It is in the large intestine that, next to the stomach, we find most varieties of struct ure. Cuvier has given the following resume of the principal facts connected with this subject, to which, however, there are numerous exceptions: 1. In man, the orangs, and the wombats, there are both etecum and vermiform appendix. 2. In the other quadrumana, the digitate carnivora, the marsupiata, the rodentia, the pachydermata, the ruminantia, the solipeds, and the amphibious mammals, there is a coecum (often in vegetable-feeders larger than the stomach, and probably subservient to the digestive process), but no vermiform appendix. 3. In the odentata, the plantigrade carnivora, and the cetacca, there is neither exeunt nor vermiform appendix. • In birds, as iu all other classes of animals, the alimentary canal varies according to the nature of the food, being long and capacious, and in some parts highly muscular, in the grauivorous tribes, while it is much more simple in those which live on fish and other animal food. We take the common fowl as a good example of the former class. The esophagus, about the middle of its course, and a little above the union of the clavicles (the furculum), presents an enlargement termed the crop or ingluries, which varies in form and structure according to the food, and is provided with numerous glandular follicles. Just before terminating in the gizzard, the esophagus again dilates to form a second but smaller cavity, known as the glandular stomach, prorentriculus, or 'centric vies succentariatus, from whence a copious secretion of gastric juice is poured out and mixed with the food, which, having previously been macerated by the_ secretion of the crop, now passes on to the gizzard, which is a muscular organ of immense strength, which grinds and crushes whatever is placed in its central cavity—a process that is facilitated by the presence of hard pebbles, which are instinctively swallowed by the bird, and act the part of millstones. There is no very marked division iu birds between the large and small intestine, the theoretical limit being indicated by the presence of two (sometimes only one) areal appendages.
There are no special points that we need notice regarding the digestive organs of reptiles, except that as the ophideans (serpents) and saurians (lizards) are mostly car nivorous, and most of the ehelonians (tortoises) are herbivorous, the apparatus in e s lion is more simple in the former than in the latter.
The amphibia afford us an excellent illustration of the close connection between the nature of the food and the development of the intestinal canal, In the young tadpole of the common frog, which lives upon the soft vegetable matter of our fresh-water ponds and ditches, the stomach is narrow and elongated, and the intestine is of extra ordinary length, and of nearly equal diameter throughout, being coiled up in a spiral manner, and distending the capacious abdomen. As the tadpole becomes metamor
phosed into a frog, it changes its vegetable food for slugs, worms, grubs, flies, etc., and, at the same time, the alimentary canal becomes very much shortened, and its divisions into stomach, etc., more distinctly marked.
In osseous fishes, the alimentary canal is generally shorter and more simple than in the higher vertebrata, in many—as, for example, the herring—being shorter than the body, and, excepting the stomach, running in nearly a straight line through it. In the cartilaginous fishes, as the sharks, rays, etc., a spiral valve winds in close turns from the pyloric to the anal extremity, leaving merely a small central aperture, along which the contents slowly progress. By this singular arrangement, the intestine, although short in proportion to the length of the animal, presents an enormous absorbing sur face. • The limits within which we must confine this article preclude us from noticing the various modifications which the digestive organs present in the various departments of the invertebrate animals; and Ave shall conclude with a few remarks upon the mode in which digestion is carried on in some of the lowest and simplest animals. The hydra or fresh-water polype is a minute animal, consisting of nothing but a bag or stomach, with tentacles surrounding its single orifice. The animalcules, etc:, which the hydra catches by these lasso-like tentacles (see HYDRA), are drawn into the interior, where they are digested, and applied to the nutrition of the organism, the insoluble portions being rejected by the aperture through which they entered. In the actinophrys sol, or sun-animalcule, there is no persistent aperture; but when its radiating filaments—from which it derives its name—have secured a particle of organized matter fit for its nour ishment, they twist over it, compress it against the globular body, which first yields, and becomes concave at the point, and finally closes over it, the prey being distinctly visible in the interior. This astomatous animalcule can thus form a mouth and stomach when it requires them. The indigestible remains are ejected by a corresponding reverse process. The amoeba or sponge-proteus neither has a mouth and stomach, nor can it con struct these organs: it simply folds itself around the solid particles from which it derives its nourishment, and imbibes their nutritious fluids through its eell-wall. We might adduce various other examples of animals devoid of a stomach, but we have brought forward sufficient evidence to show that the old doctrine, that this organ is a necessary constituent of an animal, cannot be sustained, when we approach that debat able-land which separates the two great organized kingdoms nature.