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 " gastric vacuoles " 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 frcm observations made on the digestive 

 process of Ccclentera (Actinia) that the ferment is not 

 separated, but that actual contact of the food-particle with 

 the protoplasm is necessary for a " ferment influence " 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 Ccelentera 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 Ccelentera. 



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 Gregarinse (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.g., Bacteria) from a solution of ammonium 

 tartrate in which Ciliate Protozoa are flourishing 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). 



Respiration 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 tlie 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 Krukenln-vi,' in 

 the plasmodium of the Mycetozooa Fuligo (flowers of tan). See on 

 this subject Zopf (13), p. 88. 



The contractile vacuole which exists in so many Protozoa 

 Mppoars, 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 adduced 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 " spores." Usually the Protozoon 

 before breaking up into spores forms a " cyst " (see above) 

 around itself. Frequently, but not as a necessary rule, 

 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 "conjugation;" 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 

 period of growth in numbers and form "plasmodia," and 

 after a considerable interval of feeding and growth the 

 formation of spores takes place. Still more remarkable is 

 the fact observed among the Ciliata where two individuals 

 conjugate and after a brief fusion and mixture of their 

 respective protoplasm separate, neither individual (as far 

 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 them- 

 selves and grow before further fission is carried out or not 



