5.--There are no absorbent vessels properly so called, and Plateau has long thought that the products of digestion pass by osmosis directly through the walls of the digestive tube, to mix with the blood in the perivisceral space. If we may rely upon what is now known of the process in Vertebrates, we should be led to modify this explanation. It is very likely that in Insects, as in Vertebrates, absorption is effected by the protoplasm of the epithelial cells, which select and appropriate certain substances formed out of the dissolved food. Not only do the epithelial cells transmit to the neighbouring blood-currents the materials which they have previously absorbed, but they subject certain kinds to further elaboration. The protoplasm of the epithelial cells of Vertebrates is capable of forming fat. Thus, a mixture of soap and glycerine, injected into the intestine of a Vertebrate, is absorbed by the lacteals in the form of oil-drops. Modern physiologists allow, too, that part of the peptone is similarly changed into albumen, without transport to a distance, by the activity of the epithelial lining. These facts explain why Plateau was unable to isolate the secretion of the epithelium of the chylific stomach of Insects. The cells are not secretory, but absorbent; and the secretion vainly sought for does not actually exist. CHAPTER VIII. THE ORGANS OF CIRCULATION AND RESPIRATION. _SPECIAL REFERENCES._ VERLOREN. Mém. sur la Circulation dans les Insectes. Mém. cour, par l’Acad. Roy. de Belgique, Tom. XIX. (1847). [Structure of Circulatory Organs in a number of different Insects.] GRABER. Ueb. den Propulsatorischen Apparat der Insekten. Arch. f. mikr. Anat., Bd. IX. (1872). [Heart and Pericardium.] LEYDIG. Larve von _Corethra plumicornis_. Zeits. f. wiss. Zool., Bd. III. (1852). [Valves in Heart.] LANDOIS, H. Beob. üb. das Blut der Insekten. Zeits. f. wiss. Zool., Bd. XIV. (1864). [Blood of Insects.] JAWOROWSKI. Entw. des Rückengefässes, &c., bei _Chironomus_. Sitzb. der k. Akad. der Wiss. Wien., Bd. LXXX. (1879). [Minute Structure and Development of Heart.] LANDOIS, H., and THELEN. Der Tracheenverschluss bei den Insekten. Zeits. f. wiss. Zool., Bd. XVII. (1867). [Stigmata.] PALMEN. Zur Morphologie des Tracheensystems (1877). [Morphology of Stigmata and Tracheal Gills.] MACLEOD. La Structure des Trachées et la Circulation Péritrachéenne. (Brussels, 1880.) LUBBOCK. Distribution of Tracheæ in Insects. Trans. Linn. Soc., Vol. XXIII. (1860). RATHKE. Untersuch. üb. den Athmungsprozess der Insekten. Schr. d. Phys. Oek. Gesellsch. zu Königsberg. Jahrg. I. (1861). [Experiments and Observations on Insect-respiration.] PLATEAU. Rech. Expérimentales sur les Mouvements Respiratoires des Insectes. Mém. de l’Acad. Roy. de Belgique, Tom. XLV. (1884). Preliminary notice in Bull. Acad. Roy. de Belgique, 1882.