CHAPTER XII

THE ALIMENTARY CANAL

The Alimentary Canal is the great route by which nutritive matter reaches the interior of the body. It is the most universal organ in the animal kingdom, and the rest are secondary or subservient to it. In the higher animals, it consists of a mouth, pharynx, gullet, stomach, and intestine.

It is a general law, that food can be introduced into the living system only in a fluid state. While plants send forth their roots to seek nourishment from without, animals, which may be likened to plants turned outside in, have their roots (called absorbents) directed inward along the walls of a central tube or cavity. This cavity is for the reception and preparation of the food, so that animals may be said to carry their soil about with them. The necessity for such a cavity arises not only from the fact that the food, which is usually solid, must be dissolved, so as to make its way through the delicate walls of the cavity into the system, but also from the occurrence of intervals between the periods of eating, and the consequent need of a reservoir. For animals, unlike plants, are thrown upon their own wits to procure food.

The Protozoa, as the amoeba and Infusoria, can not be said to have a digestive canal. The animal is here composed of a single cell, in which the food is digested. The jellylike amœba passes the food through the firmer outer layer (ectosarc) into the more fluid inner part (endosare), where it is digested (Fig. 1). The Infusoria,

which have a cuticle, and so a more definite form, possess a mouth, or opening, into the interior of their cell body, and at least a definite place where the excrement is passed out (Figs. 9, 11). But we can not call this cell cavity a digestive tract.

In the higher animals, the alimentary canal is a continuation of the skin, which is reflected inward, as we turn the finger of a glove.89 We find every grade of this reflection, from the sac of the hydra to the long intestinal tube of the ox. So that food in the stomach is still outside of the true body.

The simplest form of such a digestive tract is seen in the hydra (Fig. 18). Here the body is a simple bag, whose walls are

composed of two

layers of cells (ecto

derm and endoderm). A mouth leads into the cavity, and serves as well for the outlet of matter not

wanted. The en-
dodermal cells fur-
nish the juices by
which the food is
digested and ab-
sorb the nutritious
portions of it. The
polyps have also
but one external opening; but from this hangs down a
short tube, open at both ends, reaching about halfway to
the bottom of the body cavity. Such an arrangement
would be represented by a bottle with its neck turned
inward. In this suspended sac, which is somewhat con-

FIG. 236.
Dissected Actinia: a, the thick opaque skin
consisting of ectoderm, lined with muscular fibers;
c, the tubular tentacles communicating with the inter-
spaces, h, between the membranous vertical folds;
g, g', orifices in the walls allowing passage of respira-
tory water from one compartment to another; d, mouth
leading to gastric cavity, e.

stricted at the extremities, digestion takes place; but the product passes freely into all the surrounding chambers, along with the water for respiration (Fig. 236). The Medusæ, or jellyfishes, preserve the same type of a digestive apparatus; but the sac is cut off from the general cavity, and numerous canals radiate from it to a circular canal near the margin of the disk (Fig. 21). In the starfishes (Fig. 323), we find a great advance. The saclike stomach sends off two glandular branches to each arm, which doubtless furnish a fluid to aid in digestion (socalled hepatic caca). There is also an anus present in some forms, but it hardly serves to pass off the waste

matter.

Thus far we have seen but one opening to the digestive cavity, rejected portions returning by the same road by which they enter. But a true alimentary canal should have an anal aperture distinct from the oral. The simplest form of such a canal is exhibited by the sponge, in its system of absorbent pores for the entrance of liquid, and of several main channels for its discharge. The apparatus, however, is not marked off from the general cavity of the body, and digestion is not distinct from circulation.90

The sea urchin presents us with an important advance -one cavity with two orifices; and the complicated apparatus of higher animals is but the development of this type. This alimentary canal begins in a mouth well provided with teeth and muscles, and extends spirally to its outlet, which generally opens on the upper, or opposite, surface. Moreover, while in some of the worms the canal is a simple tube running through the axis of the cylindrical body from oral orifice to anal aperture, the canal of the sea urchin shows a distinction of parts, foreshadowing the pharynx, gullet, stomach, and intestine. Both mouth and vent have muscles for

constriction and expansion; and, as the vent is on the summit of the shell, and the latter is covered with

laria), and passed on from one to the other down the side of the

body, till they are dropped off into the P water.91

The worms present us with a great range of structure in the digestive tract. It is sometimes almost as simple as that of the hydra a mere sac. The earthworm has a tube running straight through the body, di

FIG. 237.-Diagrammatic Section of a Sea Urchin (Echinus): a, mouth; b, esophagus; c, stomach; d, intestine; f, madreporiform tubercle; g, stone canal; h, ambulacral ring; k, Polian vesicles, which are probably reservoirs of fluid; m, ambulacral tube; o, anus; p, ambulacra, with their contractile vesicles; r, nervous ring around the gullet; s, two nervous trunks, the right terminating, at anal pole, in an eye; t, blood-vascular rings connected by 7, the intestinal blood vessel; w, two arterial trunks radiating from the anal ring; x, an ovary opening at the anal pole in a genital plate, y; %, spines, with their tubercles.

vided into pharynx, esophagus, crop, gizzard, and sacculated intestine (Fig. 52). The leech has large sacs on each side of the intestine. The sea worms, like Nereis, have the pharynx armed with teeth, and some have glandular cœca attached to the intestine. The plan is that of a straight tube extending from mouth to anus. In myriapods and larvæ of insects, the same general plan is continued, the canal passing in a straight line from one extremity to the other, but showing a division. into gullet, stomach, and intestine.92 Crustacea, like the lobster, have a short gullet leading to a large cavity, situated in the front of the animal, which is a gizzard, rather than stomach, as it has thick muscular walls armed with teeth. A well-marked constriction

separates this organ from the intestine. The "liver," really a pancreas, is highly developed; instead of

numerous follicles, there is a large bilaterally symmetrical or

gan, divided into three lobes on

each side, pour

ing its secretion

into the upper

part of the intestine, which is the true stomach. Among insects, there is great variation in the form and length of the canal. The following parts can generally be distinguished: gullet, crop, gizzard, stomach, and large and small intestines,

with many gland

m

FIG. 238. — Anatomy of a caterpillar: g, h, esophagus; h,

i, stomach; k, hepatic vessels; 7, m, intestine; q, r, salivary glands; p, salivary duct; a, b, c, longitudinal tracheal trunks; d, e, air tubes distributed to the vis

cera; f, fat mass; v, x, y, silk secretors; z, their excretory ducts, terminating in t, the spinneret, or fusulus.

carnivorous species.

ular appendages.

The crop, gizzard, and large intes

tine

are some

times absent,

especially in the

In bees, the crop is called the

"honey-bag." The gizzard is found in insects having