The complex of sensation that in man and the higher animals urges and compels to ingestion of food is called hunger and appetite. From the standpoint of the persistence of living organisms the ingestion of food is as important as reproduction. Consequently the hunger sensation is as fundamental as the urge or appetite of sex. In fact, if we define hunger biologically as the conditions (rather than the sensation complex) that lead to taking food, hunger is even more fundamental or primitive than the sexual urge (libido), since feeding is a necessity in all forms of life, while sexual reproduction is not.
Whatever be the underlying mechanisms in the genesis of the hunger urge, in the higher animals this urge is obviously a sensation involving a more or less complex nervous organization. Hunger as a sensation or conscious process is therefore probably confined to animals having a nervous system and an alimentary canal. But all living organisms feed. What, then, are the factors that lead to the ingestion of food in unicellular animals and in the simpler metazoa having no specialized nervous system? And are there any essential connections between these primordial factors that cause the ameba to pursue and engulf another moving protozoan, and the mechanism of the hunger urge compelling a starving wolf to chase, capture, and devour a rabbit ?
All the unicellular animals live, at least during their periods of activity, in water, in animal and plant fluids, or within the living cells of other animals. In the case of the simpler organisms that are parasitic the conditions of feeding are essentially those of the tissue cells of the higher animals. That is to say, the food materials are in solution in the medium surrounding the cell or the animal. According to Putter, this also applies to all lower forms of life inhabiting the waters of the earth. It is not known to what extent the organic material in solution in the sea water actually sustains the lower animal life, but it is in all probability a very small, if not entirely a negligible factor (Biedermann, Lipschutz, Kerb, Mor-gulis). We do know that all metazoa feed on other unicellular animals and plants, whole or in fragments. That is, they take into their bodies as food other solid bodies. As regards the actual processes of ingestion of solids by the protozoa, we have practically identical conditions in the case of the special phagocytic cells in the higher animals, although the latter is probably not a feeding process primarily.
What determines the amount and the avidity of food ingestion in the unicellular animals? During the active stage the feeding appears to be on the whole as continuous as contact with food particles permits. Nutrient and non-nutrient particles are taken up somewhat indiscriminately, although there are many exceptions to this rule. In some cases minute motile organisms may by the force of their own motility penetrate into a unicellular animal only to be digested by the latter, but the taking up of solid particles by protozoa is mainly due to active ameboid movements. Augmentation of ameboid movements, increased rate of contraction of pseudopodia acting as feeders, increased ciliary motion both in free swimming and in sessile forms might be taken as external expressions of states of hunger as these would enhance the securing of food.
Are such expressions of hunger state actually present? Jennings, in his studies of the feeding processes of ameba, is silent on this point, except for a few incidental observations that an ameba may remain at rest for a few minutes after having taken up a morsel of food, but since the ameba is on the go again before this food is actually digested, the brief rest period cannot be interpreted as a state of satiety. Verworn thinks that all the phenomena of feeding in the protozoa, including a certain capacity of selection of food, involve automatic motility (chemo- and stereotropism) only, but we are not informed whether the rate of this motility varies with the degree of hunger. In vorticella Hodge and Aikens found that the cilia worked uniformly and continuously night and day, in drawing in and assorting food particles: "all efforts to surfeit the tiny animals with food produced no appreciable effect in satisfying their apparent hunger." In the presence of an abundance of food the body cilia of Paramecium beat less actively, thus bringing the animal to rest, while the oral cilia continue in activity, drawing the food particles into the mouth (Jennings). Wallengren, on the other hand, found no change in ciliary movements and vacuole contractions, or in the excitability of the Paramecium during hunger except that ciliary and vacuole activity is decreased when the organism is near death from starvation.
Schaeffer has described in stentor certain differences in behavior between the states of hunger and satiety. When stentor is gorged with food it remains somewhat contracted, the activity of the membranellae are greatly decreased, the animal is less excitable to external stimuli, and it discriminates more perfectly between food particles and indigestible particles in the water current. The degree of satiety appears to depend on other factors beside the amount of food in the body. It seems clear, then, that a state of depletion or hunger in the stentor leads to increased excitability, increased motility, and increased avidity of food ingestion. It is not unlikely that future investigations will reveal similar differences in most of the protozoa. Parker found that meat or meat extract reverses the stroke of the labial cilia in sea anemones, so that the water current carries the food particles into the esophagus. This appears to be an instance of a considerable degree of specialization. Will meat extract in the sea water induce this reversal after the anemone is gorged with meat or other forms of food ?
As regards food substances in solution, we may assume that the rate and quantity of ingestion depend on the diffusion rate of the substance and the permeability of cell surfaces. Dilution of food material within the cell may increase surface permeability, and vice versa. We know so little of the correlation of the internal processes in living cells and unicellular animals that we cannot predict what effect scarcity of nutrient material within the cell has on ameboid and ciliary movements, except that when the depletion approaches exhaustion these movements probably suffer depression. In some animals partial or complete starvation appears to accelerate metamorphosis and redifferentiation, but the factors involved in these processes are obviously more complex than that of simple and direct cell stimulation.
In the ingestion of organic particles or cells by the special phagocytes in the higher animals, certain new factors enter into play. In the first place, phagocytosis in the metazoa is not primarily a feeding process, but concerned with destruction of cellular debris and cells foreign to the organism. So far as we know, the metazoan phagocytes feed on the organic substances in solution in the body fluids, just like the cells of other tissues. Secondly, the rate and quantity of ingestion of the foreign cells depend in part, not on the condition of the phagocyte, but on certain substances (opsonins) in the body fluids that act on the foreign cells. This factor is not known to be involved in the feeding phagocytosis of the protozoa. Thirdly, even apart from the opsonin factor, certain of the phagocytic cells of the metazoa appear capable of being "trained" to increased activity. The mechanics of this capacity is a matter of conjecture. These differences in the biological meaning of phagocytosis in the metazoa, and the feeding phagocytosis in the protozoa do not imply that the essential mechanics of the phagocytic processes in the two groups are different.
There are indications, then, that relative depletion of ingested food material, at least in some of the protozoa, results in increased motility (ameboid, ciliary) and increased avidity for food or rate of food intake. These phenomena probably indicate a condition of increased cell excitability. That is to say, a state of hunger in the protozoa is a state of increased excitability. The question how a state of cell hunger causes primarily a state of increased cell excitability cannot be entered into here. To those who prefer anthropomorphic concepts the pursuit, capture, and selection of food by the protozoa become expressions of conscious states analogous to those in higher animals under like conditions. But, for the present, at any rate, simpler explanations are not only adequate, but more useful. The principles of diffusion and selective permeability or absorption suffice to account for ingestion of food in solution. The selection of solid food, so far as this principle is in evidence, is probably a matter of inherited mechanism for differential response to chemical and mechanical stimuli. The ameboid and ciliary mobility involved in the hunger state introduces more complex factors. Rhumbler endeavored to analyze the former into purely physical surface tension phenomena. Jennings has shown, at least for the ameba, that the surface-tension theory is untenable. But while we are thus forced back on unknown factors in the organization of the cell, there is no reason for believing that when once analyzed these cell processes are not, individually, quite as definitely physical and chemical phenomena as surface tension. Hamburger has recently shown that lack of oxygen acts as a primary stimulus to phagocytosis. But this is in all probability not the mechanism that induces increased cell motility in state of cell hunger, as there is no evidence that a decrease in the food material in the cell is accompanied by oxygen want.