8 



PROTOZOA 



does not constitute an essential difference. The fission of 

 the Ciliate Protozoon, Opalina (see below Fig. XXIV. 4-8), 

 is a step from the ordinary process of delayed binary divi- 

 sion towards spore-formation. In some Protozoa spores are 

 produced after encystation by a perfectly regular process 

 of cleavage (comparable to the cleavage of the egg-cell 

 of Enterozoa) first two, then four, then eight, sixteen, 

 and thirty-two fission products being the result (see 

 Fig. XX. 24, 25, <fec.). 



But more usually there is a hastening of the process, 

 and in these cases it is by no means clear what part the 

 parent cell-nucleus takes. An encysted Gregarina (or two 

 conjugated Gregarinse) suddenly breaks up into a number 

 of equal-sized spores, which do not increase in number by 

 binary division and have not been formed by any such 

 process. This multicentral segregation of the parent pro- 

 toplasm is a marked development of the phenomenon of 

 sporulation and remote from ordinary cell-division. How 

 it is related to ordinary cell-division is not known, inas- 

 much as the changes undergone by the nucleus in this 

 rapid multicentral segregation of the parent protoplasm 

 have not been determined. The spores of Protozoa may 

 be naked or encased singly or in groups in little en- 

 velopes, usually of a firm horny substance (see Fig. 

 XX. 23 to 26, and Fig. XXIV. 15 to 18). Whenever 

 the whole or a part of a Protozoon cell divides rapidly 

 into a number of equal-sized pieces which are simultane- 

 ously set free and are destined to reproduce the adult 

 form, the term spore is applied to such pieces, but the 

 details of their formation may vary and also those of their 

 subsequent history. In typical cases each spore produced 

 as the result of the fission of an encysted Protozoon (con- 

 jugated or single) has its own protective envelope, as in 

 the Mycetozoa (Fig. III.) and the Sporozoa (Fig. XVIII.), 

 from which the contained protoplasm escapes by " ger- 

 mination " as a naked corpuscle either flagellate or amcebi- 

 form. In some terminologies the word " spore " is limited 

 to such a " coated " spore, but usually the naked proto- 

 plasmic particles which issue from such " coated " spores, 

 or are formed directly by the rapid fission of the parent 

 Protozoon, are also called " spores." The former condition 

 is distinguished as a " chlamydospore, " whilst the latter are 

 termed " gymnospores." Many Protozoa produce gymno- 

 spores directly by the breaking up of their protoplasm, 

 and these are either " flagellulse " (swarm-spores) or "arnce- 

 bulse " (creeping spores). The production of coated spores 

 is more usual among the lower plants than it is among 

 Protozoa, but is nevertheless a characteristic feature of 

 the Gregarinse (Sporozoa) and of the Mycetozoa. The 

 term " gemma " or " bud-spore " is applied to cases, few 

 in number, where (as in Acinetaria, Fig. XXVI., Spiro- 

 chona, Fig. XXIII. 10, and Reticularia, Fig. X. 8) the 

 spores are gradually nipped off from the parent-cell one 

 or more at a time. This process diifers from ordinary 

 cell-division only in the facts (1) that the products of 

 division are of unequal size the parent-cell being distin- 

 guishable as the larger and more complete in structure, 

 and (2) that usually the division is not binary, but more 

 than one bud-spore is produced at a time. 



Whilst in the binary cell-division of the Protozoa the 

 two products are usually complete in structure at the 

 period of separation, spores and spore-buds are not only of 

 small size and therefore subject to growth before attaining 

 the likeness of the parent, but they are also very often of 

 simple and incomplete structure. The gap in this respect 

 between the young spore and its parent necessarily varies 

 according to the complexity of the parental form. 



In the case of the Eadiolaria, of the Gregarinse, of 

 Noctiluca, and of the Acinetaria, for instance, the spore 

 has before it a considerable process of development in 



structure and not merely of growth, before attaining the 

 adult characters. Hence there is a possible embryology 

 of the Protozoa, to the study of which the same prin- 

 ciples are applicable as are recognized in the study of the 

 embryology of Enterozoa. Embryonic forms of great sim- 

 plicity of structure, often devoid of nucleus, and consist- 

 ing of simple elongate particles of protoplasm, are hatched 

 from the spore-cases of the Gregarinee (Fig. XVII. 13, 14). 

 These gradually acquire a differentiated cortical protoplasm 

 and a nucleus. A very large number of Gymnomyxa pro- 

 duce spores which are termed " monadiform," that is, have 

 a single or sometimes two filaments of vibratile protoplasm 

 extended from their otherwise structureless bodies. By 

 the lashing of these flagella the spores (swarm-spores or 

 zoospores) are propelled through the water. The resem- 

 blance of these monadiform young (best called " flagel- 

 lulae ") to the adult forms known as Flagellata has led to 

 the suggestion that we have in them a case of recapitula- 

 tive development, and that the ancestors of the Gymno- 

 myxa were Protozoa similar to the Flagellata. Again the 

 Acinetaria produce spores which are uniformly clothed 

 with numerous vibratile cilia (Fig. XXVI.), although the 

 adults are entirely devoid of such structures ; this is 

 accounted for by the supposition that the Acinetaria 

 have been developed from ancestors like the Ciliata, whose 

 characters are thus perpetuated in their embryonic stages. 

 There can be little doubt that these embryological sugges- 

 tions are on the whole justified, and that the nucleated 

 Protozoa are the descendants of non-nucleated forms simi- 

 lar to the spores of Gymnomyxa and Sporozoa, whilst it 

 seems also extremely probable that the ancestral Protozoa 

 were neither exclusively amoeboid in the movement of 

 their protoplasm nor provided with permanent vibratile 

 filaments (flagella and cilia) ; they were neither Myxopods 

 nor Mastigopods (to use the terms which have been intro- 

 duced to express this difference in the character of the 

 locomotor processes), but the same individuals were capable 

 of throwing out their protoplasm sometimes in the form 

 of flowing lobes and networks, sometimes in the form of 

 vibratile flagella. A few such undifferentiated forms exist 

 at the present day among the Proteomyxa and in a little 

 more advanced condition among the lowest Flagellata, e.g., 

 Ciliophrys. 



Death. It results from the constitution of the Proto- 

 zoon body as a single cell and its method of multiplication 

 by fission that death has no place as a natural recurrent 

 phenomenon among these organisms. Among the Entero- 

 zoa certain cells are separated from the rest of the consti- 

 tuent units of the body as egg-cells and sperm-cells ; these 

 conjugate and continue to live, whilst the remaining cells, 

 the mere carriers as it were of the immortal reproductive 

 cells, die and disintegrate. There being no carrying cells 

 which surround, feed, and nurse the reproductive cells of 

 Protozoa, but the reproductive cell being itself and alone 

 the individual Protozoon, there is nothing to die, nothing 

 to be cast off by the reproductive cell when entering on a 

 new career of fission. The bodies of the higher animals 

 which die may from this point of view be regarded as 

 something temporary and non-essential, destined merely to 

 carry for a time, to nurse, and to nourish the more import- 

 ant and deathless fission-products of the unicellular egg. 

 Some of these fission-products of the new individual de- 

 veloped from an egg-cell namely, the egg-cells and sperm- 

 cellsare as immortal as the unicellular Protozoon. This 

 method of comparing the unicellular and the multicellular 

 organism is exceedingly suggestive, and the conception we 

 thus gain of the individuality of the Enterozoon throws 

 light upon the phenomena of reproduction and heredity in 

 those higher organisms. 



Experiment and observation in this matter are extremely 



