Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/860

Rh 83(5 PROTOZOA whole protoplasmic body of the Protozoon may even assume the form of a slowly changing network of threads of greater or less tenuity (Chlamydomyxa, Fig. VI.). Nutrition. Typically that is to say, by determinate hereditary tendency the Protozoa take solid food-particles into their protoplasm which form and occupy with the water surrounding them &quot; gastric vacuoles &quot; in the protoplasm. The food -particle is digested in this vacuole, by what chemical processes is not ascertained. It has been shown that the contents of the gastric vacuole give in some cases an acid reaction, and it is not improbable that free acid is secreted by the surrounding protoplasm. It is not known whether any ferment 1 is separated by the protoplasm, but it is probable from observations made on the digestive process of Coelentera (Actiniae) that the ferment is not separated, but that actual contact of the food-particle with the protoplasm is necessary for a &quot; ferment influence &quot; to be exerted. The digestion of a food-particle by a Protozoon is intra-cellular, and has been contrasted with the cavitary digestion of higher animals. In the latter, ferments and acids are poured out by the cells bounding the enteric cavity into that space, and digestion is extra-cellular. In the lowest Enterozoa (many Coelentera and some Planarian worms) it has been shown that food-particles are actually taken up in a solid state by the soft protoplasm of the enteric cells and thus subjected to intra cellular digestion. There appears to be a gradual transition from this process, in which close contact with living protoplasm is necessary that the solution of an albuminous food-particle may be effected, onwards to the perfectly free cavitary digestion by means of secretions accumulated in the enteron. We have not yet any satisfactory observations on the chemistry of intra-cellular digestion either of Protozoa or of Coelentera. Certain Protozoa which are parasitic do not take solid food particles ; they (like higher parasites, such as the Tapeworms) live in the nutritious juices of other animals and absorb these by their general surface in a liquid state. The Gregarinai (Sporozoa), many Ciliata, &c., are in this case. Other Protozoa are known which are provided with chlorophyll corpuscles and do not take in solid food, but, apparently as a result of exceptional adaptation in which they differ from closely -allied forms, nourish themselves as do green plants. Such are the Volvocinean Flagellata and some of the Dinoflagellata. It has also been asserted that other Protozoa (viz., some Ciliata) even some which possess a well-developed mouth can (and experimentally have been made to) nourish themselves on nitrogenous compounds of a lower grade than albumens such, for instance, as ammonium tartrate. Any such assertions must be viewed with the keenest scepticism, since experi mental demonstration of the absence of minute albuminous particles (e.y., Bacteria) from a solution of ammonium tartrate in which Ciliate Protozoa are nourishing is a matter of extreme difficulty and has not yet been effected. Undigested food-remnants are expelled by the protoplasm of the Protozoon cell either at any point of the surface or by the cell-mouth or by a special cell-anus (some Ciliata, see Fig. XXIV. 22). Inspiration and Excretion. The protoplasm of the Protozoa respires, that is, takes up oxygen and liberates carbonic acid, and can readily be shown experimentally to require a supply of oxygen for the manifestation of its activity. No special respiratory structures are developed in any Protozoa, and as a rule also the products of oxida tion appear to be washed out and removed from the proto plasm without the existence of any special apparatus. 1 The digestive ferment pepsin has been detected by Krufcenberg in the plasmodium of the Mycetozoou Fuligo (flowers of tan). See oil this subject Zopf (13), p. 88. The contractile vacuole which exists in so many Protozoa appears, however, to be an excretory organ. It has been shown to rapidly excrete in a state of solution colouring matters (anilin blue) which have been administered with food particles (8). No evidence has been adduce! to show whether traces of nitrogenous waste-products are present in the water expelled by the contractile vacuole. Chemical Metamorphosis. The form which the various products of the activity of the Protozoon s protoplasm may assume has been noted above. It will be sufficient here to point out that the range of chemical capacities is quite as great as in the cells of the higher Enterozoa. Chitin, cellulose, silicon, calcium carbonate, fats, pigments, and gases can be both deposited and absorbed by it. Owing to the minuteness of the Protozoa, we are at present unable to recognize and do justice to the variety of chemical bodies which undoubtedly must play a part in their economy as the result of the manufacturing activity of their pro toplasm. See, however, Zopf (13), p. 71. Growth and Reproduction. The Protozoon cell follows the same course as tissue-cells, in that by assimilation of nutriment its protoplasm increases in volume and reaches a certain bulk, when its cohesion fails and the viscid droplet divides into two. The coefficient of cohesion varies in different genera and species, but sooner or later the disrupting forces lead to division, and thus to multi plication of individuals or reproduction. The phenomena connected with the division of the nucleus (already alluded to) will be noticed in particular cases below. Whilst simple binary division is almost without excep tion a chief method of reproduction among the Protozoa, it is also very usual, and probably this would be found if our knowledge were complete to have few exceptions, that under given conditions the Protozoon breaks up rapidly into many (from ten to a hundred or more) little pieces, each of which leads an independent life and grows to the form and size of its parent. It will then multiply by binary division, some of the products of which division will in their turn divide into small fragments. The small fragments are called &quot; spores. &quot; Usually the Protozoon before breaking up into spores forms a &quot; cyst &quot; (see above) around itself. Frequently, but not as a necessary rule, j two (rarely three or more) Protozoon cell-individuals come together and fuse into one mass before breaking up into spores. This process is known as &quot;conjugation;&quot; and there can be no doubt that the physiological significance of the process is similar to that of sexual fertilization, namely, that the new spores are not merely fragments of an old individual but are something totally new inasmuch as they consist of a combination of the substance of indi viduals who have had different life experiences. Whilst spore-formation is not necessarily preceded by conjugation, conjugation is not necessarily followed by spore-formation. Among the Mycetozoa the young indi viduals produced from spores conjugate at a very early j period of growth in numbers and form &quot; plasmodia,&quot; and after a considerable interval of feeding and growth the formation of spores takes place. Still more remarkable is j the fact observed among the Ciliata where two individuals conjugate and after a brief fusion and mixture of their i respective protoplasm separate, neither individual (as far I as certain genera at least are concerned) breaking up into spores, but simply resuming the process of growth and recurrent binary division with increased vigour. There is certainly no marked line to be drawn between , reproduction by simple fission and reproduction by spore- formation ; both are a more or less complete dividing of the parent protoplasm into separate masses ; whether the products of the first fission are allowed to nourish thetn- i selves and grow before further fission is carried out or not