[Illustration: 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 {50}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. [Illustration: FIG. 29.—Heart of _Julus terrestris_ showing ostia (_ost_) and arteries (_Art_) magnified.] [Illustration: FIG. 30.—Heart of _Julus terrestris_ showing structure of artery (_Art._) and external coat of heart (_ext.c_), also fat body (_Fb_), highly magnified. _Ht_, The cavity of the heart. The circular muscle fibres which surrounds the heart are shown just below the external coat (_ext.c_). _ogl_, Oil globules of the fat body.] Myriapods breathe by means of _tracheae_, with the exception of the Scutigeridae, which have an elementary form of lung which resembles that of spiders, and will be mentioned further on. These tracheae, as in Insects, are tubes lined with chitin, which is arranged in spiral bands. The tracheae open to the exterior by openings called _stigmata_, through which they receive the external air, which passes into the main tracheal tubes and into their ramifications, and thus effects the aeration of the blood.