At the posterior extremity of the chylific ventricle are inserted a variable number of fine tubes, usually elongated and flexible, and terminating in _culs-de-sac_ at one end. Their colour, which depends on the liquid they may contain, is sometimes white, but more frequently brown, blackish, or green. They appear to be composed of a very slight and delicate membrane, as they are very easily torn, and nothing is more difficult than to unroll and to disengage them from the fatty or other tissues by which they are enveloped. [Illustration: Fig. 11.--Posterior extremity of the chylific ventricle, surrounded by the Malpighian vessels.] The function of these vessels is uncertain. Cuvier and Léon Dufour supposed them to be analogous to the liver, and on that account they have been called biliary vessels; and they are often termed the Malpighian vessels, after the name of their discoverer. According to M. Lacordaire, their functions vary with their position. When they enter the chylific ventricle, they furnish only bile; bile and a urinary liquid when they enter the posterior part of the ventricle and the intestine; and urine alone when they are placed near the posterior extremity of the alimentary canal. Fig. 11 represents part of the preceding figure more highly magnified, showing the manner in which these tubes enter the chylific ventricle. In our rapid description of the digestive apparatus of insects, it only remains for us to mention certain purifying organs which secrete those fluids, generally blackish, caustic, or of peculiar smell, which some insects emit when they are irritated, and which cause a smarting when they get into one's eyes. Less well developed than the salivary organs, they are often of a very complicated structure. In Fig. 12 is represented the secretory apparatus of the _Carabus auratus_, which will serve for an example: A represents the secretory sacs aggregated together like a bunch of grapes, B the canal, C the pouch which receives the secretion, D the excretory duct. [Illustration: Fig. 12.--Secretory apparatus of Carabus auratus.] Sometimes the secretion is liquid, and has a foetid or ammoniacal odour; sometimes, as in the Bombardier beetle (_Brachinus crepitans_), it is gaseous, and is emitted, with an explosion, in the form of a whitish vapour, having a strong pungent odour analogous to that of nitric acid, and the same properties. It reddens litmus paper, and burns and reddens the skin, which after a time becomes brown, and continues so for a considerable time. About the middle of the seventeenth century Malpighi at Bologna, and Swammerdam at Utrecht, discovered a pulsatory organ occupying a median line of the back, which appeared to them to be a heart, in different insects. Nevertheless, Cuvier, having declared some time afterwards that there was no circulation, properly so called, among insects, his opinion was universally adopted.