1. Mouth. 2. Alimentary tubes. 3. Sucker.
1. Proboscis in a retracted state. 2. Interior of digestive cavity. 3, 3. Cœcal appendages opening into it.
557. Up to this point in the animal series the digestive sacs and the alimentary canal are merely cavities formed in the common cellular tissue of the body, without any lining membrane, without teeth, or without any instruments for dividing and preparing the aliment, and without a single gland, as far as has been ascertained, to assist the digestive process. All the assimilative functions, the respiratory as well as the digestive, appear to be performed by this single surface. But in the ascending scale not only is an apparatus appropriated to digestion, perfectly distinct from that assigned to respiration, but even the stomach and the alimentary canal are separate organs, distinguished from each other, both in structure and function. Still higher in the scale new organs are successively added, as the process becomes more complex and refined, in order to assist the main operations carried on in particular parts of the apparatus; and as that apparatus approaches its highest degree of perfection, not only do the several parts of which it is composed increase in number and complexity, but each part becomes more and more isolated from the rest, a specific office being assigned to each in the division of labour that is made. Viewing, however, the digestive apparatus as a whole, whether simple or complex, whether consisting of a single uninterrupted surface, or divided into many separate portions, its nature is universally and invariably the same, and from the monad to man is endowed with analogous vital energies.
558. Comparative anatomy, which has succeeded in tracing through the different classes, orders, genera, and countless tribes of animals, the modifications in form and structure of the digestive apparatus, has shown that those modifications are invariably in strict adaptation to the kind of food on which the apparatus is destined to act and to the extent of the elaboration requisite to convert crude aliment into proper animal substance. To trace this adaptation through the rising and ever-varying series, is a most interesting and instructive study, not only exhibiting, in the very organs that elaborate its food, the physical and even the mental qualities assigned by the hand of nature to each individual, but oftentimes shedding a clear and bright light on the complex structures of the highest and most perfect organization. Striking and beautiful illustrations are afforded by these investigations of the principle formerly insisted on (vol. i. chap. i. p. 28, 3), that the communication of the higher faculties exalts the apparatus even of the very lowest processes, that the latter may work in harmony with the former. In conformity with this principle, as the nobler endowments exalt the animal in the scale of organization, so even its very digestive apparatus becomes extended, isolated, complex and refined.
559. The highest and most perfect form of the digestive apparatus is that which is disposed in a series of chambers in free communication with each other. In these chambers the food undergoes a succession of changes, by which it is progressively assimilated to the nature of animal substance. This assimilation, however, is never effected by the sole agency of the chambers themselves; it is accomplished, to a great extent, by the influence of special organs placed in the neighbourhood of the digestive chambers. In the lowest animal there is but one substance and one surface for every function; in the highest, even for the performance of the lowest function, there is the combination of many substances which are arranged in complex modes.
560. In man, the digestive chambers are five; the auxiliary organs are many.