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 cœlenteria one single system of organs, the gastro-canal system, discharges the whole (or most part) of the partial functions of nutrition; while in the cœlomaria 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 cœlenteria 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 cœlenteria (or cœlenterata 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 cœlenteria 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 (cœloma) 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 cœlenteria. 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 cryptocœla and acæla) come very close to the gastræa. But most of the platodes are distinguished from the rest of the cœlenteria 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 cœlomaria differ from the simpler cœlenteria 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 cœlenteria—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 cœlomaria the gastric canal has usually two openings, the mouth and the anus. Finally, they all have a special body-cavity (cœloma); 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 (cœlom-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 cœlomaria, 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 cœlomaria 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.