PROTOZOA 



difficult ; 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 

 Huxley (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 

 hsematoxylin) 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 



Erotoplasm which assists the observer in its detection. Nuclei 

 ave been discovered in many Reticularia (Foraminifera), a group 

 in which they were supposed to be wanting, by Schultze (11) and 

 the Hertwigs (12) and more recently in the Mycetozoa and in 

 Vampyrella and Protomonas (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 

 Homogenea are really so. In the meantime these forms will find 

 their places alongside of the Nueleata most nearly allied to them 

 in other characters. 



The Protozoa with a definite permanent cortical substance of 

 differentiated protoplasm are undoubtedly to be regarded as evolved 

 from 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 Gj-mnomyxa, 

 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 

 into " pseudopodia " of very varied shapes, produce spores which 

 swim by means of one or two flagella of vibratile protoplasm 

 (monadiform young or flagellulae). These flagellate young forms 



1 The " exoplasm " and "endoplasm" described in Amoeba, &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. 



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 affinity and divergence in genetic 

 relationships inter se, 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, which 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, Protomyxa, Archerina. 

 Class II. MYCETOZOA. 



Ex. The Eu-mycetozoa of Zopf. 

 Class III. LOBOSA. 



Ex. Amteba, Arcella, Pelomyxa. 

 'Class IV. LABYRINTHULIDEA. 



Ex. Labyrinthula, Chlamydomyxa. 

 Class V. HELIOZOA. 



Ex. Actinophrys, Raphidiophrys, Clathrulina, 

 Class VI. RETICULARIA. 



Ex. Gromia, Lituola, Astrorhiza, Globigerina. 

 Class VII. RADIOLARIA. 

 . Ex. Thalassicolla, Eucyrtidium, AcantJwmetra. 



GRADE B. CORTICATA. 

 ( Class I. SPOROZOA. 

 J Ex. Gregarina, Coccidium. 

 Class II. FLAGELLATA. 



Ex. Monas, Salpingoeca, Euglena, Volmx. 

 Class III. DINOFLAGELLATA. 



Ex. Prorocentrum, Ceratium. 

 Class IV. RHYNCHOFLAGELLATA. 



Ex. Noctiluca. 

 Class V. CILIATA. 



Ex. Vortieella, Paramcecium, Slentor. 

 Class VI. ACINETARIA. 

 . Ex. Acineta, Dendrosoma. 

 The genetic relationships which probably obtain among these 

 groups may be indicated by the following diagram : 



Claa Aciuetaria. 



CUM 



Rbjncho-flagellata. 

 Claw 

 Dino-flagellata. 



Sections. 

 Proteana. 



Plasmodiata. 

 Lobosa. 



Filosa. 



Lipostoma. 



Stomato- 

 phora. 



CUas 

 Beliozoa. 



rowomyra. / 



/ 

 / 



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 O. F. Muller, 

 Animalcula Infusoria, 1786; Ehrenberg, Infusionsthierchen, 1838; 



B 



