6. In many of the Vertebrata — such as Birds and Edentata—there are czeca to which, as to the smaller vermiform appendix of man. the above explanation cannot apply ; since the ordinary accessory' glands are also present. And some of the tubes seen in Insects are probably quite as supplementary. The struc ture of all these tubes seems to indicate that they are true organs of secretion. But whe ther this is their main function—or if so, what is the nature of their product—is utterly unknown. The supposition of their possess ing a special absorptive function only increases this obscurity, by leaving it doubtful whether the lower parts of such tubes reclaim a por tion of the secretion poured out by the upper —just as the intestine absorbs the bile after its entry into the duodenum— or whether they absorb materials derived from the general cavity of the intestine. But that increase of surface which facilitates mere absorption is effected by folds and projections so much better than by tubes, that, even supposing this latter re absorption to obtain, we ought at least to concede some modifying power to the secret ing surface. The ordinary situation of their apertures — near the junction of the small and large intestine — scarcely assists our spe culations. It may, however, indicate an ex posure of their secretions to the long and energetic absorption effected by the large in testine.
7. Finally, we may close these vague con jectures by attempting to include, in one for mula, most of the varieties seen in the whole animal kingdom. The complexity of the di gestive apparatus varies with that of the digestive function. And this is again the product of two chief elements :—the kind of food used; the nature of the animal to be nourished.
In respect to the food, we might almost form a scale of decreasing simplicity, begin ning with the rich chylous fluid that bathes the intestinal parasite, and passing through the various gradations of liquor sanguinis, blood, flesh more or less decomposed, vege table juices, fruits, v egetables, and grains: — gradations which, however -increased in number and minuteness, would all find their corresponding representatives in Natural His tory. And we have already seen that, through
out the Vertebrate series, there is a constant association of a long intestine or a compli cated stomach, with a vegetable diet.
As reg,ards the nature of the animal, the Acalephan, Crustacean, Cephalopod, Fish, Bird, Cetacean, all prey upon fish. Yet not only are their organs of digestion most di verse, but they even exhibit a certain corre spondence with the general development of each animal. Nor is it difficult to imagine why this is the case. Looking only at the unity of the organism, we might si priori expect, that a high development of the whole would imply an equal advance in the complexity of its chief parts. To this we may add, that one organ seems in a certain sense comple mentary to another,—the necessary, and not merely the formal, result of an increased evolution of its fellow. And, in conclusion, it is not unlikely that the complexity of the digestive organ in the higher animals may be referred to causes even more immediate than either of the preceding : —viz., to the inore composite chemistry of their structure, and the more rapid and energetic change of their substance. The structure of every animal is so far self-regulative,as to determine the perma nence of its own composition, by a process of which the blood is one main agent, and the tis sues generally another. But there is no rea son why we should exclude a third — why we may not suppose that the chemical assimila tion or likening of the foreign substances taken as food is commenced in the course of the digestive act — why, in short, the absorp tion of more numerous, abundant, and com plex aliinentary principles may not necessitate the co-operation of a more highly developed digestive organ.