Functions of nutrition—Assimilation and disassimilation—Plasmodoma and plasmophaga—Phytoplasm and zooplasm—Plasmodomism of plants—Chlorophyll granules and nitro-bacteria—Plasmophagism of fungi and animals—Metasitism (conversion of metabolism)—Nutrition of the monera (chromacea, bacteria, rhizomonera)—Nutrition of the protophyta and metaphyta (cell-plants and tissue-plants)—Nutrition of the metazoa—- GastrÆa theory—Gastro-canal system of the coelenteria (gastrÆads, sponges, cnidaria, platodes)—Nutrition of the coelomaria (digestion, circulation, respiration, evacuation)—Saprositism—Parasitism—Symbiosis.

The wonder of life which we call, in the widest sense of the word, "nutrition" is the chief factor in the self-maintenance of the organic individual. It is always bound up with a chemical modification of the living matter, an organic metabolism (circulation of matter), and a corresponding circulation of force. In this chemical process plasm is used up, built up afresh, and once more disintegrated. The metabolism which lies at the root of this chemistry of food is the essential feature in the manifold processes of nutrition. A large part of the several nutritive processes are explained without further trouble by the known physical and chemical properties of inorganic bodies; for another part of them we have not yet succeeded in doing this. Nevertheless, all impartial physiologists now agree that it is possible in principle, and that we have no reason to introduce a special vital principle. All the trophic (nutritive) processes, without exception, are subject to the law of substance.

In all the higher plants and animals the chemical process of metabolism, with the stream of energy that accompanies it, is a very complex vital activity, in which many different functions and organs co-operate with the common aim of self-maintenance. As a rule, they are distributed in four groups—namely: (1) Intussusception of food and digestion: (2) distribution of the food in the body, or circulation; (3) respiration, or exchange of gases; and (4) excretion of unusable matter. In most of the histona, either tissue-plants or tissue-animals, a number of organs are differentiated for the accomplishment of these tasks. At the lower stages of life this division of labor is not found, the entire process of nutrition being accomplished by a single layer of cells (lower algÆ, gastrÆads, sponges, lower polyps). In the protists, again, it is the single cell that does all these things itself; in the simplest cases, the monera, a homogeneous plasma-globule. As a long gradation uninterruptedly unites these lowest forms of nutrition with the more complicated forms, we must regard the latter no less than the former as physico-chemical processes.

When we take the whole of the metabolic functions in organisms together, we may look upon them as the outcome of two opposite chemical processes—on the one hand the building-up of living matter by taking in food (assimilation), and on the other the breaking-down of it in consequence of its vital activity (disassimilation). As in every case the plasm is the active living matter, we may say: Assimilation (or plasma-production) consists in the conversion within the organism into the special plasm of the particular species of food that has been received from without; disassimilation (or plasma-destruction) is the result of the work done by the plasm, which is the cause of its partial decomposition or breakdown. In both respects there is a striking difference between the two great kingdoms of organic nature. The plant kingdom is, on the whole, the agent of assimilation, forming new plasm by synthesis and reduction from inorganic matter. In the animal world, on the contrary, disassimilation preponderates, the plasm received being resolved by oxydation, and the actual energy taken out of it by analysis being converted into heat and motion. Plants are plasmodomous; animals, plasmophagous.

Of all the chemical processes the most important, because the most indispensable, for the origin and maintenance of organic life is the constant reconstruction of plasm. We give it the name of plasmodomism (domeo=to build up), or carbon-assimilation. Botanists have the habit of late of calling it briefly assimilation, and have thus caused a good deal of misunderstanding. The more common and older meaning of assimilation in animal physiology is, in the widest sense, the intussusception and preparation of the food received. But the carbon-assimilation in plants—what I call plasmodomism—is only the first and original form of plasma-production. It means that the plant is able, under the influence of sunlight, to form carbohydrates, and from these new plasm, out of simple inorganic compounds (water, carbonic acid, nitric acid, and ammonia) by synthesis and reduction. The animal is unable to do this. It has to take its plasm in its food from other organisms—plant-eaters directly, and animal-eaters indirectly. We therefore give the title of plasmophagous to these animal "plasma-eaters." In working up the foreign plasm it has eaten, and converting it into its own specific form of plasm, the animal also accomplishes assimilation; but this animal albumin-assimilation is totally different from the vegetal carbon-assimilation. The fresh-formed animal plasm is then broken up by oxydation, and by this analysis the energy needed for the vital movements is obtained.

The physiological contrast which we thus find between the two principal forms of living matter, the synthetic plasm of the plant and the analytic plasm of the animal, is of great importance for the lasting maintenance of the whole organic world. It depends on a reversal of the molecular movement in the plasm, the intimate nature of which is just as little known to us as the chemical constitution of the albumins in general, and that of living albumin, the plasm, in particular. As I mentioned in chapter v., modern physiological chemistry has good reason to believe that the invisible albumin-molecule is, comparatively speaking, gigantic, and is composed of more than a thousand atoms. These are in such an unstable equilibrium, so complicated and impermanent an arrangement, that the slightest push or stimulus suffices to alter them and form a new kind of plasm. As a fact, the number and variety of kinds of plasm are immense. This is seen at once from the ontogenetic fact that the ovum and sperm-cell of each species (and each variety) have a specific chemical constitution. In reproduction this is transmitted to the offspring. But, setting aside these countless finer modifications, we may distinguish two chief groups of kinds of plasm: the phytoplasm of the plant, with the synthetic property of plasmodomism, and the zooplasm of the animal, which is destitute of this property, and so confined to plasmophagy.

The remarkable synthetic process of building up the plasm, to which we give the name of plasmodomism, or carbon-assimilation, usually needs as its first condition the radiant energy of sunlight. Every green plant-cell contains in its chlorophyll-granules so many tiny laboratories, their green plasm being able to form new plasm out of inorganic compounds under the influence of light. The water that is needed for this, besides nitrogenous compounds (nitric acid, ammonia), is drawn from the earth by the roots; the carbonic acid is taken from the atmosphere by the green leaves. The immediate products of the synthesis, due to the separation of the carbonic acid, is, as a rule, a non-nitrogenous starch-flour (amylum). This is further used for the composition of the nitrogenous albumin by an as yet unknown synthetic process, with the aid of nitrogenous mineral compounds. In this process of reduction the separated free oxygen is returned to the atmosphere. The carbohydrates that chiefly co-operate in this are glucoses and maltoses: the mineral substances, especially salts of potassium and magnesium, and compounds of these elements with nitric acid, sulphuric acid, and phosphoric acid. Iron is also found to be an important element in the process, though in a very small quantity. As a rule, the ferruginous chlorophyll can only form new plasm with the help of light-waves. The most important part of the spectrum for this purpose is that containing the red, orange, and yellow waves.

The chief factor in plasma-formation in the organic world is the photosynthesis, or ordinary carbon-assimilation by chlorophyll, the wonderful green matter that amounts to only a very small percentage (about one-tenth) of the weight of the chlorophyll-granules, and can be separated from their plasmatic substance by certain methods. Even when the plant has some other color than green the chlorophyll is still the real plasmodomous substance. Its green color is then masked by some other color—diatomin in the yellow diatomes, phycorhodin in the red rhodophyceÆ, phycophÆin in the brown phÆophyceÆ, and phyocyan in the blue-green chromacea or cyanophyceÆ. The latter have an especial interest for us, because in the simplest specimens the entire organism is merely a globular bluish-green granule of plasm. Moreover, in the simplest forms of nucleated primitive plants (algariÆ)—many of the so-called unicellular algÆ—the metabolism is effected by a single grain of chlorophyll. There is usually a large number of them in the plasm of the plant-cells.

Another kind of plasm-synthesis, quite different from the ordinary plasmodomism by chlorophyll and sunlight has lately been discovered in some of the lowest organisms (by Heraeus, Winogradsky, and others). The nitro-bacteria (or nitromonades) are tiny monera (unnucleated cells) that live in complete darkness underground. Their globular colorless plasma-bodies contain neither chlorophyll nor nucleus. They have the remarkable capacity of forming carbohydrates, and from these plasm, by a peculiar synthesis out of purely inorganic compounds—water, carbonic acid, ammonia, and nitric acid. Pfeffer has called this carbon-assimilation, on account of its purely chemical nature, "chemosynthesis," in opposition to the ordinary photosynthesis by means of sunlight. There are also other bacteria (sulphur-bacteria, purple-bacteria, etc.) that show various peculiarities of metabolism. The nitro-bacteria must belong to the oldest monera, and represent a transition from the vegetal chromacea to the animal bacteria.

The extensive class of the fungi (or mycetes) resembles a part of the bacteria in regard to metabolism. These organisms are, it is true, generally regarded as plants, but they have not the capacity of the green, chlorophyll-bearing plants to supply themselves with carbon from the carbonic acid in the atmosphere. They have to take it from organic substances, such as albumin, carbohydrates, etc., like the animals. But while the animals have to derive their nitrogen from the latter, the fungi can obtain it from inorganic matter in the earth. Fungi cannot support life without the addition of organic compounds; but we can make them grow in a food solution consisting of sugar and purely inorganic nitrogenous salts. Thus they are on the border that separates the plasmodomous plants from the plasmophagous animals. Like the latter, the fungi have evolved from the plants through changed food conditions. We find this process even among the unicellular protists in the phycomycetes, which descend from the siphonea. In the same way the real multicellular fungi (ascomycetes and basimycetes) may be traced to the tissue-forming algÆ.

All true animals have to derive their food from the plant kingdom, the vegetal feeders directly, and the flesh feeders indirectly, when they consume vegetal feeders. Hence the animals are, in a certain sense, as the older natural philosophy put it four hundred years ago, "parasites of the plant world." From the point of view of phylogeny, the animal kingdom is, therefore, clearly much younger than the plant kingdom. The development of the animals from the plants was determined originally by a change in the method of nutrition which we call metasitism.

The chemical modification of the living matter which is connected with the loss of plasmodomism—in other words, the conversion of the reducing phytoplasm into oxidizing zooplasm—must be regarded as one of the most important changes in the history of organic life. This "reversal of metabolism" is polyphyletic; it has been repeated many times in the course of biological history, and has taken place independently in very different groups of the organic world—whenever a plasmodomous cell or group of cells (=tissue) had occasion to feed directly on ready-made plasm, instead of giving itself the trouble of building it up out of inorganic compounds. We see this particularly among the unicellular protists in the independent ciliated cells. The longer plasmophagous flagellata, which are colorless, and have no chlorophyll (monodina, conoflagellata), closely resemble in form and movement the older plasmadomous and chlorophyll-bearing mastigota, from which they are descended (volvocina, peridinia); they only differ in the manner of nutrition. The colorless flagellata feed on ready-formed plasm, which they obtain either by means of their lashes or by a special cell mouth in their cell body. On the other hand, their ancestors, the green or yellow mastigota, form new plasm by photosynthesis like true cells. But there are also complete intermediate forms between the two groups—for instance, the chrysomonades and the gymnodinia; these may behave alternately as protozoa or protophyta. In the same way we can derive the phycomycetes by metasitism from the siphonea, the fungi from the algÆ; and, finally, the process is also found in many of the higher parasitic plants (orchids, orobanches, etc.). (See under "Parasitism.")

As is the case with every other vital function, so for the function of metabolism we find a starting-point in the lowest and simplest group of the protophyta, the chromacea. In their oldest forms, the chroococcacea, the whole body is merely a blue-green, structureless, globular plasma particle, growing by means of its plasmodomous power, and splitting up as soon as it reaches a certain stage of growth. There the miracle of life consists merely of the chemical process of plasmodomism by photosynthesis. The sunlight enables the blue-green phytoplasm to form new plasm of the same kind out of inorganic compounds (water, carbonic acid, ammonia, and nitric acid). We may look upon this process as a special kind of catalysis. In this case there is absolutely nothing to be done by Reinke's "dominants," or conscious and purposive vital forces. There are, as yet, no differentiated physiological functions in these organisms without organs, and no anatomically distinct members; and so their one vital activity, growth, may very well be compared to the simple growth of inorganic crystals.

It has been pointed out repeatedly that the remarkable monera which now play so important a part in biology as bacteria stand, in many respects, quite apart from the ordinary vital phenomena of the higher organisms. This is especially true of their metabolism, which has the most striking peculiarities. Morphologically, many of the bacteria cannot be distinguished from their nearest relatives and direct ancestors, the chromacea, differing from them only in the absence of coloring matter in the plasm. Many of them are simple, globular, ellipsoid, or rod-shaped plasma particles, without any visible organization or movement. Others move about by means of one or more very fine lashes (like the flagellata). No real nucleus can be discovered in the structureless plasma body. The very fine granules which are found in some species, and the vacuole-formation that we see in others, may be regarded as products of metabolism; and the same may be said of the thin membrane or the thicker gelatinous envelope which many of the bacteria secrete. This makes all the more remarkable the peculiarity of their chemical constitution and the metabolism determined thereby. The nitro-bacteria we have mentioned previously are plasmodomous; the anaËrobe bacteria (of butyric acid and tetanus) only flourish where oxygen is excluded; the sulphur bacteria (beggiatoa) secrete—by the oxydation of sulphuretted hydrogen—pure regulation sulphur in the form of round granules. The ferruginous bacteria (leptothrix ochrocea) store up oxyhydrate of iron (by the oxydation of carbonic protoxide of iron). The saprogenetic bacteria cause putrefaction, and the zymogenetic fermentation. Finally, we have the very interesting pathogenetic bacteria which cause the most dangerous diseases by the secretion of special poisons—toxins—festering, small-pox, tetanus, diphtheria, typhus, tuberculosis, cholera, etc. On account of their great practical importance, these bacteria have of late been taken over by a special branch of biology, bacteriology. But only a few of the many experts in this department have pointed out the extreme theoretical significance which these zoomonera have for the important questions of general biology. These structureless plasma bodies show unmistakably that their vital activity is a purely chemical phenomenon. Their great variety proves how manifold and complicated must be the molecular composition of the plasm, even in these simplest organisms.

The unicellular protophyta exhibit the same form of metabolism and plasmodomism as the familiar green cells of the tissue-plants; but in most of the protozoa we find special features of nutrition and plasmophagy. The great class of the rhizopods is distinguished by the fact that their naked plasma body can take in ready-formed solid food at any point of its surface. On the other hand, most of the infusoria have a definite mouth-opening in the outer wall of their unicellular body, and sometimes a gullet-tube as well. Besides this cell-mouth (cytostoma) we usually find also a second opening for the discharge of indigestible matter, a cell-anus (cytopyge).

Metabolism in the tissue plants (metaphyta) forms a long gradation from very simple to very complicated arrangements. The lowest and oldest thallophyta, especially the simplest algÆ, are not far removed from the communities of protophyta, and, like these, are merely definitely grouped colonies of cells. The social cells which form their most rudimentary tissue are quite homogeneous, with no differentiation beyond that of sex. The thallus or bed-formation consists in the simplest specimens of plain or branched fine threads, consisting of rows or chains of homogeneous cells (so conferva among the green, ectocarpus among the brown, and callithamnion among the red algÆ). Other algÆ (such as the ulva) form thin leaf-shaped forms of the thallus, a number of homogeneous cells lying side by side along a level. In the larger algÆ compact tissue-bodies are formed, in which frequently firmer rows of cells exhibit the rudiments of fibres; and the thallus divides, as in the cormophyta, into root, stalk, and leaves. There is also a trophic differentiation, the fibres undertaking special functions of nutrition (the conduction of the sap). The same must be said of the mosses (bryophyta). Their lowest forms (ricciadinÆ) are close akin to the algÆ; the highest mosses (the mnium and polytrichum, for instance) approach the cormophyta. Many botanists comprise these lower plants—algÆ, fungi, and mosses—under the title of "cell-plants" (cytophyta), and oppose the higher plants—ferns and flowering-plants—to them as "vascular plants" (angiophyta), because they have complex fibres or sap vessels. This distinction has a phylogenetic significance similar to the division between coelenteria and coelomaria in the animal kingdom.

While most of the cell-plants either live in the water (algÆ) or are very simply organized on account of their saprophytic or parasitic habits (fungi), the vascular plants mostly live on land, and have to adapt themselves to much more complicated conditions. Their nutrition is accordingly distributed among different functions, and special organs have been evolved to discharge them. This is equally true of the crytogam ferns (pteridophyta) and the phanerogam flowering plants (anthophyta). The most important later acquisition which distinguishes both groups from the lower cell-plants is the possession of vascular or conducting fibres. These organs for conducting water pass through the entire body of the vascular plant in the shape of long tubes, formed by the combination of rows of cells; the cells themselves die off, and their plasma content disappears. The stream of water that rises constantly in these tubes is taken up by the roots, conducted by the fibres to all parts, and given off (transpiration) by the pores of the leaves. But these pores also serve for the breathing of plants, being connected with the air-containing intercellular passages; through these air-spaces, which serve for the aËration of the higher plant-body, air and moisture can enter, and oxygen be given off in respiration. Finally, many of the vascular plants have special glands that serve for secretion (of oil, resin, etc.). In the higher flowering plants this division of work among the various digestive organs gives rise to a very complicated apparatus for nutrition. Among the many remarkable structures that have been developed in this way by adaptation to special conditions we may particularly note the organs for catching and digesting insects in the insect-eating plants, the European drosera and utricalaria, and the tropical nepenthas and dionÆa.

The long scale of evolutionary forms which we find in the tissue animals (metazoa) leads up uninterruptedly from the simplest to the most elaborate physiological functions and a corresponding morphological complexity of organs. The two principal divisions of the metazoa are chiefly distinguished by the circumstance that in the coelenteria one single system of organs, the gastro-canal system, discharges the whole (or most part) of the partial functions of nutrition; while in the coelomaria they are usually distributed among four different systems of organs, each of which is made up of a number of organs. To an extent, we find once more in each great division characteristic types of organization. However, comparative ontogeny teaches us that all these various structures have been developed from one simple fundamental form, as I have shown in my theory of the gastrÆa (1872).

The older research into the origin of the nutritive apparatus in the metazoa—especially its chief part, the alimentary or gastric canal—had led to the erroneous belief that in several groups of the metazoa it owed its origin to very different growth-processes, and that particularly in the higher vertebrates (the amniotes) it was a comparatively late product of evolution. On the other hand, the comparative study of the embryology of the lower and higher animals led me thirty-four years ago to the opposite conclusion, that a simple gastric sac was the first and oldest organ of all the metazoa, and that all the different forms of it had been developed from this primitive type. I gave this view in my Biology of the Sponges in 1872; and I developed and established it in my Studies of the GastrÆa Theory in 1873. In the latter book I also worked out the important conclusions that follow from this monistic reform of the theory of germinal layers for the phylogenetic natural classification of the animal kingdom. I began with the consideration of the simplest sponges (olynthus) and cnidaria (hydra). The whole body of these lowest and oldest of the coelenteria is in essence nothing but a round, oval, or cylindrical gastric vesicle, a digestive sac, the thin wall of which consists of two simple layers of cells. The outer layer (the ectoderm or skin-layer) is the covering layer of the external skin (epidermis); it is the instrument of sensation and movement. The inner layer of cells (entoderm or gastric layer) serves for nutrition; it clothes the simple cavity of the sac, which admits the food by its opening and digests it. This opening is the primitive mouth (prostoma or blastoporus), the inner cavity itself the primitive gut (progaster or archenteron). I proved that there was the same composition in the young embryos or larvÆ of many of the lower animals, and showed that the manifold and apparently very different embryonic form of all the higher animals may be reduced to the same common type. To this I gave the name of the "cup-embryo" or gastric larvÆ (gastrula), and concluded, in virtue of the biogenetic law, that it is the palingenetic reproduction of a corresponding ancestral form (the gastrÆa) maintained until the present by heredity. It was not until much later (1895) that Monticelli discovered a modern gastrÆad (pemmatodiscus) which corresponds completely to this hypothetical ancestor (see the last edition of my Anthropogeny, fig. 287). The simplest living forms of the sponges (olynthus) and the cnidaria (hydra) only differ from this hypothetical primitive form of the gastrÆa by a few secondary and subsequently acquired features.

The classes of the lower animals which we comprise under the name coelenteria (or coelenterata in the widest sense) generally agree in having all the functions of nutrition accomplished exclusively (or for the most part) by a single system of organs, the gastro-canal or gastro-vascular system. From their common stem-group, the gastrÆads, three different stems have been evolved—the sponges, cnidaria, and platodes. All these coelenteria have three features in common: (1) The gastric canal or tube has only one opening—the primitive mouth, which serves at once for admitting food and ejecting indigestible matter; there is no anus; (2) there is no special body-cavity (coeloma) distinct from the gastric tube; (3) there is also no trace of a vascular system. All cavities that are found in these lower animals besides the digestive gut-cavity are direct processes from it (with the exception of the nephridia in the platodes).

While the simple digestive gut is the sole organ of nutrition in the stem-group of the gastrÆads, we find other structures co-operating in the rest of the coelenteria. The characteristic stem of the sponges is distinguished by the piercing of the wall of the gastric vesicle with several holes. Through these water pours into the body, bringing with it the small particles of food which are received and digested by the ciliated cells of the entoderm; the water emerges again by the mouth-opening (osculum). The best-known of the sponges is the common bath-sponge (euspongia officinalis), the horny skeleton of which we use daily in washing. In these and most other sponges the large, unshapely body is traversed by a number of branching canals, on which there are thousands of tiny vesicles, produced by the multiplication of a simple gastric vesicle of the primitive sponge (olynthus). Each of these ciliated chambers is really a tiny gastrÆa, a "person" of the simplest character (cf. chapter vii.). Hence we may regard the whole sponge-body as a gastrÆad-stock (cormus).

The large group of the cnidaria offers a long series of evolutionary stages, from very small and simple to very large and elaborate forms. Some of them remain at a very low stage, as does our common green fresh-water polyp (hydra viridis), which only differs from the gastrÆa by a few variations in tissue and the formation of a crown of feelers about the mouth. Most of the polyps form stocks (cormi), the individuals shooting out buds which remain joined to the mother animal. In these and all the other stock-forming animals the nutrition is communistic; all the food that the individuals get and digest is conducted by tubes to the common fund and equally distributed. In all the larger cnidaria the body-wall becomes thicker, and is traversed by branching gastro-canals; these convey the nutritive fluid to all parts of the body.

While the fundamental type in the cnidaria is radial (determined by the crown of radiating feelers or tentacles that surrounds the mouth), it is bilateral-symmetrical in the platodes or "flat-worms" (plathelminthes). In this animal-stem, moreover, the lowest forms, the platodaria (also called cryptocoela and acÆla) come very close to the gastrÆa. But most of the platodes are distinguished from the rest of the coelenteria by the formation of a pair of nephridia (renal canals or water-vessels), thin tubes which, as excretory organs, remove from the body the unusable products of metabolism, the urine. Here we have a second organ of nutrition, the gut tube, added to the first. In the lower platodes this remains very simple. As a rule, a gullet tube (pharynx) is formed by the hollowing out of the mouth, as in the corals; and as in the case of the latter branched canals, which conduct the nutritive sap from the stomach to distant parts of the body, grow out of the stomach, in the larger coil-worms (turbellaria) and suction-worms (trematodes). On the other hand, the gut atrophies in the tape-worms (cestodes); as these parasites live in the intestines or other organs of animals, they can obtain their nutritive sap directly from them through the surface of the skin.

The more highly organized coelomaria differ from the simpler coelenteria chiefly by the greater complexity in the structure and functions of their apparatus of nutrition. As a rule, these functions are divided between four groups of organs, which are not yet differentiated in the coelenteria—namely: 1, organs of digestion (gastric system); 2, organs of circulation (vascular system); 3, organs of breathing (respiratory system); and 4, organs of excretion (renal system). Moreover, in the coelomaria the gastric canal has usually two openings, the mouth and the anus. Finally, they all have a special body-cavity (coeloma); this is quite separate from the gastric canal, which is suspended in it, and serves for the formation of the sexual cells. It is formed in the embryo by the hollowing out and cutting off of a pair of sacs (coelom-pouches) from the gut near the mouth; the pouches touch, and then coalesce, as their division-walls break down. If a part of the dividing wall remains, it serves as mesentery to fasten the gut to the body-wall. The action of the four groups of alimentary organs remains very simple in the lowest and oldest coelomaria, the worms (vermalia); but in the other higher animals, which have been evolved from these, they have very varied and often complicated features.

In the great majority of the coelomaria the gastric system forms a highly differentiated apparatus, composed, as in man, of a number of different organs. The food is usually taken in by the mouth, ground up by the jaws or the teeth, and softened with saliva, which the salivary glands pour into the cavity of the mouth. From the mouth the pulpy food passes in swallowing into the gullet, which often has glandular appendages, and from this through the narrow esophagus into the stomach. This most important part of the alimentary apparatus is often divided into several sections, one of which (the masticating stomach) is armed with teeth and prepared for a further triturition of solid pieces, while the other (the glandular stomach) produces the dissolving gastric juice. The liquefied food (chylus) then passes into the small intestine (ileum), which has to absorb it, and is as a rule the longest section of the alimentary canal. A number of different digestive glands open into this intestine, the most important of them being the liver. The small intestine is often sharply distinguished from the large intestine (colon), the last large section of the alimentary canal; into this also a number of glands and blind intestines open. The last portion of it is called the rectum, and this removes the indigestible remnants of the food (fÆces) through the anus.

This general plan of the alimentary system, which is common to most of the coelomaria in its chief features, is very much modified in the various groups of these animals and adapted to their several conditions of nutrition. The simplest structures are found in many of the vermalia; the lowest forms of these, the rotifers, and especially the gastrotricha, still closely resemble their platode ancestors, the turbellaria. The higher type of animal-stems which have been evolved from them are partly distinguished by special structures. Thus the mollusks have a characteristic masticating apparatus; on their tongue there is a hard plate (radula) armed with a number of teeth, which grinds against a hard upper jaw, and so breaks up the food. In most of the articulates this work is done by side-jaws, which consist of hard rods and represent modified bones. The vertebrates and the closely related tunicates are distinguished by the conversion of the first sections of the alimentary canal into a characteristic respiratory apparatus (gills). But the construction of the various sections of the gastro-canal also varies a good deal in the small groups of the coelomaria, as it depends to a great extent on the nature of the food and the conditions in which it is got and prepared. The largest expenditure of mechanical and chemical energy is needed for a voluminous solid vegetal diet. Hence the alimentary canal and its many appendages are longest and most complicated in the plant-eating snails, leaf-eating insects, and grass-eating ruminants. On the other hand, they are shortest and simplest in parasitic coelomaria, which derive their fluid food already prepared from the contents of another animal's intestines. In these cases the gut may altogether atrophy; as in the acanthocephala among the vermalia, the entoconcha among the mollusks, and the sacculina among the crustacea.

The greater the extent of the body, and the more complex the organization of the higher animals, the more necessary it is to have an orderly and regular distribution of the nutritive fluid to all parts. In the coelenteria this work is accomplished by the gastric canals (side branches from the gut, opening into its cavity) but in the coelomaria it is done much better by means of blood-vessels (vasa sanguifera). These canals do not communicate directly with the gastro-canal, but are formed independently of it in the surrounding parenchyma of the mesoderm. They take up the filtered and chemically improved food-fluid, which transudes through the intestinal walls, and conduct it in the form of blood to all parts of the body. This blood generally contains millions of cells, which are of great importance in metabolism. The blood-cells of the lower coelomaria are usually colorless (leucocytes), while those of the vertebrates are mostly red (rhodocytes).

The circulation of the blood in most of the coelomaria is effected by a heart, a contractile tube, formed by the local thickening of a skin-vessel, which contracts and beats regularly by means of its muscular bands. Originally two of these skin-vessels were developed in the abdominal wall—a dorsal vessel in the upper and ventral vessel in the lower wall (as in many of the vermalia). The heart is formed from the dorsal vessel in the mollusks and articulates, but from the ventral in the tunicates and vertebrates. The arteries are the vessels which conduct the blood from the heart; those which conduct it from the body to the heart are the veins. The finest branchlets of both kinds of vessels, which form the connecting link between them, are called capillaries; these immediately effect the interchange of matter in the tissues by osmosis. The blood-vessels co-operate very closely with the respiratory organs.

The interchange of gases in the organism, which we call breathing or respiration—the taking in of oxygen and giving out of carbonic-acid gas—does not require special organs in the lower animals. In these it is accomplished by epithelial cells, which clothe the surface of the body—the ectoderm of the outer skin layer and the entoderm of the inner gut-covering. As nearly all these coelenteria live in the water, or (as parasites) in some fluid that contains air, and as these fluids are constantly pouring in and out of the body, the exchange of gases is accomplished at the same time. But in the higher animals this is rarely found, only in the small animals of simple construction (such as the rotifers and other vermalia, and the smallest specimens of the mollusca and articulata). The majority of these coelomaria attain a considerable size, and so require special organs; these afford a larger surface for the exchange of gases in the limited space, and accomplish a very peculiar chemical work as the localized organs of respiration. They fall into two groups according to the nature of the environment; gills for breathing in water and lungs for breathing on land. The latter take the oxygen directly from the atmosphere, and the former from the water, in which atmosphere air is contained in solution.

The instruments of water-respiration which we call gills (branchiÆ) are generally attenuated parts or processes of the outer skin or the inner gastric skin; hence we distinguish the two chief forms, external and internal gills. Both are richly provided with blood-vessels which bring the blood from the body for the purpose of aËration. Cutaneous or external gills are especially found in the vertebrates, in the form of threads, combs, leaves, pencils, tufts of feathers, etc., which are drawn out from the entoderm as local processes of the outer skin, and afford a wide surface for the interchange of gases between the body and the water. In the mollusca there are usually a pair of comb-shaped gills near the heart; in the articulates there are several pairs, repeated in the different segments of the body. Gastric or internal gills are peculiar to the vertebrates and the next-related tunicates, with a small group of the vermalia, the enteropneusta. In these the fore-gut or head-gut is converted into a gill-organ, the wall of which is pierced with gill-fissures; the water that has been taken in by the mouth passes away through the outer openings of these fissures. In the lower aquatic vertebrates (acrania, cyclostoma, and fishes) the gills are the sole organs of breathing; in the higher animals, that live in the air, they fall into disuse, and their place is taken by lungs. Nevertheless, heredity is so tenacious that we find from three to five pairs of rudimentary gill-clefts in the embryo right up to man, though they have long since ceased to have any function. This is one of the most interesting of the palingenetic facts that prove the descent of the amniotes (including man) from the fishes.

The group of the aquatic echinoderms has some very peculiar features of respiration. Their body possesses an extensive water-duct, which takes in the sea-water and returns it by special openings (skin-pores or madreporites). The many branches of these water-vessels or ambulacral vessels fill with water, especially the tiny feelers or feet, which extend from the skin in thousands; they serve at once for movement, feeling, and breathing. But many of the echinoderms have also special gills—the star-fish have small finger-shaped cutaneous gills on the back, the sea-urchins special leaf-shaped ambulacral gills, the sea-cucumbers internal gastric gills (tree-shaped branching internal folds of the rectum).

The organs of air-breathing are called, in general, lungs (pulmones). Like the organs of water-breathing, they are formed sometimes from the external and sometimes from the internal covering of the body. Cutaneous or external lungs are found in several groups of the vertebrates. Among the mollusks the land-dwelling lung-snails have acquired a lung-sac by change in the work of the gill cavity: among the articulata the lung-spiders and scorpions have two or more trachea-lungs; that is to say, cutaneous sacs, in which are enclosed fanwise a number of trachea-leaves. In the other air-breathing articulates (tracheata) we find, instead of these simple or branched, and often bushlike, air-tubes (tracheÆ), which spread through the whole body and conduct the air direct to the tissues. They take the air from without by special air-holes in the skin (stigmata and spiracula). The myriapods and insects generally have numbers of air-holes; the spiders only one or two, more rarely four, pairs. When these air-tube animals return to an aquatic life (as happens with the larvÆ of various groups of insects), the outer air-holes close up, and new thread-shaped or leaf-shaped trachea-gills are formed, which take the air from the surrounding water by osmosis. The oldest and lowest tracheata are the primitive air-tube animals, or protracheata, and form the link between the older annelids and the myriapods. They have a number of clusters of short air-tubes distributed over the whole skin, and it is clear that these have been evolved from simple skin-glands by change of function.

Gastric or internal lungs are only found in the higher animals, to which we give the name of quadrupeds (or tetrapoda), the amphibia and amniotes, and their fishlike ancestors, the dipneusta. These internal lungs are sac-shaped folds of the fore-gut, formed originally from the swimming-bladder (nectocystis) of the fishes by change of function. This air-filled bladder, a sac-shaped appendage of the gullet, merely serves the purpose of a hydrostatic organ, by varying the specific weight, in the fishes. When the fish wishes to descend it contracts the bladder and becomes heavier; it rises to the top by inflating it again. The lungs were formed by the adaptation of the blood-vessels in the wall of the swimming-bladder to the interchange of gases. In the oldest living lung-fishes (ceratodus) it is still a simple sac (monopneumones=one-lunged); in the others the simple gullet-cavity divides early into a pair of sacs (dipneumones, two-lunged). The wind-pipe (trachea—not to be confused with the organ of the same name in the tracheata) is formed by the lengthening of their stalk and strengthening of it with cartilaginous rings. At the anterior end of the trachea we find already formed in the amphibia the larynx, the important organ of voice and speech.

The function of removing unusable matter is not less important to the organism than breathing. Just as breathing gets rid of the poisonous carbonic acid, so the kidneys remove fluid and solid excreta in the shape of urine; these are partly acid (uric acid, hippuric acid, etc.), partly alkaline (urea, guanine, etc.). In most of the coelomaria special organs for removing these would be superfluous, as this is accomplished (like breathing) by the stream of water that is constantly passing through the whole body. But with the platodes we begin to find important excretory organs in the nephridia, a pair of simple and ramified canals which lie on either side of the gut, and open outward. These primitive renal canals are transmitted by the platodes to the vermalia, and by these to the higher stems of the coelomaria. In the latter they generally open by special funnels into the inner body-cavity, which serves as first receptacle for the urine. Their outer opening sometimes (primarily) goes through the outer skin at the back (excretory pores), sometimes (secondarily) to the rectum, and so out through the anus. The oldest articulates, the annelids, have a pair of nephridia in each segment of the body; each renal canal, or segmental canal, consists of three sections, an inner funnel which opens into the body-cavity, a middle glandular section, and an external bladder that ejects the urine by contraction. The disposition of the renal system in the internally articulated vertebrates is very similar to this; but now complicated structures begin to appear, a pair of compact kidneys (renes), which are made up of a number of branching nephridia. Three generations of kidneys succeed each other, as phylogenetic stages of evolution—first the primary fore-kidneys (protonephros), in the middle the secondary primitive kidneys (mesonephros), and last the tertiary after-kidneys (metanephros). The latter are only reached in the three highest classes of vertebrates, reptiles, birds, and mammals. Mollusks also have a couple of compact kidneys. They are developed from a pair of nephridia, the funnels of which open internally into the heart-pouch (the remainder of the reduced body-cavity); at the back they open outward. The crustacea also have generally a pair of renal canals. On the other hand, the protracheata (the stem-forms of the air-tube animals) have segmental nephridia, a pair to each joint inherited from their annelid ancestors. The rest of the tracheata, the myriapods, spiders, and insects, have, instead of these, Malpighi tubes, funnel-shaped glands that arise from the entodermal rectum, sometimes one pair or less, sometimes a number in a cluster.

While most plants are purely plasmodomous, and most animals plasmophagous, there are nevertheless in both organic kingdoms a number of species (especially the lower) whose metabolism has assumed peculiar forms by their relations to other organisms. To this class belong especially the saprosites and parasites. By saprosites are understood those plants and animals which feed entirely or mostly on the corpses of other animals, or the decomposed matter which is unfit for the food of higher animals. Among the unicellular protists many of the bacteria, especially, belong to this class, and also many fungilla (phycomycetes); among the metaphyta the fungi (mycetes), and among the metazoa the sponges. I have already spoken of the many peculiarities of metabolism in the ubiquitous bacteria; while many of them cause putrefaction, they at the same time feed on the parts of other organisms which have died. The fungi feed for the most part on the decayed remains of plants and the products of putrefaction which accumulate on the ground. In this character of scavengers they play the same important part on land as the sponges do at the bottom of the sea. But a number of small groups of the higher plants and animals have, as a secondary habit, turned to saprositism. Among the metaphyta we have especially the monotropea (to which our native asparagus, monotropa hypopitys, belongs) and many orchids (neottia, corallorhiza). As they find their plasm directly in the decayed matter in the woods, they have lost their chlorophyll and green leaves. Among the metazoa many of the vermalia, and some of the higher animals, such as the rain-worm and many tube-dwelling annelids (the mud-eaters, limicolÆ), etc., live on putrid matter. The organs which their nearest relatives use for obtaining, breaking up, and digesting food (eyes, jaws, teeth, digestive glands) have been entirely or mostly lost by these saprosites. Many of them form a transitional type to the parasites.

By parasites, in the narrower sense, we understand, in modern biology, only those organisms which live on others and derive their nourishment from them. They are numerous in all the chief divisions of the plant and animal kingdoms, and their modifications are of great interest in connection with evolution. No other circumstance has so profound an influence on the organism as adaptation to a parasitic existence. Moreover, there is no other section in which we can follow, step by step, the course of the degeneration which is caused, and show clearly the mechanical nature of the process. Hence the science of parasites—parasitology—is one of the soundest supports of the theory of descent, and provides an abundance of the most striking proofs of the much-contested inheritance of acquired characteristics.

Among the unicellular organisms, the bacteria are the most conspicuous instances of manifold adaptation to parasitic habits. As we count these unnucleated protozoa among the oldest and simplest organisms, and trace them directly by metasitism to the plasmodomous chromacea, it is very probable that they turned to parasitism very early in the history of life. Even a part of the monera (in which group we must place the bacteria on account of their lack of a nucleus) found it convenient and advantageous to prey on other protists and assimilate their plasm directly, instead of going through the laborious process of carbon assimilation themselves in the hereditary fashion. This is also true of the large class of the sporozoa or fungilla (gregarinÆ, coccidia, etc.), real nucleated cells, which have adapted themselves in various ways to parasitic habits. Many of them live in the rectum, the coelum, or other organs of the higher animals (the gregarinÆ, especially in the articulates); others in the tissues (for instance, the sarcosporidia in the muscles of mammals, the coccidia and myxosporidia in the liver of vertebrates). A good many of them are "cell-parasites," and live inside the cells of other animals, which they destroy; such are the hoemosporidia, which destroy the blood-cells in man, and so cause intermittent fever.

Among the multicellular metaphyta it is particularly the fungi that have taken to parasitism in various ways. Many of them are, as is known, the most dangerous enemies of the higher animals and plants. The various species of fungi cause certain diseases by their poisonous (chemical) action on the tissues of their host. It is well known how our most important cultivated plants, the vine, potato, corn, coffee, etc., are threatened by fungoid diseases; and this is also true of many of the lower and higher animals. It is probable that the fungi have been evolved polyphyletically by metasitism from the algÆ.

Among the higher metaphyta we find parasitism in many different families, especially orchids, rhinanthacea (orobranche, lathraca), convolvulacea (cuscuta), aristolochiacea, loranthacea (viscum, loranthus), rafflesiacea, etc. These various kinds of flowering-plants often grow to resemble each other by convergence (that is to say, by their common adaptation to parasitic life); they lose their green leaves, the plasmodomous chlorophyll of which is of no further use to them. Frequently rudimentary leaves are left on them in the form of colorless scales. For the purpose of clinging to the plants they live on, and penetrating into their tissues, they evolve special clinging apparatus (haustoria, suctorial cups, creepers). Their stalks and roots are also modified in a characteristic way. The whole productive force of these parasites is expended on their sexual organs; rafflesia has the largest flowers there are, more than a yard in diameter.

Parasitism in the metazoa (in all groups) is even more frequent and interesting than in the metaphyta. The mollusks and echinoderms show the least disposition for it, and the platodes, vermalia, and articulates the most. Even among the gastrÆada, the common ancestral group of the metaphyta, we find parasites (kyemaria and gastremaria). The protection they find inside their hosts is probably the reason why these oldest of the metazoa have remained unchanged to the present day. Real parasites are not numerous among the sponges and cnidaria. But they are very numerous among the platodes. The suctorial worms (trematodes) live partly externally (as ectoparasites) on other animals and partly inside them (as endoparasites), and produce serious diseases in them. They have lost the vibratory coat of their free-living ancestors, the turbellaria, and acquired clinging apparatus instead. The tape-worms (cestodes), which live entirely in the interior of other animals, and are descended from the suctorial worms, have lost their gastro-canal; they are nourished by imbibition through the skin. The same degeneration is found in the itchworms (acanthocephala) among the vermalia, the parasitic snails (entoconcha) among the mollusks, and the root-crabs (rhizocephala) among the crustacea.

The class of crustacea affords the most numerous and most instructive examples of degeneration through parasitism, because in this class it is found polyphyletically in very different orders and families, and because their highly organized body shows every stage of degeneration together in the different organs. The free-living crustacea generally move about very rapidly and ingeniously; their numerous bones are well jointed and excellently adapted for the most varied methods of locomotion (running, swimming, climbing, digging, etc.); their organs of sense are highly developed. As these are no longer used when they take to parasitism, they atrophy and gradually disappear. The younger crustacea all proceed from the same characteristic form of the nauplius, and swim freely about; later, when they settle down to parasitic habits, their organs of sense and locomotion atrophy. As Fritz MÜller-Desterro showed in his famous little work, For Darwin (1864), forty years ago, the crustacea afford most luminous proofs of the theory of descent and selection, and of progressive heredity and the biogenetic law. These facts are the more important as the crab undergoes the same degeneration by parasitic habits in a number of different orders and families.

From parasitism we must entirely distinguish that intimate life-union of two different organisms which we called symbiosis or mutualism. Here we have an association of two living things for their mutual benefit, while the parasite lives entirely at the expense of his host. Symbiosis is found among the protista, being very wide-spread among the radiolaria. In the gelatinous envelope (calymma) which encloses the central capsule of their unicellular bodies we find a number of motionless yellow cells (zooxanthella) scattered. These are protophyta or (as it is said) "unicellular algÆ" of the class of paulotomea (palmellacea). They receive protection and a home from the radiolaria, grow plasmodomously, and multiply by rapid segmentation. A large part of the starch-flour and the plasm which they form by carbon-assimilation goes as food directly to the radiolarium-host; the other part of the xanthella goes on growing and multiplying. Similar yellow zooxanthella or green zoochlorella are found as symbionta in the tissues of many animals. Our common fresh-water polyp (hydra viridis) owes its green color to the zoochlorella which live in great numbers on the ciliated cells of its entoderm (the digestive gut-epithelium). In general, however symbiosis is rarer in the metazoa than in the metaphyta. In the latter case it is the fundamental feature of a whole class of plants, the lichens. Each lichen consists of a plasmodomous plant (sometimes a protophyte, sometimes an alga) and a plasmophagous fungus. The latter affords a home, protection, and water to the green alga, which repays the service by providing food.

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