EXPLANATION OF THE PLATE. FIG. 1.-Longitudinal median section of Caloptenus femur-rubrum, female, to show the course and divisions of the digestive canal. M, mouth; Oe, œsophagus; Cr1 anterior, Cr2 posterior part of crop; P, proventriculus; Div, blind sack or diverticulum of the stomach; Ven, ventricle; Il, ileum; col, colon; R, rectum; An, anus. FIG. 2.-Optical section of Malpighian tube. FIG. 3.-Section of the epithelium of the rectum of Caloptenus spretus. FIG. 4.-Transverse section of the hind part of the crop; ss, spines of the cuticula; rid, ridges; L, longitudinal; muc. C, circular muscular coat. FIG. 5.-Epithelium of the ileal folds; A, middle of folds; B, furrow between folds; L, longitudinal muscular band. FIG. 6.—Epithelial cells of the gastro-ileal valve of Edipoda sordida. FIG. 7.-Surface view of the gastro-ileal valves; Ven, ventricle; Bd, circular muscular band underneath the folds; I, ileum. FIG. 8.-Transverse section of a diverticulum. Tr, trachea; muc, circular mus cular coat. FIG. 9.-Transverse section through the furrow; F, between two ileal folds; cu, cuticula; Ep, epithelium; conn, connective tissue; muc. C, circular muscles; L, longitudinal muscular band. FIG. 10.-Transverse section of the gastro-ileal folds of Edipoda sordida: muc, muscular band, Bd, of Fig. 7. THE AMERICAN NATURALIST. VOL. XII.— JUNE, 1878. — No. 6. A LESSON IN COMPARATIVE HISTOLOGY. BY CHARLES SEDGWICK MINOT. NE of the first scraps of information which a young naturalist ΟΝΕ is apt to acquire, when he learns something of the history of zoology, is the knowledge of the great names of Linné and Cuvier. The reforms effected by these two great men in the methods of studying animated nature would alone secure them enduring fame; when we reflect, however, that they also labored industriously to add to our knowledge, and were successful as investigators as well as reformers, we must yield to the conviction that the honorable prominence assigned to them among naturalists is their just due. After both Linnæus and Cuvier had been dead for years, a German savant made a discovery concerning the elementary structure of plants, which revealed the fact that all the parts of plants are built up out of the same units, which have ever since been called cells. This great generalization is now taught to every botany class as one of the fundamental principles of vegetable anatomy and physiology, and the name of its great discoverer, Schleiden, is generally coupled with it. Schleiden made the results of his investigations generally known in the year 1837; and it was only two years later that his friend and countryman, Schwann, published a memoir showing that the same units-the cells-are found in all animals as well. It is a familiar fact to all, in these days when natural science has penetrated to the schoolhouse and the magazines, that matter consists of molecules, or minute particles, which are themselves composed of other particles still more minute-the atoms. Every mass of matter is made up of molecules, and indefinite numbers of molecules may be added to a mass without essentially |