PROTOZOA 



ing Protozoa from Protophyta, and the insuperable diffi- 

 culty in really accomplishing the feat satisfactorily, has led 

 at various times to the suggestion that the effort should be 

 abandoned and a group constituted confessedly containing 

 both unicellular plants and unicellular animals and those 

 organisms which may be one or the other. Hacckel has 

 proposed to call this group the Protista (I). 1 On the 

 whole, it is more satisfactory to make the attempt to dis- 

 criminate those unicellular forms which belong to the 

 animal line of descent from those belonging to the veget- 

 able line. It is, after all, not a matter of much conse- 

 quence if the botanist should mistakenly claim a few 

 Protozoa as plants and the zoologist a few Protophyta 

 as animals. The evil which we have to avoid is that some 

 small group of unattractive character should be rejected 

 both by botanist and zoologist and thus our knowledge of 

 it should unduly lag. Bearing this in mind the zoologist 

 should accord recognition as Protozoa to as wide a range 

 of unicellular organisms as he can without doing violence 

 to his conception., of probability. 



A very interesting and very difficult subject of speculation forces 

 itself on our attention when we attempt to draw the line between 

 the lowest plants and the lowest animals, and even comes again 

 before us when we pass in review the different forms of Protozoa. 



That subject is the nature of the first protoplasm which was 

 evolved from not-living matter on the earth's surface. Was that 

 first protoplasm more like animal or more like vegetable proto- 

 plasm as we know it to-day ? By what steps was it brought into 

 existence ? 



Briefly stated the present writer's view is that the earliest proto- 

 plasm did not possess chlorophyll and therefore did not possess the 

 power of feeding on carbonic acid. A conceivable state of things 

 is that a vast amount of albuminoids and other such compounds 

 had been brought into existence by those processes which cul- 

 minated in the development of the first protoplasm, and it seems 

 therefore likely enough that the first protoplasm fl upon these 

 antecedent steps in its own evolution just as animals feed on 

 organic compounds at the present day, more especially as the 

 large creeping plasmodia of some Mycetozoa feed on vegetable 

 refuse. It indeed seems not at all improbable that, apart from their 

 elaborate fructification, the Mycetozoa represent more closely than 

 any other living forms the original ancestors of the whole organic 

 world. At subsequent stages in the history of this archaic living 

 matter chlorophyll was evolved and the power of taking carbon 

 from carbonic acid. The "green" plants were rendered possible 

 by the evolution of chlorophyll, but through what ancestral forms 

 they took origin or whether more than once, i.e., by more than 

 one branch, it is difficult even to guess. The green Flagellate Pro- 

 tozoa (Volvocineaj) certainly furnish a connecting point by which 

 it is possible to link on the pedigree of green plants to the primi- 

 tive protoplasm ; it is noteworthy that they cannot be considered 

 as very primitive and are indeed highly specialized forms as com- 

 pared with the naked protoplasm of the Mycetozoou's plasmodium. 



Thus then we are led to entertain the paradox that though the 

 animal is dependent on the plant for its food yet the animal 

 preceded the plant in evolution, and we look among the lower 

 Protozoa and not among the lower Protophyta for the nearest 

 representatives of that first protoplasm which was the result of a 

 long and gradual evolution of chemical structure and the starting 

 point of the development of organic form. 



The Protozoan Cell-Individual compared with the Typical 

 Cell of Animal and Vegetable Tissues. 



MORPHOLOGY. 



The Protozoon individual is a single corpuscle of proto- 

 plasm, varying in size when adult from less than the 

 TU 1 uj J th of an inch in diameter (some Sporozoa and Flagel- 

 lata) up to a diameter of an inch (Nummulites), and even 

 much larger size in the plasmodia of Mycetozoa. The sub- 

 stance of the Protozoa exhibits the same general properties 

 irritability, movement, assimilation, growth, and division 

 and the same irremediable chemical alteration as the result 

 of exposure to a moderate heat, which are observed in 

 the protoplasm constituting the corpuscles known as cells 

 which build up the tissues of the larger animals and 



1 These numbers refer to the bibliography at p. 866. 



plants. There is therefore no longer any occasion to make 

 use of the word " sarcode " which before this identity was 

 established was very usefully applied by Dujardin (2) to 

 the substance which mainly forms the bodies of the 

 Protozoa. Like the protoplasm which constitutes the 

 " cells " of the Enterozoa and of the higher plants, that 

 of the Protozoon body is capable of producing, by chemical 

 processes which take place in its substance (over and above 

 those related merely to its nutrition), a variety of distinct 

 chemical compounds, which may form a deposit in or 

 beyond the superficial protoplasm of the corpuscle or may 

 accumulate centrally. These products are therefore either 

 ectoplastic or entoplastic. The chemical capacities of 

 protoplasm thus exhibited are very diverse, ranging from 

 the production of a denser variety of protoplasm, probably 

 as the result of dehydration, such as we see in the nucleus 

 and in the cortical substance of many cells, to the chemical 

 separation and deposition of membranes of pure chitin or 

 of cellulose or of shells of pure calcium carbonate or quasi- 

 crystalline needles of silica. 



NUCLEUS. The nucleus is probably universally present in 

 the Protozoon cell, although it may have a very simple struc- 

 ture and be of very small size in some cases. The presence 

 of a nucleus has recently been demonstrated by means of 

 appropriate staining reagents in some Protozoa (shell- 

 bearing Reticularia or Foraminifera and many Mycetozoa) 

 where it had been supposed to be wanting, but we are not 

 yet justified in concluding absolutely that there are not 

 some few Protozoa in which this central differentiation of 

 the protoplasm does not exist ; it is also a fact that in the 

 young forms of some Protozoa which result from the 

 breaking up of the body of the parent into many small 

 " spores " there is often no nucleus present. 



In contrast to this it is the fact that the cells which 

 build up the tissues of the Enterozoa are all derived from 

 the division of a nucleated egg-cell and the repeated 

 division of its nucleated products, and are invariably 

 nucleated. The same is true of tissue-forming plants, 

 though there are a few of the lowest plants, such as the 

 Bacteria, the protoplasm of which presents no nucleus. In 

 spite of recent statements (3) it cannot be asserted that 

 the cells or protoplasmic corpuscles of the yeast-plant 

 (Saccharomyces) and of the hyphs of many simple moulds 

 contain a true nucleus. We are here brought to the 

 question " What is a true nucleus 1 " The nucleus which 

 is handed on from the egg-cell of higher plants and 

 Enterozoa to the cells derived from it by fission has lately 

 been shown to possess in a wide variety of instances such 

 very striking characteristics that we may well question 

 whether every more or less distinctly outlined mass or 

 spherule of protoplasm which can be brought into view by 

 colouring or other reagents, within the protoplasmic body 

 of a Protozoon or a Protophyte, is necessarily to be con- 

 sidered as quite the same thing as the nucleus of tissue- 

 forming egg-cell-derived cells. 



Kesearches, chiefly due to Flemming (4), have shown 

 that the nucleus in very many tissues of higher plants 

 and animals consists of a capsule containing a plasma of 

 " achromatin " not deeply stained by reagents, ramifying 

 in which is a reticulum of "chromatin" consisting of fibres 

 which readily take a deep stain (Fig. I., A). Further it is 

 demonstrated that, when the cell is about to divide into 

 two, definite and very remarkable movements take place 

 in the nucleus, resulting in the disappearance of the 

 capsule and in an arrangement of its fibres first in the 

 form of a wreath (Fig. I., D) and subsequently (by the 

 breaking of the loops formed by the fibres) in the form of a 

 star (E). A further movement within the nucleus leads to 

 an arrangement of the broken loops in two groups (F), the 

 position of the open ends of the broken loops being reversed 



