeny and homoplasy as two See also:species under it, and carried the See also:analysis into See also:great detail, distinguishing at first twenty-five, but later fifteen, kinds of correspondence: (1) parts similar in function only, e.g. legs of See also:lizard and See also:lobster; (2) parts similar both in function and relative position, wings of See also:bat and bird; (3) parts of common descent, fore-limb of See also:horse and See also:rhinoceros; (4) parts of similar embryonic origin, whatever be their racial genetic relations, e.g. occipitals of See also:panther and See also:perch; (5) parts of dissimilar embryonic origin, whatever be their racial genetic relations, e.g. legs of See also:Diptera; (6, 7, 8, 9, 10) laterally, vertically, serially, anteroposteriorly and radially homologous parts; (11) subordinate serial homologues, e.g. See also:joints of antenna; (12 and 13) secondary and See also:tertiary subordinate serial homologues; (14 and 15) special and general homologies (in Owen's sense). In his Kalkschwamme Haeckel proposed to term homophyly the truly phylogenetic homology in opposition to homomorphy, to which genealogic basis is wanting; and finally Von See also:Jhering has published a repetition of Lankester's view. In this discussion, as in that of individuality, it is evident that we are dealing with numerous logical See also:cross-divisions largely corresponding, no doubt, to the complex See also:web of inter-relations presented by nature, yet remaining in need of disentanglement. Though we must set aside analogies of functional activity, the resemblances in See also:external shape or geometric ground-form which correspond to these, e.g. See also:Hydrozoa and Bryozoa, Fishes and Cetaceans, mimetic FfG. 5.—Starfish, an example of Heteropola homostaura. Ground-form a regular single pyramid of five sides. organisms, are nevertheless, as our historic survey showed, the first which attract See also:attention; and these homoplastic or homomorphic forms, as Haeckel has shown, come as fairly within the See also:province of the promorphologist as do isomorphic crystals within that of his an-organological colleague the crystallographer. Here, too, would be considered " radial," " vertical," lateral " homology, " homotypy of antimeres," and all questions of symmetry, for which Haeckel's nomenclature of homaxonial, homopolic, &c., is distinctly preferable. Entering the See also:field of tectology or morphology in the See also:ordinary sense, we may next consider whether two organisms compared are of the same See also:category of individuality—are homocategoric; and under this serial homology, for instance, would come as a See also:minor See also:division, the correspondence between the See also:units or parts of units of a linear dyad-deme or triad. From a third point of view, that of the embryologist, we trace the development of each multi-cellular organism (I) from the embryonic layers and systems into which the secondary unit (gastrula or plant embryo) differentiates, (2) from a unit-deme or unit of the inferior See also:order or orders of individuality. The parts and units thus recognized by ontogenetic See also:research, respectively or successively homodermic, homosystemic and homodemic, may then conveniently be termed (indifferently See also:save for considerations of priority) either " specially homologous, (indifferently " homogenous," " homophylic," or " homogenetic," in the See also:language of phylogenetic theory.

These three great classes of morphological correspondence—promorphological, tectological and embryological —may or may not coincide. But the completest homology, in which all forms of resemblance unite and from which they differentiate, is that expressed in the cell theory, or rather in that ovum theory which underlies it, and which Agassiz therefore not unjustly regarded as " the greatest See also:

discovery in the natural sciences of See also:modern times." See also:Orientation and Subdivisions of Morphology.—The position of morphology in the classification of the sciences and the proper mode of subdividing it cannot be discussed within these limits, although the latter is especially the subject of much disagreement. The position above assumed, that of including under morphology the whole statical aspects of the organic See also:world, is that of Haeckel, See also:Spencer, Huxley and most See also:recent animal morphologists; botanists frequently, however, still use the term under its earlier and more limited significance (see See also:PLANTS: Morphology). (P. GE.; P. C.

End of Article: ANAXONIA

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