The digestive canal, which is a straight tube, extends throughout the whole length of the body, and terminates in the last segment of the body. It may be divided into the following parts:—
1. A narrow oesophagus, beginning with the mouth or buccal cavity, and receiving the contents of two or more salivary glands (d).
2. A wide mesenteron or mid-gut (n) extending throughout almost the whole length of the body.
3. A rectum which at its junction with the mid-gut receives the contents of two or four Malpighian tubes (g, h) which function as kidneys. Their function was for a long time unknown, but the discovery of crystals of uric acid in them placed the matter beyond doubt.
The heart has the form of a long pulsating dorsal vessel which extends through the whole length of the animal. It is divided into a number of chambers, which are attached to the dorsal wall of the body, and are furnished with muscles of a wing-like shape, which are known as the alary muscles, and which govern its pulsations. The chambers are furnished with valves and arteries for the exit of the blood, and slits known as ostia for the return of the blood to the heart. The blood enters the chambers of the heart from the body cavity through the ostia, and passes out through the arteries to circulate through the organs of the body and to return by the ostia.
Fig. 28.—Lithobius dissected. (After Vogt and Yung.) a, antennae. b, poison claws. c, brain. d, salivary glands. e, legs. f, nerve cord. g, Malpighian tube. h, Malpighian tube. i, vesicula seminalis. j, accessory gland. k, accessory gland. l, testis. m, thigh gland. n, digestive tube.
The two figures below (Figs. 29 and 30) show the position of the arteries and the ostia in a single segment of the body. The heart is too small and delicate to be seen with the naked eye; it therefore requires the aid of the microscope. A freshly-killed animal was therefore taken and prepared in the manner known to all microscopists, and extremely thin slices or sections cut horizontally from its back. One of these sections cut the whole length of the heart in one segment, which was accordingly drawn under the microscope (Fig. 29), and shows a longitudinal horizontal section through the whole length of the heart in a single segment, with the two ostia at each end of the segment and the two arteries in the middle.
The arteries, when they leave the body, pass into masses of fatty tissue on either side of the heart, and the other figure (Fig. 30) is intended to show the artery leaving the heart and penetrating into the fatty tissue. The figure is taken from the same section as the former one, but is much more highly magnified, so as to show more detail. The delicate coats of the heart are shown, the artery being covered with a clothing of large cells.
