838 PROTOZOA difficult j but we have no reason to suppose that there is any inherent limit to the process of nutrition, growth, and fission, by which continuously the Protozoa are propagated. The act of conjugation from time to time confers upon the protoplasm of a given line of descent new properties, and apparently new vigour. Where it is not followed by a breaking up of the conjugated cells into spores, but by separation and renewed binary fission (Ciliata), the result is described simply as " rejuvenescence." The protoplasm originated by the successive division of substance traceable to one parent cell has become specialized, and in fact too closely adapted to one series of life-conditions ; a fusion of substance with another mass of protoplasm equally specialized, but by experience of a somewhat differing character, imparts to the resulting mixture a new com bination of properties, and the conjugated individuals on separation start once more on their deathless career with renewed youth. CLASSIFICATION* OF THE PROTOZOA. In attempting a scheme of classification it would be most in accordance with the accepted probabilities of the ancestral history of the Protozoa to separate altogether those forms devoid of a nucleus from those which possess one, and to regard them as a lower "grade" of evolution or differentiation of structure. By some systematists, notably Biitschli (9), the presence or absence of a nucleus has not been admitted as a basis of classifica- tory distinction, whilst on the other hand both Haeckel (1) and Huley (10) have insisted on its importance. The fact is that during recent years many of those Protozoa which were at one time supposed to be devoid of nucleus even in a rudimentary form, and furnished therefore the tangible basis for a lowest group of "Protozoa Homogenea" or "Monera," have been shown by the application of improved methods of microscopic investigation to possess a nucleus, that is to say, a differentiated corpuscle of denser protoplasm lying within the general protoplasm, and capable when the organism is killed by alcohol or weak acids of taking up the colour of various dyes (such as carmine and hiematoxylin) more readily and permanently than is the general protoplasm. In such cases the nucleus may be very small and exhibit none of the typical structure of larger nuclei. It is usually surrounded by a clear (i.e., non-granular) halo of the general protoplasm which assists the observer in its detection. Nuclei have been discovered in man} 7 Reticularia (Foraminifera), a group in which they were supposed to be wanting, by Schultzc (11) and the Hertwigs (12) and more recently in the Mycetozoa and in Vampyrella and Protomouas (Zopf, 13), where so excellent an observer as Cienkowski had missed them. It seems therefore not improbable that a nucleus is present though not observed in Protomyxa, Myxastrum, and other similar forms which have been by Haeckel and others classed as " Monera " or " Homogenea." The recently described (14) Archerina (Fig. II. 8, 11) certainly possesses no nucleus in the usual sense of that term, but it is possible that the chlorophyll-coloured corpuscles of that organism should be considered as actually representing the nucleus. Whilst then refraining from asserting that there are no existing Protozoa devoid of nucleus corresponding in this character with non-nucleate Protophyta, such as the Bacteria, we shall not in our scheme of classification institute a group of Homogenea, but shall leave the taking of that step until it has been shown after critical examination that those forms now regarded by some observers as Ho nogenea are really so. In the meantime these forms will find tli rir places alongside of the Nucleata most nearly allied to them in other characters. The Protozoa with a definite permanent cortical substance of differentiated protoplasm are undoubtedly to bt; regarded as evolved fro:n forms devoid of such differentiation of their substance, and we accordingly take this feature as the indication of a primary division of the Protozoa. 1 The lower grade, the Gymnomyxa, afford in other respects evidence of their being nearly related to the ancestral forms from which the Corticata (the higher grade) have developed. The Gymnomyxa all or nearly all, whilst exhibiting amoeboid movement and the flowing of their protoplasm in o " pstudopodia" of very varied shapes, produce spores which 8 -vim by means of one or two flagella of vibratile protoplasm (monadiform young or flagelluhe). These flagellate young forms 1 The " exoplasm " and " cndoplasm " described in Amoebae, &c. , by some authors are not distinct layers but one and the same con tinuous substance what was internal at one moment becoming ex ternal at another, no really structural difference existing between them. Sections. Proteana. Plasmodiata. Lobosa. are closely related to the Flagellata, a group of the Corticata from which it seems probable that the Dinoflagellata, the Ciliata, and the Acinetaria have been derived. The Gymnomyxa themselves cannot, on account of the small number of structural features which they offer as indications of aflinity and divergence in genetic relationships inter sc, be classified with anything like confidence in a genealogical system. We are obliged frankly to abandon the attempt to associate some of the simpler forms with their nearest genetic allies and to content ourselves with a more or less artificial system, .hich is not, however, artificial in so far as its main groups are concerned. Thus the genetic solidarity of each of the large classes Heliozoa, Reticularia, Mycetozoa, and Radiolaria is not open to question. The Lobosa on the other hand appear to be a more artificial assemblage, and it is difficult to say that genetically there is any wide separation between them and the Mycetozoa or between the Mycetozoa and some of the simpler forms which we bring together under the class Proteomyxa. The scheme of classification which we adopt is the following : PROTOZOA. GRADE A. GYMNOMYXA. Class I. PROTEOMYXA. Ex. Vampyrella, Frotomyxa, Archerina. Class II. MYCETOZOA. Ex. The Eu-mycetozoa of Zopf. Class III. LOBOSA. Ex. Amoeba, Arcella, Pelomyxa. ( Class IV. LABYRINTHULIDEA. I Ex. Labyrinthula, Chlamydomyxa. j Class V. HELIOZOA. ., J Ex. Actinophrys, Raphidiophrys, C lathrulina. 1 Class VI. RETICULARIA. Ex. Gromia, Lituola, Asti orhi~a, Globigerina.
Class VII. RADIOLARIA.
L Ex. Thalassicolla, Eucyrtidium, Acanthomctra. GRADE B. CORTICATA.
Class I. SPOROZOA. Ex. Gregarina, Coccidium.
Class II. FLAGELLATA. Ex. Monas, Salpingoeca, Euglcna, Volwx. Class III. DINOFLAGELLATA. Ex. Prorocentrum, Ceratium. Class IV. RHYNCHOFLAGELLATA. Ex. Noctiluca. Class V. CILIATA. Ex. Vorticella, Paramcecium, Stentor. Class VI. ACINETARIA. Ex. Acincta, Dendrosoma. The genetic relationships which probably obtain among these groups may be indicated by the following diagram : Class Acinetaria. Class Rhyncbo tiagellata. Class Dino-flagellata Lipostoma. Stomato- phora.
Class Cporozoa (1 rotonucleala ) Hujnogenea Literature. Certain works of an older date dealing with micro scopic organisms, and therefore including many Protozoa, have historical interest. Among these we may cite 0. F. Miiller,
Animalcula Infusoria, 1786; Ehrenberg, Infusionsthicrchcn, 1838;