PROTOZOA 



hydrogen and oxygen to form fat, sugar, and starch. 

 Albumens and fats are not soluble in water and diffusible ; 

 they have to be seized by the animal in the condition 

 of more or less solid particles, and by chemical processes 

 superinduced in the living protoplasm of the animal by 

 the contact of these particles they are acted upon, chemic- 

 ally modified, and rendered diffusible. Hence the animal 

 is provided with a mouth and a digestive cavity, and with 

 organs of locomotion and prehension by which it may search 

 out and appropriate its scattered nutriment. Further the 

 albumens, fats, sugars, and starch which are the necessary 

 food of an animal are not found in nature excepting as 

 the products of the life of plants or of animals ; accord- 

 ingly all animals are in a certain sense parasitic upon 

 either plants or other animals. It would therefore seem 

 to be easy to draw the line between even the most minute 

 unicellular plants and the similarly minute unicellular 

 animals assigning those which feed on the albumens, &c., 

 of other organisms by means of a mouth and digestive 

 apparatus to the animal series, and those which can appro- 

 priate the elements of ammonia, nitrates, and carbonates 

 to the plants. 



Such absolute distinctions lending themselves to sharp 

 definitions have, however, no place in the organic world ; 

 and this is found to be equally true whether we attempt 

 to categorically define smaller groups in the classification 

 of plants and animals or to indicate the boundaries of the 

 great primary division which those familiar names imply. 

 Closely allied to plants which are highly and specially 

 developed as plants, and feed exclusively upon ammonia, 

 nitrates, and carbonates, we find exceptionally modified 

 kinds which are known as " insectivorous plants " and are 

 provided with digestive cavities (the pitchers of pitcher- 

 plants, &c.), and actually feed by acting chemically upon 

 the albumens of insects which they catch in these diges- 

 tive receptacles. No one would entertain for a moment 

 the notion that these insectivorous plants should be con- 

 sidered as animals. The physiological definition separat- 

 ing plant from animal breaks down in their case ; but the 

 consideration of the probable history of their evolution as 

 indicated by their various details of structure suffices at 

 once to convince the most sceptical observer that they 

 actually belong to the vegetable line of descent or family 

 tree, though they have lost the leading physiological char- 

 acteristic which has dominated the structure of other 

 plants. In this extreme case it is made very obvious that 

 in grouping organisms as plants or as animals we are not 

 called upon to apply a definition but to consider the 

 multifarious evidences of historical evolution. And we 

 find in the case of the Protozoa and the Protophyta that 

 the same principle holds good, although, when dealing 

 with extremely simple forms, it becomes much more diffi- 

 cult to judge of the genetic relationship of an organism in 

 proportion as the number of detailed points of possible 

 agreement with and divergence from other forms to which 

 it may be supposed to be related are few. 



The feeding of plants upon carbonic acid is invariably 

 accompanied by the presence of a peculiar green-colouring 

 matter chlorophyll. In virtue of some direct or indirect 

 action of this chlorophyll the protoplasm of the plant is 

 enabled to seize the carbon of the mineral world the car- 

 bon which has sunk to the lowest resting stage of combina- 

 tion and to raise it into combination with hydrogen and 

 oxygen and ultimately with nitrogen. There are plants 

 which have no chlorophyll and are thus unable to feed 

 upon carbonic acid. They are none the less plants since 

 they agree closely with particular chlorophyll-bearing 

 plants in details of form and structure, mode of growth 

 and reproduction. A large series of these are termed 

 Fungi. Though unable to feed on carbonic acid, they do 



not feed as do animals. They can take their carbon from 

 acetates and tartrates, which animals cannot do, and their 

 nitrogen from ammonia. Even when it is admitted that 

 some of these colourless plants, such as the Bacteria 

 (Schizomycetes), can act upon albumens so as to digest 

 them and thus nourish themselves, it is not reasonable to 

 place the Bacteria among animals, any more than it would 

 be reasonable so to place Nepenthes, Sarracenia, and 

 Drosera (insectivorous Phanerogams). For the structure 

 and mode of growth of the Bacteria is like that of well- 

 known chlorophylligerous minute Algaj from which they 

 undoubtedly differ only in having secondarily acquired 

 this peculiar mode of nutrition, distinct from that which 

 has dominated and determined the typical structure of 

 plants. 



So we find in a less striking series of instances amongst 

 animals that here and there the nutritional arrangements 

 which we have no hesitation in affirming to be the leading 

 characteristic of animals, and to have directly and perhaps 

 solely determined the great structural features of the 

 animal line of descent, are largely modified or even alto- 

 gether revolutionized. The green Hydra, the freshwater 

 Sponge, and some Planarian worms produce chlorophyll 

 corpuscles in the protoplasm of their tissues just as green 

 plants do, and are able in consequence to do what animals 

 usually cannot do namely, feed upon carbonic acid. The 

 possibilities of the protoplasm of the plant and of the 

 animal are, we are thus reminded, the same. The fact 

 that characteristically and typically plant protoplasm ex- 

 hibits one mode of activity and animal protoplasm another 

 does not prevent the protoplasm of even a highly developed 

 plant from asserting itself in the animal direction, or of a 

 thoroughly characterized animal, such as the green Hydra, 

 from putting forth its chlorophylligenous powers as though 

 it belonged to a plant. 



Hence it is not surprising that we find among the 

 Protozoa, notwithstanding that they are characterized by 

 the animal method of nutrition and their forms determined 

 by the exigencies of that method, occasional instances of 

 partial vegetable nutrition such as is implied by the deve- 

 lopment of chlorophyll in the protoplasm of a few members 

 of the group. It would not be inconsistent with what is 

 observed in other groups should we find that there are 

 some unicellular organisms which must, on account of 

 their structural resemblances to other organisms, be con- 

 sidered as Protozoa and yet have absolutely given up alto- 

 gether the animal mode of nutrition (by the ingestion of 

 solid albumens) and have acquired the vegetable mode of 

 absorbing ammonia, nitrates, and carbonic acid. Experi- 

 ment in this matter is extremely difficult, but such " veget- 

 able" or "holophytic nutrition " appears to obtain in the 

 case of many of the green Flagellata, of the Dinoflagellata, 

 and possibly of other Protozoa. 



On the other hand there is no doubt that we may fall 

 into an error in including in the animal line of descent all 

 unicellular organisms which nourish themselves by the 

 inception of solid nutriment. It is conceivable that some 

 of these are exceptional creophagous Protophytes parallel 

 at a lower level of structure to the insectivorous Phanero- 

 gams. In all cases we have to balance the whole of the 

 evidence and to consider probabilities as indicated by a 

 widely-reaching consideration of numerous facts. 



The mere automatic motility of unicellular organisms 

 was at one time considered sufficient indication that such 

 organisms were animals rather than plants. We now know 

 that not only are the male reproductive cells of ferns and 

 similar plants propelled by vibratile protoplasm, but such 

 locomotive particles are recognized as common products 

 (" swarm-spores " and " zoospores ") of the lowest plants. 



The danger of dogmatizing erroneously in distinguish- 



