stomach, and receives no nourishment from the time of its cell or egg condition, till it dies.
The tape-worm, which has a comparatively high organization, has no mouth, and takes in food already prepared for it through its entire surface. What it lives on is first digested by the animal in which it happens to be located, and it thrives as a parasite through the activity of an organ not its own.
We must now go still further, and say that neither the presence of a stomach nor the existence of that process which we call digestion, is characteristic of all animals; but that the two grand divisions of all created living forms are distinguished from one another by the nature of the food which contributes to their growth and maintenance.
The higher animals in the embryonic condition pass through stages of development which are the mature permanent states of those lower in the scale; and, as the grade of an animal is in a great measure determined by the degree of specialization of its parts, it is to be expected that the lower we descend the fewer will be the organs devoted to certain functions, till we reach the simplest forms in which the whole body is equally endowed with nervous, circulatory, digestive, and reproductive powers.
Thus much on the existence in animals of a stomach and of digestion; we now have a few words to add on the nature of the latter.
In the higher animals this is quite a complicated process, calling in the aid of the mouth, teeth, pharynx, œsophagus, stomach, intestines, pancreas, liver, mucous follicles, salivary glands, lungs, and heart; whereas, in the lowest animals, the power or function exerted by all these parts combined, seems to reside in every granule or individual cell. The gastric fluid, which we have already mentioned in connection with the names of Reaumur, Spallanzani, and others, and through the action of which digestion is chiefly accomplished, is of so complicated a nature that chemists and physiologists are not yet fully agreed as to its composition. For some time muriatic acid was believed to give to it its acidity; but at present it is quite generally conceded that the free acid in this secretion is of quite a different nature, namely, lactic acid, and that when free muriatic acid is found to be present, it results from the method of analysis employed.
An organic substance, called pepsin, is held to be an essential ingredient of the gastric juice, and various salts of soda, potash, lime, ammonia, magnesia, and iron contribute to its complicated nature.
Beaumont, who made a series of interesting experiments on a man who had a permanent gastric fistula, or opening through his left side into the stomach, caused by a gunshot wound, has shown that digestion in man goes on at a temperature of about 100° Fahrenheit, and that when pieces of ice are introduced into the stomach this process is checked.
Whether the amœbæ and other similar animals secrete a digestive fluid or not, we have no direct means of deciding; but the food on which they live has been seen, by the aid of the microscope, to slowly disappear without the exercise of any triturating or mechanical action, and the non-nutritious portions, as shells, to work their way out of the body. We are thus, it would seem, justified in concluding that digestion is carried on in them by means of some fluid secreted by their jelly-like substance, though it is quite probable that it differs greatly in composition from the gastric juice of the higher animals; for many of these minute beings live and flourish in water which has a temperature of
