EXPLANATION OF THE FIGURES. FRONTISPIECE. -The diagram opposite the title page is intended to present, at one view, the distribution of the principal types of animals, and the order of their successive appearance in the layers of the earth's crust. The four Ages of Nature, mentioned at page 221, are represented by four zones, of different shades, each of which is subdivided by circles indicating the number of formations of which they are composed. The whole disk is divided by radiating lines into four segments, to include the four great departments of the Animal Kingdom; the Vertebrates, with Man at their head, are placed in the upper compartment, the Articulates at the left, the Mollusks at the right, and the Radiates below, as being the lowest in rank. Each of these compartments is again subdivided to include the different classes belonging to it, which are named at the outer circle. At the centre is placed a figure to represent the primitive egg, with its germinative vesicle and germinative dot, (278,) indicative of the universal origin of all animals, and the epoch of life when all are apparently alike, (275, 276.) Surrounding this, at the point from which each department radiates, are placed the symbols of the several departments, as explained on page 155. The zones are traversed by rays which represent the principal types of animals, and their origin and termination indicates the age at which they first appeared or disappeared, all those which reach the circumference being still in existence. The width of the ray indicates the greater or less prevalence of the type at different geological ages. Thus, in the class of Crustaceans, the Trilobites appear to commence in the earliest strata, and to disappear with the carboniferous formation. The Ammonites also appeared in the Silurian formation, and did not become extinct before the deposition of the Cretaceous rocks. The Belemnites appear in the lower Oölitic beds; many forms commence in the Tertiary; a great number of types make their appearance only in the Modern age; while only a few have continued from the Silurian, through every period to the present. Thus, the Crinoids were very numerous in the Primary Age, and are but slightly developed in the Tertiary and Modern Age. It is seen, at a glance, that the Animal Kingdom is much more diversified in the later than in the earlier Ages. Below the circle is a section, intended to show more distinctly the rel ative position of the ten principal formations of stratified rocks (461) composing the four great geological ages; the numerals corresponding to those on the ray leading to Man, in the circular figure. See also figure 154. THE CHART OF ZOOLOGICAL REGIONS, page 195, is intended to show the limits of the several Faunas of the American Continent, corresponding to the climatal regions. And as the higher regions of the mountains correspond in temperature to the climate of higher latitudes, it will be seen that the northern temperate fauna extends, along the mountains of Mexico and Central America, much farther towards the Equator than it does on the lower levels. In the same manner, the southern warm fauna extends northward, along the Andes. FIG. 1. Simple cell, magnified, as seen in the house-leek. 2. Cells when altered by pressure upon each other; from the pith of elder. 3. Nucleated cells, (a,) magnified; b, nucleolated cells. 4. Cartilaginous tissue from a horse, magnified 120 diameters. 5. Osseous tissue from a horse, magnified 450 diameters. 6. Nervous fibres, showing the loops as they terminate in the skin of a frog. 7. Gray substance of the brain, magnified. 8. Head of an embryo fish, to show its cellular structure throughout 9. Diagram, to show the nervous system of the Vertebrates, as found in a monkey. 10. Diagram of the nervous system of the Articulates, as seen in a lobster. 11. Diagram of the nervous system of the Mollusks, as found in Natica heros. 12. Diagram of the nervous system of the Radiates, as found in Scutella, (Echinarachnius parma.) 13. Section of the eye. a, optic nerve; b, sclerotic coat; c, choroid coat; d, retina; e, crystalline lens ; fƒ, cornea; g, iris; h, vitreous body; ¿, chamber, divided by the iris. 14. Diagram, showing the effect of the eye on rays of light. 15. Position of the eye of the snail. 16. Eyes (ocelli) of the spider. 17. Eye-spots of a star-fish, (Echinaster sanguinolentus.) 18. Compound eyes, showing the arrangement of the facettes, and their connection with the optic nerve, as seen in a crab's eye. 19. Diagram of the human ear, to show the different chambers, canals, and bones. 20. Tympanum and small bones of the ear, twice the natural size; a tympanum; m, malleus; n, incus; o, orbiculare: s, stapes. 21 Section of the brain of a crow, showing the origin of the nerves of the special senses. 22. Diagram of the larynx, in man. 23. Larynx of the merganser, (Mergus merganser.) 24. Nests of Ploceus Philippinus, male and female. 25. Distribution of nerves to the muscular fibres. 26. Test. or crust-like covering of an Echinoderm, (Cidaris.) FIG 27. Muscular ribbons of the willow-moth, (Cossus ligniperda.) 28. Vertebra of a cod-fish. 29. Disposition of the muscles of the trout, (Salmo trutta.) 30. Disposition of the muscles of an owl, (Strix brachyotis.) 31. Jelly-fishes, (Stomobrachium cruciatum, Hippocrene Bougainvillıs. ) 32. Leech, showing the terminal cups. 33. Portion of a Nereis, showing the gills as organs of motion. 34-43. Modifications of the fore-arm. 34. Monkey. 35. Deer. 36. Tiger. 37. Whale. 38. Bat. 39. Pigeon. 40. Turtle. 41. Sloth. 42. Mole. 43. Whale. 44. Leg of a beetle. 45. Leg of a lizard. 46 Skeleton of a tiger. 47 Cuttle-fish, (Loligo illecebrosa.) 18 Sea-anemone, (Actinia marginata ;) a, mouth; b, stomach; c, general cavity of the body. 49. Planaria, showing the mouth, stomach, and its branches. 50. Jaws, stomach, and intestine of a sea-urchin, (Echinus lividus.) 51. Plan of the digestive organs of an insect. 52. Plan of the digestive organs of a land-slug, (Tebennophorus Carolini ensis.) 53. Globules of chyle. 54. Portion of intestine, showing the lacteals of man, and their entrance into a vein. 55. Jaws of an Echinoderm, (Echinarachnius parma 56. Jaws of a sea-urchin, (Echinus granulatus.) 57. Beak of a cuttle-fish. 58. Portion of the tongue of a mollusk, (Natica heros,) magnified. 59. Jaws of an Annelide, (Nereis.) 60. Trophi (organs for taking food) of a beetle. 67. Skull of a tiger, showing the muscles for mastication. 68. Head of a snapping-turtle, (Emysaurus serpentina.) 69. Head of a Whale, showing the whalebone. 70. Head of an ant-eater. 71. Head of an alligator. 72. Head of a skate-fish, (Myliobatis,) showing the palate bone. 73. Head of a monkey, showing the three different kinds of teeth. 74. Teeth of an insectivorous animal, the mole. 75. Teeth of a carnivorous animal, the tiger. 76. Teeth of a rodent. FIG. 77. A polyp, (Tubularia indivisa;) m, mouth; o, ovaries; p. tantacles. 78. Blood disks in man, magnified. 82 Portion of a vein opened, to show the valves 83. Network of capillary vessels. 84. Dorsal vessel of an insect, with its valves. 85 Cavities of the heart of mammals and birds. 88. Heart and bloodvessels of a gasteropod mollusk, (Natica.) 89. Tracheæ, or air tubes of an insect; s, stigmata; t, trachea. 90. Relative position of the heart and lungs in man. 91. Respiratory organs of a naked mollusk, (Polycera illuminata.) 92. Respiratory organs (gills) of a fish. 93. Vesicles and canals of the salivary glands. 94. Section of the skin, magnified, to show the sweat glands; a, the cut'a b. blood-layer; c,epidermis; g, gland imbedded in the fat-layer,(f. 95. Egg of a skate-fish, (Myliobatis.) 96. Egg of hydra. 97. Egg of snow-flea, (Podurella.) 98. Section of an ovarian egg; d, germinative dot; g, germinative vesi cle; s, shell membrane; v, vitelline membrane. 99. Egg cases of Pyrula. 100. Monoculus bearing its eggs, a a. 101. Section of a bird's egg; a, albumen; c, chalaza; e, embryo; s, shell y, yolk. 102. Cell-layer of the germ. 103. Separation of the cell-layer into three layers; 8, serous or nervou layer; m, mucous or vegetative layer; v, vascular or blood layer. 104. Embryo of a crab, showing its incipient rings. 105. Embryo of a vertebrate, showing the dorsal furrow. 106-8. Sections of the embryo, showing the formation of the dorsal canal 109. Section, showing the position of the embryo of a vertebrate, in re lation to the yolk. 110 Section, showing the same in an articulate, (Podurella.) 111 -22. Sections, showing the successive stages of development of the embryo of the white-fish, magnified. 123. Young white-fish just escaped from the egg, with the yolk not yet fully taken in. 124, 125. Sections of the embryo of a bird, showing the formation of the allantois; e, embryo; xx, membrane rising to form the amnios; a, allantois; y, yolk. 126. The same more fully developed. The allantois (a) is further de |