the air-pipe; and, to keep food and drink from taking the wrong road, there is an effective system of valves. The parts act spasmodically whenever they are irritated by the pressure of solids and liquids.

Saliva is found in nearly all animals. Its universal office is to lubricate the food and so help it to glide easily along the pharynx and

FIG. 1.-MEDIAN ANTERO-POSTERIOR SECTION OF THE HUMAN FACE: a, septum of nose, with section of hard palate below it; b, tongue; c, section of soft palate; d, d, lips; u, uvula; r, anterior arch, or pillar of fauces; i, posterior arch; , tonsil; p, pharynx; h, hyoid bone; k, thyroid cartilage; n, cricoid cartilage; 8, epiglottis; v, glottis; 1, posterior opening of nares; 3. isthmus faucium: 4, superior opening of larynx; 5, passage into œsophagus; 6, orifice of right Eustachian tube."

gullet. This kind of saliva is a glairy mucus, and is the only kind in animals which do not chew the food-for example, birds, reptiles, and fishes. As an aid to digestion, the saliva of mammals will be considered later.

The most astonishing feats in swallowing are performed by the snakes. The boa can certainly swallow a goat or deer. Our common little snakes, the size of a finger, can swallow a large frog, a performance sufficiently remarkable. The process is very slow and tedious, and one would suppose painful. The boa first kills its prey by crushing it in its tightening coils, which break down the ribs and limbs and reduce the victim to a shapeless mass. By this horrible proceeding the carcass is gotten into condition to be more easily swallowed. After coating it with mucus, the boa begins the difficult operation of forc

ing the huge mouthful down its throat. But how shall the act be accomplished with no limbs to assist? As the under jaw divides in front, and articulates with the skull by the intervention of extra movable bones, the mouth and throat can stretch enormously. The sharp, conical teeth are recurved, acting like the barb of an arrow to hold whatever position is gained. Each side of the jaw is pushed forward in turn and gains a new and further hold on the carcass, which by successive slight movements is slowly pulled head first down the gullet. The common striped snake seizes a toad or frog, however he can catch him, usually by one or both hind legs, and immediately proceeds to "take him in," despite all protests and struggles.

D DDDDD

FIG. 2.-SKULL of a Serpent (Python): b, articular portion of the lower jaw; a, quadrate bone; c, squamosal portion of the temporal bone.

As the opposite of the enormous throat of the snake, the bulky whalebone whale has the smallest throat, proportionate to its size, of any animal-just large enough to admit the tiny creatures which are its food. We see in this a fine example of Nature's economy.

The alligator has a curious way of preventing the admission of water when swallowing prey. Seizing a fish or other small creature, the reptile rises to the surface of the water and flings it into the air; then, before it reaches the water, catches it and gulps it down. If the prey is too large to handle in this manner, it is carried to the shore to be devoured.

INGLUVIATION.-Many animals can not procure their particular food at all times. Such either can endure fasting, like members of the cat tribe, or have a special reservoir, as shown in the crop of a fowl. This crop is only a dilatation of the gullet. In the cormorant, the whole gullet is very capacious, for the purpose of storing fish; on account of which habit the bird has become a type of voracity. The pigeon has its crop divided into two-perhaps to give a better form for flight. The pelican has a bag beneath the lower jaw. Many small animals, insects especially, have crops. Similar in purpose is the first stomach, or paunch, of a cud-chewer. Birds which eat fruit, insects, or other food readily procured, and of a character which needs no delay in digestion, have usually no crop, or but a rudimentary one.

While the whole digestive tract serves the purpose of a reservoir, the special reservoirs have indeed a digestive function, serving to delay the food, that it may be acted upon for a sufficient time by the chemical fluids. Thus the crop of a bird secretes a fluid which softens and prepares the hard grain for subsequent trituration and digestion. ORGANS OF CHEMICAL DIGESTION.-As the organ of digestion proper is the one most nearly universal, it consequently affords the

FIG. 3.-Tania solium, OR SOLITARY
WORM: a, head, or scolex; b, tape

formed of many individuals, the last
of which, completely sexual, separate
represent the adult and complete ani-
mal. Each solitary worm is a colony.

under the name of proglottides, and

finest example of specialization and development. From the improvised cavity of the amoeba, there is a steady progress by minute steps to the complex apparatus of the mammals. Digestion is not more perfect, however, in the latter than in the former. The simple nutritive act of the amoeba is as perfect for itself as the differentiated process of the highest animals is for them. In the lowest animals, the function is single, and so simple that no special organ is necessary. As we rise in the animal scale, the function is divided into secondary functions, which require for their performance a corresponding number of special organs. Indeed, the complex functions of prehension, mastication, digestion, and circulation are only subdivisions of nutrition which begins in the lowest life as a single act. The present purpose, however, is not to trace the evolution of specialization of the digestive function further than to illustrate its general principles and methods, and present some of its peculiar and interesting features.

The tape-worm has no digestive organs whatever, having no use for them. A robber subsisting on the labors of its victim, it takes food in the same manner as a plant, by absorption from the outside. This is also the case with many lower protozoa.

The digestion of the amoeba is only one remove higher than that of the tape-worm-with no permanent organs, but extemporizing a stomach from the skin as required. A step higher still we find the hydra, with a permanent body cavity serving the purpose of a stomh. But it is not distinctively a stomach, as it is the common organ

of all the other vegetative functions. A single opening serves both to receive the food and expel the waste matter. Within even this narrow limit of structure, we find a host of low animals which exhibit a great variety of forms; so that from the hydra to the ctenophore is a progressive series showing a gradual specialization of this common

organ. In the higher part of the series, as for example the sea-anemone, there is a digestive cavity somewhat separated from the body cavity, though still connecting; and all the excretions have to find their way out through the oral aperture.

In the compound hydrozoans, produced by budding and division, such as sertularia and the so-called corals, the body cavity is continuous through the whole community. Hence each individual (though

h

FIG. 5. PERPENDICULAR SECTION OF Actinia holsatica (after Frey and Leuckart): a, mouth; b, gastric cavity; c, common cavity, into which the gastric cavity and the intermesenteric chambers open; d, intermesenteric chambers; e, thickened free margin, containing thread-cells of, f, a mesentery; g, reproductive organ; h, tentacle.

it is scarcely correct to regard it as such) has its stomach connected with the stomachs of all the others. Whatever food one digests serves to nourish the whole colony. They are absolute communists.

At this point should be presented the fact that in all animals the lining or secreting membrane of the food-canal is essentially but a continuation of the skin. That such is true of the amoeba is evident, for what was the outside of the body-mass becomes when food is enveloped the lining of the new cavity. The cup-shaped body of the hydra can

be turned inside out without interference with the business of digestion. The skin in these cases must have a chemical digestive power,

h

FIG. 6.-a, Sertularia (Diphasia) pinnata, natural size: a', fragment of the same enlarged, carrying a male capsule (a), and showing the hydrothecæ (h); b. fragment of Campanularia neglecia (after Hincks), showing the polypites contained in their hydrothecæ (h), and also the point at which the cœnosarc communicates with the stomach of the polypite (0).

as the food taken in mass, and frequently living substance, or even whole animals, is dissolved without trituration or mechanical aid. The

human skin has powers of absorption, and in some slight degree a person may be fed through it. The continuation of the skin which lines the digestive canal is supplied with new powers of secretion; so that, instead of producing perspirations, oils, etc., it manufactures chemicals for changing food. Hence the principle of digestion and the character of the organs are fundamentally the same in all animals, the lowest with the highest. Only in the amœba, hydra, etc., digestion is accomplished with the least possible expenditure.

FIG. 7.-DIGESTIVE SYSTEM OF A BEETLE (Carabus auratus): a, œsophagus; b,

crop; gizzard; d, chylific stomach; e, Malpighian tubes; f, intestine; g, cloaca; h, supposed renal vessels.

A true stomach must be wholly devoted to the elaboration of food, leaving other functions to other organs. This requires that it be wholly shut off from other cavities of the body; and it were better to have two openings, one for reception of food, the other an outlet for Waste matter, in order to give the food a single direction and prevent the mingling