Part 5

=Parasitic Bacteria= live in other living organisms; but the relation between “host” and parasite may vary in considerable degree. Some parasites do no injury to their host, others produce dangerous contagious diseases; some choose only a special kind as host, others again live equally well in many different ones. There are further specific and individual differences with regard to the _predisposition_ of the host, and every individual has not the same receptivity at all times.

THE HARMLESS PARASITES OF HUMAN BEINGS. Several of the above mentioned saprophytes may also occur in the alimentary canal of human beings; _e.g._, the Hay-bacillus, the Butyric-acid-bacillus, etc.; but the gastric juice prevents the development of others, at all events in their vegetative condition. _Sarcina ventriculi_, “packet-bacterium,” is only known to occur in the stomach and intestines of human beings, and makes its appearance in certain diseases of the stomach (dilation of the stomach, etc.) in great numbers, without, however, being the cause of the disease. It occurs in somewhat cubical masses of roundish cells (Fig. 25).

LESS DANGEROUS PARASITES. In the mouth, especially between and on the teeth, a great many Bacteria are to be found (more than fifty species are known), _e.g. Leptothrix buccalis_ (long, brittle, very thin filaments which are united into bundles), Micrococci in large lumps, _Spirochæte cohnii_, etc. Some of them are known to be injurious, as they contribute in various ways to the decay of the teeth (_caries dentium_); a _Micrococcus_, for instance, forms lactic acid in materials containing sugar and starch, and the acid dissolves the lime salts in the external layers of the teeth: those parts of the teeth thus deprived of lime are attacked by other Bacteria, and become dissolved. Inflammation in the tissues at the root of a tooth, is probably produced by septic materials which have been formed by Bacteria in the root-canal.

DANGEROUS PARASITES. In a large number of the infectious diseases of human beings and animals, it has been possible to prove that parasitic Bacteria have been the cause of the disease. Various pathogenic Bacteria of this nature, belonging to the coccus, rod, and spiral Bacteria groups, are mentioned in the following:--

=Pathogenic Micrococci.= _Staphylococcus pyogenes aureus_ produces abscesses of various natures (boils, suppurative processes in internal organs). The same effects are produced by--

_Streptococcus pyogenes_, which is the most frequent cause of malignant puerperal fever; it is perhaps identical with--

_Streptococcus erysipelatis_, which is the cause of erysipelas in human beings.

_Diplococcus pneumoniæ_ (A. Fränkel) is the cause of pneumonia, and of the epidemic cerebro-spinal meningitis.

_Gonococcus_ (Neisser) is the cause of gonorrhea and inflammation of the eyes.

=Pathogenic Rod-Bacteria.= _Bacterium choleræ gallinarum_, an aerobic, facultative parasite which produces fowl-cholera among poultry; it is easily cultivated on various substrata as a saprophyte. The disease may be conveyed both through wounds and by food, and may also be communicated to mammals.

_Bacillus anthracis_, the _Anthrax bacillus_ (Fig. 32), chiefly attacks mammals, especially herbivorous animals (house mice, guinea-pigs, rabbits, sheep, cattle), in a less degree omnivorous animals (including human beings), and in a still less degree the Carnivores. Aerobic. Cylindrical cells, 3–4 times as long as broad, united into long rod-like bodies, which may elongate into long, bent, and twisted filaments. Not self-motile. Endosporous. Germination takes place without the throwing off of any spore-membrane (compare Hay-bacillus p. 37 which resembles it). Contagion may take place both by introduction into wounds, and from the mucous membrane of the intestines or lungs, both by vegetative cells and by spores; in intestinal anthrax, however, only by spores. The Bacillus multiplies as soon as it has entered the blood, and the anthrax disease commences. The Bacilli not only give off poison, but also deprive the blood of its oxygen. Vegetative cells only occur in living animals. This species is a _facultative parasite_ which in the first stage is a saprophyte, and only in this condition forms spores.

_Bacillus tuberculosis_ produces tuberculosis in human beings, also in domestic animals (_perlsucht_). It is a distinct parasite, but may also live saprophytically. It is rod-formed, often slightly bent, and is recognised principally by its action with stains (when stained with an alkaline solution of methyl-blue or carbolic fuchsin, it retains the colour for a long time even in solutions of mineral acids, in contrast with the majority of well-known Bacteria): it probably forms spores which are able to resist heat, dryness, etc.

_Bacillus lepræ_ produces leprosy; _Bacillus mallei_ produces glanders; _Bacillus tetani_, tetanus (the tetanus bacillus is very common in soil; anaerobic); _Bacillus diphtheriæ_, diphtheria; _Bacillus typhosus_, typhoid fever, etc.

=Pathogenic Spiral Bacteria.= _Spirochæte obermeieri_ (Fig. 24) produces intermittent fever (febris recurrens); it makes its appearance in the blood during the attacks of fever, but it is not to be found during intervals when there is no fever. Obligate parasite.

_Spirillum choleræ asiaticæ_ (_Microspira comma_) without doubt produces Asiatic cholera; an exceedingly motile spirillum, which is also found in short, bent rods (known as the “Comma-bacillus”), it lives in the intestines of those attacked by the disease, and gives off a strong poison which enters the body. It is easily cultivated as a saprophyte.

A great many circumstances seem to show that a number of other infectious diseases (syphilis, small-pox, scarlet-fever, measles, yellow-fever, etc.) owe their origin to parasitic Bacteria, but this has not been proved with certainty in all cases.

It has been possible by means of special cultivations (ample supply of oxygen, high temperature, antiseptic materials) to produce from the parasitic Bacteria described above (_e.g._ the fowl-cholera and the anthrax Bacteria) _physiological varieties_ which are distinct from those appearing in nature and possess a less degree of “virulence,” _i.e._ produce fever and less dangerous symptoms in those animals which are inoculated with them. The production of such physiological varieties has come to be of great practical importance from the fact that they are used as vaccines, _i.e._ these harmless species produce in the animals inoculated with them _immunity_ from the malignant infectious Bacteria from which they were derived. This immunity is effected by the change of the products of one or more of the Bacteria, but we do not yet know anything about the way in which they act on the animal organism. The white blood corpuscles, according to the Metschnikoff, play the part of “Phagocytes” by absorbing and destroying the less virulent Bacteria which have entered the blood, and by so doing they are gradually enabled to overcome those of a more virulent nature.

Class 5. =Conjugatæ.=

The Algæ belonging to this class have chlorophyll, and pyrenoids round which starch is formed. The cells divide only in one direction, they live solitarily, or united to form filaments which generally float freely (seldom attached). Swarm-cells are wanting. _The fertilisation is isogamous (conjugation) and takes place by means of aplanogametes._ The zygote, after a period of rest, produces, immediately on germination, one or more new vegetative individuals; sometimes akinetes or aplanospores are formed in addition. They only occur in fresh or slightly brackish water.

Order 1. =Desmidiaceæ.= The cells generally present markings on the outer wall, and are mostly divided into two symmetrical halves by a constriction in the middle, or there is at least a symmetrical division of the protoplasmic cell-contents. The cell-wall consists nearly always of two layers, the one overlapping the other (Fig. 35 _C_). The cells either live solitarily or are united into unbranched filaments. The mass of protoplasm formed by the fusion of the two conjugating cells becomes the zygote, which on germination produces one (or after division 2, 4 or 8) new vegetative individual. The chromatophores are either star-, plate-, or band-shaped, and regularly arranged round the long axis of the cell.

The Desmidiaceæ are not able to swim independently, many species, however, show movements of different kinds by rising and sliding forward on the substratum. These movements, which are partly dependent upon, and partly independent of light and the force of gravitation, are connected with the protrusion of a mucilaginous stalk. The mucilage, which sometimes surrounds the whole individual, may acquire a prismatic structure, it is secreted by the protoplasmic threads which project through certain pores definitely situated in the walls (Fig. 35 _A_, _B_).

VEGETATIVE MULTIPLICATION takes places by division. A good example of this is found in _Cosmarium botrytis_ (Fig. 36 _A-D_). The nucleus and chromatophores divide, and simultaneously the central indentation becomes deeper, the outer wall is then ruptured making a circular aperture through which the inner wall protrudes forming a short, cylindrical canal between the two halves to which it is attached (Fig. 36 _C_). After elongation the canal is divided by a central transverse wall, which commences as a ring round its inner surface and gradually forms a complete septum. The dividing wall gradually splits, and the two individuals separate from each other, each one having an old and a new half. The two daughter-cells bulge out, receive a supply of contents from the parent-cells, and gradually attain their mature size and development (Fig. 36 _B-D_). Exceptions to this occur in some forms.

CONJUGATION takes place in the simplest way in _Mesotænium_, where the two conjugating cells unite by a short tube (conjugation-canal), which is not developed at any particular point. The aplanogametes merge together after the dissolution of the dividing wall, like two drops of water, almost without any trace of preceding contraction, so that the cell-wall of the zygote generally lies in close contact with the conjugating cells. The conjugating cells in the others lie either transversely (_e.g. Cosmarium_, Fig. 37 _d_; _Staurastrum_, etc.), or parallel to one another (_e.g. Penium_, _Closterium_, etc.), and emit a short conjugation-canal (Fig. 37 _d_) from the centre of that side of each cell which is turned towards the other one. These canals touch, become spherical, and on the absorption of the dividing wall the aplanogametes coalesce in the swollen conjugation-canal (Fig. 37 _e_), which is often surrounded by a mucilaginous envelope. The zygote, which is often spherical, is surrounded by a thick cell-wall, consisting of three layers; the outermost of these sometimes bears thorn-like projections, which in some species are simple (Fig. 37 _f_), in others branched or variously marked; in some, however, it remains always smooth (_e.g._ _Tetmemorus_, _Desmidium_). Deviation from this mode of conjugation may occur within certain genera (_e.g._ _Closterium_, _Penium_). Upon germination the contents of the zygote emerge, surrounded by the innermost layers of the wall (Fig. 37 _g_, _h_) and generally divide into two parts which develop into two new individuals, placed transversely to each other (Fig. 37 _i_); these may have a somewhat more simple marking than is generally possessed by the species.

The most frequent genera are:--

_A._ Solitary cells: MESOTÆNIUM, PENIUM (Fig. 38 _B_), CYLINDROCYSTIS, EUASTRUM (Fig. 38 _D_), MICRASTERIAS (Fig. 38 _C_), COSMARIUM (Fig. 36, 37), XANTHIDIUM, STAURASTRUM (Fig. 38 _E_), PLEUROTÆNIUM, DOCIDIUM, TETMEMORUS, CLOSTERIUM (Fig. 38 _A_), SPIROTÆNIA.

_B._ Cells united into filaments: SPHÆROZOSMA, DESMIDIUM, HYALOTHECA, GYMNOZYGA, ANCYLONEMA, GONATOZYGON.

Order 2. =Zygnemaceæ.= Cell-wall without markings. The cells are cylindrical, not constricted in the centre, and (generally) united into simple, unbranched filaments. The whole contents of the conjugating cells take part in the formation of the zygote, which on germination grows out directly into a new filament.

_Spirogyra_ is easily recognised by its spiral chlorophyll band; _Zygnema_ has two star-like chromatophores in each cell (Fig. 40); both these genera are very common Algæ in ponds and ditches.

The conjugation among the Zygnemaceæ takes place in the following manner: the cells of two filaments, lying side by side, or two cells, the one being situated above the other in the same filament (Fig. 41), push out small protuberances opposite each other (Fig. 39 _A_, _a_, _b_); these finally meet, and the dividing wall is absorbed so that a tube is formed connecting one cell with the other; the protoplasmic contents round off, and the whole of these contents of one of the cells glides through the conjugation-tube and coalesces with that of the other (Fig. 39 _B_), the aggregate mass then rounds off, surrounds itself with a cell-wall, and becomes a zygote. A distinct difference may be found between the cells in the two filaments, those in the one whose protoplasmic contents pass over being cylindrical, while those of the recipient one are more barrel-shaped, and of a larger diameter. The former may be regarded as a male, the latter as a female plant. The zygote germinates after a period of rest, and grows out into a new filament (Fig. 42).

Order 3. =Mesocarpaceæ.= The cell-walls are glabrous, unconstricted in the centre, and united into simple unbranched filaments. The chromatophore consists of an axial chlorophyll-plate, with several pyrenoids. The zygote is formed by the coalescence of two cells (Fig. 43) (sometimes three or four), but the whole protoplasmic contents of the cells do not take part in this process, a portion always remaining behind; the aplanogametes coalesce in the conjugation-canal. The zygote thus formed appears incapable of germination until after 3–5 divisions. Of the cells so formed, only one is fertile, the sterile cells, according to Pringsheim, constituting a rudimentary sporocarp. The germinating cells grow out into a new filament. In this order, conjugation has been observed between two cells of the same filament. The Mesocarpaceæ thrive best in water which contains lime.

Class 6. =Chlorophyceæ (Green Algæ).=

These Algæ are coloured green by chlorophyll, seldom in combination with other colouring matter, and then especially with red. The product of assimilation is frequently starch, which generally accumulates round certain specially formed portions of protoplasm termed pyrenoids. The thallus is uni- or multicellular; in the higher forms (certain Siphoneæ) the organs of vegetation attain differentiation into stem and leaf. The asexual reproduction takes place in various ways; the sexual reproduction is effected by conjugation of motile gametes, or by oogamous fertilisation. The swarm-cells (zoospores, gametes, and spermatozoids) are constructed symetrically, and have true protoplasmic cilia, these generally being attached to the front end of the swarm-cells. Most of these Algæ live in water (fresh or salt); some are found upon damp soil, stones, or tree-stems, and some live enclosed in other plants.

The Class is divided into three families:--

1. PROTOCOCCOIDEÆ: Volvocaceæ, Tetrasporaceæ, Chlorosphæraceæ, Pleurococcaceæ, Protococcaceæ, Hydrodictyaceæ.

2. CONFERVOIDEÆ: Ulvaceæ, Ulothricaceæ, Chætophoraceæ, Mycoideaceæ, Cylindrocapsaceæ, Œdogoniaceæ, Coleochætaceæ, Cladophoraceæ, Gomontiaceæ, Sphæropleaceæ.

3. SIPHONEÆ: Botrydiaceæ, Bryopsidaceæ, Derbesiaceæ, Vaucheriaceæ, Phyllosiphonaceæ, Caulerpaceæ, Codiaceæ, Valoniaceæ, Dasycladaceæ.

Family 1. =Protococcoideæ.=

The Algæ which belong to this group are uni- or multicellular with the cells more or less firmly connected, sometimes in a definite, sometimes in an indefinite form (Fig. 47). Colonies are formed either by division or by small unicellular individuals becoming united in a definite manner; the colonies formed in this latter way are termed _Cœnobia_. Apical cells and branching are absent. Multiplication by division; asexual reproduction by zoospores, rarely by akinetes. Sexual reproduction may be wanting, or it takes place by isogamous, rarely by oogamous fertilisation.

Some are attached by means of a stalk to other objects (_Characium_, Fig. 49), others occur as “Endophytes” in the tissues of certain Mosses or Phanerogams, _e.g. Chlorochytrium lemnæ_, in _Lemna trisulca_; _Endosphæra_, in the leaves of _Potamogeton_, _Mentha aquatica_, and _Peplis portula_; _Phyllobium_, in the leaves of _Lysimachia nummularia_, _Ajuga_, _Chlora_, and species of Grasses; _Scotinosphæra_ in the leaves of _Hypnum_ and _Lemna trisulca_; the majority, however, live free in water and in damp places. Many species which were formerly considered to belong to this family have been proved to be higher Algæ in stages of development.

Order 1. =Volvocaceæ.= The individuals in this order are either uni- or multicellular, and during the essential part of their life are free-swimming organisms. They are generally encased in a mucilaginous envelope, through which 2–6 cilia project from every cell. The vegetative reproduction takes place by the division of all, or a few, of the cells of the individual; in some a palmella-stage is found in addition. The sexual reproduction takes place by isogamous or oogamous fertilisation.

The Volvocaceæ may be considered to include the original forms of the Chlorophyceæ, because, among other reasons, the motile stage is here the most prominent; they also form the connecting link between the animal Flagellata, and forms intermediate to the _Syngeneticæ_ may perhaps be found amongst them. Three series of green Algæ may be supposed to have taken their origin from the Volvocaceæ: CONJUGATEÆ (_Desmidiaceæ_) which have lost the swarming stage, but whose conjugation is the nearest to the fertilisation in _Chlamydomonas pulvisculus_: the PROTOCOCCACEÆ in which the vegetative divisions have disappeared, while the swarming stage continues to be present, though of shorter duration; and TETRASPORACEÆ, in which the vegetative divisions are more prominent, whilst the swarming stage is less so.

A. UNICELLULAR INDIVIDUALS. The principle genera are: _Chlamydomonas_, _Sphærella_, _Phacotus_.--_Sphærella nivalis_ is the Alga which produces the phenomenon of “Red Snow,” well known on high mountains and on ice and snow fields in the polar regions. The red colouring matter which appears in this and other green Algæ, especially in the resting cells, is produced by the alteration of the chlorophyll.

_Phacotus lenticularis_ has an outer covering incrusted with lime, which, at death, or after division, opens out into two halves. Species may be found among _Chlamydomonas_, in which conjugation takes place between gametes of similar size without cell-wall, but in _C. pulvisculus_ conjugation takes place between male and female aplanogametes which are surrounded by a mucilaginous envelope.

B. MULTICELLULAR INDIVIDUALS. The most important genera are _Gonium_, _Stephanosphæra_, _Pandorina_, _Eudorina_, _Volvox_.--_Gonium_ has 4 or 16 cells arranged in a definite pattern in a flat plate (Fig. 44). _Pandorina_ (Fig. 45), has 16 cells arranged in a sphere (Fig. 45 _A_). The vegetative reproduction takes place in this way: each cell, after having rounded off, and after the withdrawal of the cilia, divides itself into 16 new ones (Fig. 45 _B_), each forming a new individual, which soon grows to the size of the mother-individual. It was in this Alga that the conjugation of self-motile gametes was first discovered by Pringsheim, 1869. When conjugation is about to take place, each cell divides into sixteen, as in vegetative reproduction, but the 16 × 16 cells all separate from one another (Fig. 45 _C_, female gametes, and _D_, male gametes), and swarm solitarily in the water. The male are, most frequently, smaller than the female, but otherwise they are exactly alike; they are more or less pear-shaped, with a colourless anterior end, 2 cilia, a red “eye-spot,” etc. After swarming for some time they approach each other, two and two, generally a large and a smaller one, and come into contact at their colourless end; in a few moments they coalesce and become one cell (Fig. 45 _E_, _F_), this has at first a large colourless anterior end, 4 cilia, and 2 “eye-spots” (Fig. 45 _G_), but these soon disappear and the cell becomes uniformly dark-green and spherical, and surrounds itself with a thick cell-wall, losing at the same time its power of motion: the zygote (Fig. 45 _H_) is formed, and becomes later on a deep red colour. On the germination of the zygote, the protoplasmic cell-contents burst open the wall (Fig. 45 _J_), and emerge as a large swarmspore (Fig. 45 _K_) which divides into 16 cells, and the first small individual is formed (Fig. 45 _L_, _M_).

_Eudorina_ is like _Pandorina_ in structure, but stands somewhat higher, since the contrast between the conjugating sexual cells is greater, the female one being a motionless oosphere.