842 MORPHOLOGY and other monographers substantially concurred, the application of the microscope led to the view suggested by James Clark, and still stoutly supported by Saville Kent, that the Sponge is a city of amoeboid or infusorian individuals. Carter looked upon the separate -ampullaceous sacs as the true individuals, while Schmidt, defining the individual by the possession of a single exhalent aperture, dis tinguishes Sponges into solitary and social. Later, however, he terms them Zoa impcrsonalia. For the higher animals the problem, though perhaps really even more difficult, is less prominent. As Haeckel points out, the earlier -discussions and even the comparatively late essay of Johannes M tiller take an almost purely psychological or at least a physiological point of view ; and the morphological aspect of the inquiry only came forward when the study of much lower forms, such as Cestoid Worms (see PLATYHELMINTHES) or Siphouophores (see HYDROZOA), had raised the difficulties with which botanists had so long been familiar. With the rapid progress of embryology, too, arose new problems ; and in 1842 Steenstrup introduced the conception of an alternation of generations" as a mode of origin of distinct individuals by two methods, for him fundamentally similar, the sexual from im pregnated females and the asexual from unimpregnated "nurses," .a view adopted by Edward Forbes and many other naturalists, but keenly criticized by Carpenter and Huxley. In Leuckart s remark able essay on polymorphism (1853) the Siphonophora were analysed into colonies, and their varied organs shown to be morphologically equivalent, while the alternate generations of Steenstrup were reduced to a case of polymorphism in development. Leuckart further partly distinguished individuals of different orders, as well AS between morphological and physiological individuals. In 1852 Huxley proposed the view which he still substantially maintains (see BIOLOGY). Starting from such an undoubted homo- logy as that of the egg-producing process of Hydra with a free- swimming Medusoid, he points out that the title of individual, if applied to the latter, must logically be due to the former also, and avoids this confusion between organ and individual by defining the individual animal, as Gallesio had done the plant, as the entire product of an impregnated ovum, the swarm of Aphides or free Medusa which in this way might belong to a single individual being termed Zooids. In Carus s System of Animal Morphology (1853) another theory was propounded, but the problem then seems to have fallen into .abeyance until 1865, when it formed the subject of a prolonged and fruitful discussion in the Principles of Biology. Adopting the cell (defined as an aggregate of the lowest order, itself formed of physio logical units) as the morphological unit, Spencer points out that these may either exist independently, or gradually exhibit unions into aggregates of the second order, like the lower Algae, of which the individuality may be more or less pronounced. The union of such secondary aggregates or compound units into individuals of a yet higher order is then traced through such intermediate forms as are represented by the higher seaweeds or the Liverworts, from the thallus of which the axes and appendages of Monocotyledons and Dicotyledons are ingeniously derived. The shoot of a flowering- plant is thus an aggregate of the third order ; it branches into an aggregate of the fourth or higher order, and finally as a tree acquires a degree of composition too complex to be any longer defined." Proceeding to animals, the same method is applied. The Protozoa are aggregates of the first order. These, like plants, exhibit transitions, of which Radiolarians, Foraminifera, and Sponges are taken as examples, to such definite compound wholes as Hydra ; and such secondary aggregates multiply by gemmation into permanent aggregates of the third order, which may exhibit all degrees of integration up to that of the Siphonophora, where the individualities of the Polyps are almost lost in that of the aggregate form. The whole series of articulated animals are next interpreted as more or less integrated aggregates of the third order, of which the lower Annelids are the less developed forms, the Molluscs order. In 1866 appeared the latest morphological classic, the Gcncrclle Morphologic of Haeckel. Here pure morphology is distinguished into two sub-sciences, the first purely structural, tcctology, which regards the organism as composed of organic individuals of different -orders ; the second essentially stereometric, promorphology. To tectology, defined as the science of organic individuality, a large section of the work is devoted. Dismissing the theory of absolute individuality as a metaphysical figment, and starting from the view of Schleiden, De Candolle, and Nageli of several successive categories of relative individuals, he distinguishes more clearly than heretofore the physiological individual (or bion), characterized by definiteness and independence of function, from the morphological individual (or morphon), characterized similarly by definiteness of form ; of the latter he establishes six categories, as follows : 1. Plastides (cytodes and cells), or elementary organisms. 2. Organs (cell-stocks or cell -fusions), simple or homoplastic or gans (tissues), or heteroplastic organs. Organ-systems, organ- apparatuses. Arthropods the more highly integrated and individualized. M and Vertebrates are regarded as aggregates of the second ore 3. Antimeres (opposite or symmetrical or homotypic parts), e.g., rays of radiate animals, halves of bilaterally symmetrical animals." 4. Metameres (successive or homodynamous parts), e.g., stem- segments of Phanerogams, segments or zoonites of Annelids or Vertebrates. 5. Persona, shoots or buds of plants, polyps of Ccelenterates, &c., " individuals " in the narrowest sense among the higher animals. 6. Corms (stocks or colonies), e.g., trees, chains of Salp, polyp- stocks, &c. In his subsequent monograph on calcareous Sponges, and in a final paper, he somewhat modifies these categories by substituting one category of extreme comprehensiveness, that of the idorgan, in place of the three separate orders of organs, antimeres, and meta meres. The idorgan (of course clearly distinguished from the physiological organ or Morgan) is finally defined as a morphological unit consisting of two or more plastids, which does not possess the positive character of the person or stock. These are distinguished into homoplasts or homo-organs and alloplasts or alloc-organs, the former including, as subdivisions, plastid-aggrcgates and plastid- fusions, the latter idomeres, antimeres, and metameres. The former definition of the term antimere, as denoting at once each separate ray of a radiate, or the right and left halves of a bilaterally sym metrical animal, is corrected by terming each ray a paramere, and its symmetrical halves the antimeres. Thus an ordinary Medusoid has four parameres and eight antimeres, a Star-fish five and ten. The con ception of the persona is largely modified, not only by withdrawing the comparison of the animal with the vegetable shoot and by omit ting the antimere and metamere as necessary constituents, but by taking the central embryonic form of all the Metazoa the gastrula (fig. 1) and its assumed ancestral representative, the gastraea as the simplest and oldest form of per sona. The different morphological stages to which it may attain are clas sified into three series : (1) Monax- onial 1 inarticulate persons, i.e., uni- axial and unsegmented without anti meres or metameres, as in Sponges, or lowest Hydroids ; (2) Stauraxonial l inarticulate persons with antimeres, but without metameres, e.g., Coral, Medusa, Turbellarian, Trematode, Bry- ozoon ; (3) Stauraxonial articulate per sons with antimeres and metameres, c. g. , Annelids, Arthropods, Vertebrates. The colonies of Protozoa are mere idor- FIG. 1. Gastrula in optical sec- gans. True corms, composed of united ai?d dicest i ve^clavMty (i>lTs" ^ persons, occur only among Sponges, j, re and arch-enteron), as also Hydroids, Siphonophores, Corals, Bry- outer and inner layers, ectoderm ozoa, Tunicatcs, and Echinoderms, of andendoderm. (After Haeckel.) which the apparent parameres are regarded as highly centralized per- sonse of a radially-budded worm colony; and these can be classified according to the morphological rank of their constituent pcrsonce. They usually arise by gemmation from a single persona, yet in Sponges and Corals occasionally by fusion of several originally distinct persons or corms. The theory of successive subordinate orders of individuality being thus not only derived from historical criticism of previous theories but brought into conformity with the actual facts of development and descent, various groups of organisms being referred to their several categories, the remaining problem of tectology, that of the relation of the morphological to the physio logical individuality, is finally discussed. Of the latter, three cate gories are proposed : (1) the "actual bion or complete physiological individual," this being the completely developed organic form which lias reached the highest grade of morphological individuality proper to it as a representative of, e.g., its species ; (2) the "virtual bion or potential physiological individual," including any incompletely developed form of the former from the ovum upwards ; and (3) the "partial bion or apparent physiological individual," such frag ments of the actual or virtual bion as may possess temporary inde pendence without reproducing the species this latter category having, however, inferior importance. 2 Haeckel s theory, indeed in its earlier form, has been adopted by Gegenbaur and other morphologists, also in its later form by Ja ger, who, however, rejects the category of idorgan on the ground of the general morphological principle that every natural body which carries on any chemical changes with its environment becomes differentiated into more or less concentric layers ; but the subject, especially as far as animals are concerned, is again recently dis cussed in a large work by Terrier. Starting from the cell or plastid, he terms a permanent colony a meride, and these may remain
isolated like Sagitta or Rotifer, or may multiply by gemmation to