351. In this simple apparatus is apparent the rudiment of the more perfect aërial respiration by the organs termed tracheæ, minute air tubes which ramify like blood-vessels through the body (fig. CXXX. 3). These air tubes open on the external surface by distinct apertures termed _spiracula_ or _spiracles_ (fig. CXXX. 2), which are commonly placed in rows on each side of the body (fig. CXXX. 2), with distinct prominent edges (fig. CXXX. 2), often surrounded with hairs; sometimes guarded by valves to prevent the entrance of extraneous bodies, and capable of being opened and closed by muscles specially provided for that purpose. These tubes, as they proceed from the spiracles to be distributed to the different organs of the body, often present the appearance of radii (fig. CXXX. 3), and when traced to their terminations are found to end in vesicles of various sizes and figures, but commonly of an elongated and oblong form. These minute vesicles, when examined by the microscope, are seen to afford still minuter ramifications, which are ultimately lost in the tissues of the body. 352. The tracheæ are composed of three tunics, the external dense, white and shining; the internal soft and mucous, between which is placed a middle tunic, dense, firm, elastic, and coiled into a spiral. By this arrangement the tube is constantly kept in a state of expansion, and is therefore always open to the access of air. A great part of the blood of the body, in the extensive class of creatures provided with this form of respiratory apparatus, including the almost countless tribes of insects, is not contained in distinct vessels, but is diffused by transudation through the several organs and tissues of the body. All the creatures of this class live in air, and possess great activity; they therefore require a high degree of respiration; yet they are commonly small in size, and often some portions of their body consist of exceedingly dense and firm textures; hence to have localized the function of respiration, by placing the seat of it in a single organ, would have been impossible, on account of the disproportionate magnitude which such an organ must have possessed; in this case it was easier to carry the air to the blood, than the blood to the air, and accordingly the air is carried to the blood, and, like the blood in creatures of higher organization, is diffused through every part of the system. [Illustration: Fig. CXXXI.—_Respiratory Organs of the Scorpion._ 1. Spiracles. 2. Integument of one half of the body turned back. 3. Branchial organs. 4. Cells or pouches in which they are lodged. _a._ One of the respiratory organs removed and magnified, showing its resemblance to the branchial leaflets, and presenting the pectinated appearance described in the text.] [Illustration: Fig. CXXXII.—_Apparatus of Respiration in the Frog._ 1. Trachea. 2. Vesicular lungs. 3. Stomach.]